It is the coastal zone stretching from southern Oregon into northern California, and out a hundred miles or so into the ocean.

Here is the sustained wind forecast of the UW WRF model for 5 PM this afternoon.  Notice the greenish area along the West Coast, with sustained winds of 30 kts or more?

Now examine the closer view from the 12-km domain.  Strong northerly winds stretch from just north of Brookings, Oregon (on the southern OR coast) to just north of San Francisco. You will also note a large change in sea level pressure over than area (solid lines), which has a lot to do about why we have strong winds there. 

So why the strong winds in this area during the warm season?  In the spring the East Pacific High strengthens and moves northward.   At the same time, the desert southwest of the U.S., warms and pressure falls.   So with rising pressure over the NE Pacific and falling pressure over the U.S. Southwest, an area of very strong pressure difference develops between them….over northern CA and southern Oregon.   Large pressure differences result in strong winds.

But as the infomercials always say, WAIT, THERE’S MORE!  The area of large pressure difference produces northerly winds over the coastal ocean, which in turns leads to upwelling…water coming up from below in the coastal ocean.  Those upwelling waters are COLD.  So we have very cold water offshore, and the land gets warm during the day.   The result is a large temperature gradient (peaking during the warmest time of the day), which enhances the pressure gradient even more, producing a super sea breeze.     So the morning is windy, but the afternoons are savage.

Lets take a look at the coastal winds for the past few days.  Here are the peak gusts at Buoy 46027…just offshore of the CA/OR border.  Wow…winds right now are gusting to NEARLY 45 knots (52 mph).

Or how about Cape Arago, Oregon (not far from North Bend)?  Almost 35 knots over land.  Can you imagine what that would be like on beach?   Similar winds are also found at Crescent City, CA (see below).

The northerly summer winds are so strong along the southern Oregon coast that a lot of trees are bent southward.  Want proof?  Check out this picture taken at Brookings:

I better stop before I get into trouble with the Brookings, Oregon Chamber of Commerce.  You might think this would be a good area for wind turbines, but the lack of transmission facilities and excessive strength of the winds might make it a poor choice.  But the cold water does help produce lots of coastal fog and that gives us the magnificent redwoods, which depend on fog drip for summertime water.

A reminder for those near Portland, Oregon…I will be giving a free public lecture on the Future of Weather Forecasting at the OMSI science center (more info to the right or here), at 10 AM on Saturday.

Figure 1. The ten warmest 12-month periods in the contiguous U.S. since record keeping began in 1895. Image credit: NOAA/NCDC.

Figure 2. The average temperature of 45.4°F during January – April 2012 was the warmest on record: 5.4°F above the 20th century average for the period, and was 1.6°F above the previous record set in 2006. January – April temperatures have been rising at about 1.9°F per century since 1895. Image credit: NOAA/NCDC.

April 2012: 3rd warmest on record
April 2012 was the third warmest April in the contiguous U.S. since record keeping began in 1895. Ten states had a top-ten warmest April, and no states were cooler than average. But what’s really remarkable about April was that eight states–Illinois, Indiana, Kentucky, Michigan, Wisconsin, Ohio, Pennsylvania, and West Virginia–had average April temperatures cooler than their March temperatures, even though their April temperatures were still above the long-term average for the month. These statistics show just how remarkably warm March 2012 was. Most extreme was Illinois, where April 2012 temperatures ranked in the top 20% for warmest Aprils, yet were cooler than March 2012 temperatures.

Figure 3. Temperature rankings for April 2012 in the Contiguous U.S. Ten states had a top-ten warmest April, and no states were cooler than average. Image credit: NOAA/NCDC.

Figure 4. NOAA’s U.S. Climate Extremes Index (CEI) for January – April shows that 2012 had the most extreme weather on record.

Most extreme January – April on record
NOAA’s U.S. Climate Extremes Index (CEI), an index that tracks the highest 10 percent and lowest 10 percent of extremes in temperature, precipitation, and drought, was 42% during the January-April period, over twice the average value, and the greatest on record. Remarkably, 82% of the contiguous U.S. had maximum temperatures that were in the warmest 10% historically, and 68% had warm minimum temperatures in the top 10%, with records going back to 1910. The previous records were 56% (2000) and 57% (1992) for maximum and minimum temperature, respectively. The percentage area of the U.S. experiencing top-10% drought conditions during January – April was 19%, which was the 17th greatest since 1910. Extremes in precipitation as computed by the CEI were near average for the January – April period.

Jeff Masters

Why are my seeds not germinating and rotting in the soil?
When will the soil be warm enough to sow my seeds or transplant my plants?

Well, I don’t pretend to be an expert in this, but I suspect it has something to do so with the temperature of the soil.

You can purchase or adapt a thermometer for taking soil temperature measurements

Lets talk about soil temperatures and how this compare to air temperatures, which is interesting beyond gardening.

The first thing to keep in mind is that the temperatures you hear on TV or read about in the news paper are for the air temperature, in shade, at a height of roughly 2 meters (6 feet).   So when meteorologists talk about surface air temperatures…that is what we mean.  And such temperatures SHOULD be taken above native vegetation, which is increasingly rare.

Now the temperature of the ground surface is often quite different than the air temperature.  During sunny days, ground temperatures can be much warmer than the air temperature (10-40F is not unusual), and on a cold, clear winter night, when the ground radiates heat to space, the ground temperature can be 1-8F cooler than air temperature.  After relatively warm periods, heat conduction from the warmed soil below can keep the surface temperature warmer than the air temperature. 

Several of these characteristics are evident in this plot of surface and ground temperatures at the WSDOT road weather site at Silica Road (near Quincy in eastern Washington, along I90).  Time advances to the left in this figure.   The road temperature zoomed up to roughly 105F, while the air temperature was in the low to mid 60s!  Wow.  And last night the air temperature was cooler than ground temperature as the former dropped in the upper 30s.

Now what about the soil temperatures?  The deeper you go the weaker the daily temperature variation becomes, with the soil temperature reflecting the average temperatures of the weeks and months before.  Here an example of the soil temperatures at roughly 1 inch (2.4 cm), 6 inches (15 cm), and 12 inches (30 cm).  Lots of daily variation in the top inch (in this case a range of 30F), but only a 2F range at 12 inches.  And you can it takes a while for the warming to propagate down into the soil.

So what are the current soil temperatures around the State?   Lets look at soil temps at 8 inches down from the highly useful Washington State University AgweatherNet network.  Here are the values today…mid 50s over western Washington and the mid to upper 60s over the western portion of eastern Washington.

 

Now lets look at  the 8-inch deep soil temperatures in Seattle (see graph) since January 1.  A steady rise until the cold spell this week, and now it is rising again with the warmer days. Roughly upper 50s F.

 Ok, so what does this have to do with the germination of vegetable seeds….and particularly why my bean seeds just rotting in the soil?

Take a look at the typical soil temperature required for germination of various seeds (see below).  Big variations.   Here is western Washington we are good to go for corn, spinach, carrots, and peas (not shown, but ok at 50F).  But beans need soil temps in the 60s.  Boy did I make a mistake!   Next time I will check the soil temperatures before I sow.

Vegetables or not, soil and ground temperatures are fascinating, and during the winter knowledge about them can save your life when roadway icing is threatening.

Figure 1. Volunteers in the city of Salvador, Brazil, have connected the dots have and drawn people’s attention to sea level rise and what it impacts in our life.

Figure 2. Madaba, Jordan. “Drops (of water) are dots of hope”. A beautiful message from King’s Academy in drought-prone Jordan.

Figure 3. Activists hold a banner in front of a damaged coral reef in the vulnerable Marshall Islands. Rising temperatures and increased CO2 uptake are raising the acidity of the ocean, which bleaches and ultimately kills fragile coral reefs.

Figure 4. One thousand students in Bekaa, Lebanon make their dots into the wheels of a giant bicycle to raise awareness about the threat of air pollution, and to advocate for bike lanes.

Figure 5. In 2009, at 17,785 feet in Bolivia’s Cordillera Oriental was the Chacaltaya Glacier. Before its unexpected melting, it was home to Bolivia’s only ski resort and the first tow-rope ever to be built in South America. Today all that remains is a rocky mountain-top that only receives seasonal snowfall. Photo by Lauren Farnsworth.

Figure 6. Ausable Valley, NY, USA: Young people in New York understand the first-hand impacts of climate change. Hurricane Irene, the third five-hundred-year climate event in the last twelve months, devastated communities in the region and pummeled the beachfront with debris.

Jeff Masters

 The reason?  A persistent trough over the Northwest, as shown by this upper level map (500 mb).   You will notice that while we have a trough, the central/eastern U.S. has had ridging, and yes, their temperatures have been well above normal, a pattern we have

seen a lot of this spring. Here is the mean and anomaly (difference from normal) temperatures for the first three days of May.  We are cold and purple–that seems appropriate, purple for cold, and that is the UW Husky color.  The eastern U.S. is brown, with warm anomalies approaching 8F.

But this will change as ridging is now occurring aloft, as illustrated by the latest European Center upper air prediction for Sunday at 5 PM.   This ridging will hold at least through Monday, so expect lots of sun and much warmer temperatures on Sunday and Monday–in fact, many of you will be in the 70s Monday afternoon.

Here are the latest UW WRF model forecasts for surface air temperatures (remember these are for 2-m above the surface).   On Sunday, mid-60s on BOTH sides of the Cascades (except for NW WA and coastal residents near the water, sorry).

And 70s in that Willamette Valley, a natural hot box due to its isolation from the ocean influence.

But as good as Sunday will be Monday will be spectacular, with seventies extending to Seattle and the torrid Willamette Valley getting near 80F. 

Finally, let me end by noting that we have had strong Puget Sound convergence zones over the past few days, with some being unusually broad.   A few samples from the Camano Island radar for your amusement.   The nice thing about such situations is that you know where to go to escape the rain.

Let me end with a reminder that I will be giving a talk in Portland (OMSI Science Museum) at 10 AM on Saturday on the future of NW weather prediction.  More info to the right.  I will bring a few books to sign as well.

Figure 1. Women who work on a tea farm in Assam, India hold up a dot in honor of Climate Impacts Day (May 5, 2012), to urge people to connect the dots between climate change and the threat to their livelihood. Chai is one of the most consumed beverages in India, but a prolonged dry spell and extreme heat has affected tea plantations in Assam and Bengal with production dropping by 60% as compared to the same period in 2011. Image credit: 350.org.

May 5: Climate Impacts Day
On Saturday, May 5 (Cinco de Mayo!), the activist group 350.org, founded by Bill McKibben, is launching a new effort to “connect the dots between climate change and extreme weather.” They’ve declared May 5 Climate Impacts Day, and have coordinated an impressive global effort of nearly 1,000 events in 100 countries to draw attention to the links between climate change and extreme weather. Their new climatedots.org website aims to get people involved to “protest, educate, document and volunteer along with thousands of people around the world to support the communities on the front lines of the climate crisis.” Some of the events planned for Saturday: firefighters in New Mexico will hold posters with dots in a forest ravaged by wildfires; divers in the Marshall Islands take a dot underwater to their dying coral reefs; climbers on glaciers in the Alps, Andes, and Sierras will unfurl dots on melting glaciers with the simple message: “Melting”; villagers in Northeastern Kenya will create dots to show how ongoing drought is killing their crops; in San Francisco, California, aerial artist Daniel Dancer and the Center for Biological Diversity will work with hundreds of people to form a giant, moving blue dot to represent the threat of sea level rise and ocean acidification; and city-dwellers in Rio de Janeiro hold dots where mudslides from unusually heavy rains wiped out part of their neighborhood. I think its a great way to draw attention to the links between climate change and extreme weather, since the mainstream media coverage of climate change has been almost nil the past few years. A report by Media Matters for America found out that nightly news coverage about climate change on the major networks decreased 72% between 2009 and 2011. On the Sunday shows, 97% of the stories mentioning climate change were about politics in Washington D.C. or on the campaign trail, not about extreme weather or recent scientific reports. You can check out what Climate Impacts Day events may be happening in your area at the climatedots.org website.

Figure 2. Front Street Bridge on the Susquehanna River in Vestal, NY, immediately following the flood of September 8, 2011. Image credit: USGS, New York. In my post, Tropical Storm Lee’s flood in Binghamton: was global warming the final straw? I argue that during September 8, 2011 flood, the Susquehanna River rose twenty feet in 24 hours and topped the flood walls in Binghamton by 8.5 inches, so just a 6% reduction in the flood height would have led to no overtopping of the flood walls and a huge decrease in damage. Extra moisture in the air due to global warming could have easily contributed this 6% of extra flood height.

Jeff Masters

Figure 1. True-color MODIS image of Hurricane Irene over North Carolina taken at 11:35 am EDT August 27, 2011. At the time, Irene was a Category 1 hurricane with 85 mph winds. Image credit: NASA.

At last month’s 30th Conference on Hurricanes and Tropical Meteorology of the American Meteorological Society, Paul Ruscher of Florida State University explained how Irene’s storm surge came within 8 inches of flooding New York City’s subway system, which would have caused devastating damage. At the current global rate of sea level rise of 3.1 mm/year, a repeat of Irene 65 years from now would be capable of flooding the subway system, if no action is taken. To read more about New York City’s vulnerability, see Andrew Freedman’s analysis at Climate Central, Climate Change Could Cripple New York’s Transportation, or my November 2011 blog post, Hurricane Irene: New York City dodges a potential storm surge mega-disaster.

Jeff Masters

One can not be considered a Northwesterner if you don’t grouse about our endless cool, showery weather during Spring.  And it turns out that our pioneer ancestors—more than 150 years ago–were doing the same thing.

Let’s go back to 1855 and the town of Steilacoom on Puget Sound’s waters.  In a hard-hitting commentary in the Puget Sound Courier published at the end of May,  Honest John Tompkins chastised his fellow citizens for their “growling” about the weather.

He noted that Spring 1855 started out well:

“Throughout the almost entire month of March last, the people everywhere in the Territory, were rejoicing at the beautiful summer-like weather … Every one seemed to be perfectly happy and contented, and there was not a countenance to be met with between the Columbia River and the Straits of Fuca, that was not the index of a joyous heart.”

But then the weather shifted:

“Well, March went out, April came in, and with it, cold, wet, disagreeable weather, and a universal spirit of discontent, and a disposition to “growl”"

“Throughout the entire month, and even up to this, the last day of May, it has been precisely the same, and some amongst us profess to be so thoroughly disgusted with the weather …. that they threaten to leave the Territory altogether.”

Well, I have heard that one before….people threatening to move to California or some other warmer clime! 

By the end of May the weather had improved and Honest John made the false claim I have heard so often….the idea of balance…if the weather is bad for a while, it will surely be good later (and vice versa).  That some kind of omnipotent weather god is ensuring that the yin and yang of weather will be in rough equilibrium.   Sounds profound and true.  It isn’t.  And I must admit I have heard some TV weathercaster types saying the same kind of thing.

“Is it not fair to presume or reasonable to expect, that there will be a large number of wet disagreeable days in the Spring, if there be but few in the Winter, the season in which it was natural to expect them? Most assuredly. And again, now that really fine weather has broke upon us—may we not reasonably anticipate, for a series of months, the same clear and bracing atmosphere, the same un-cloudy days, and deliciously cool nights for which the summers of Washington Territory are so justly and widely celebrated?”

Then Honest John takes on God’s work in putting the complainers in their place.  The kind of work I try to model in my blog.

“Men from New England… You have “growled” a good deal at the weather this Spring, have you not?  … a few short years since, among the bleak hills of your native States, you shivered beneath the fierce Northern blasts that swept from their summits, for five long winter months in each, and then that for three more you were almost constantly subjected to those damp, unwholesome, and consumptive-engendering fogs which are driven by the raw East winds, from “Newfoundland’s Banks” into every “nook and corner of the Northeastern section?” 

I wish I could write like that.  “Damp, unwholesome, consumptive-engendering fogs”!!

And then he took out the heavy guns for those from the warm, tropical southeast:

“Men from the “sunny South!”…  for you are … no freer from blame. Has not the truth many times occurred to your minds … that the enervating influence upon both mind and body of your own tropical climate, and the sluggish vapors arising from your extensive swampy lands, impregnating the entire atmosphere with contagious diseases, are more to be feared and deprecated, than the rains and comparatively cold weather of Washington Territory”

Early Steilacoom

 He ends his reproach to complainers by noting:

“let us have no more of your foolish and wicked repinings at the season.” 

Wise words for all of you to remember.

But some of you insist on complaining.  Surely, the spring of 2011 was worse than any of ancient memory.   Fine, let’s check the numbers, with the assistance of Mark Albright, past State Climatologist.

The average temperature in 1855 for April/May was 51.6 F at Steilacoom which lies between SeaTac and Olympia. The April/May 2011 mean temperature averaged over SeaTac/Olympia was 48.0 F, considerably cooler than Steilacoom experienced 157 years ago in 1855. The coldest April at Steilacoom over the 19 years between 1850 and 1868 was 45.2 F in 1859. This is warmer than the 44.6 F recorded in April 2011 at SeaTac/Olympia.  (Please…no global warming skeptic comments!)

So complainers among you take heart…last spring’s cool/dampness was far worse than the weather that unsettled our tough, stout pioneer ancestors.  

This spring is a pussycat in comparison.

 Nice mini wall cloud with the funnel (the wider part hanging from the cloud base).

Today, I received an impressive picture of two funnels caught over the tulip fields on Sunday at around 12 PM.  This photo was taken by Chris Evans.

http://spindriftphoto.photoshelter.com/image/I0000L1WR_zt.Y50

To be honest with you, I as a little surprised by these funnels.  Yesterday was not very unstable and there was very little precipitation or convection.  Here is the NWS radar at that 12:29 PM. UNIMPRESSIVE.

And here is a blown up view over the area of interest 10 minutes later…a developing cell near Mt. Vernon (green color)…but nothing significant.

And take a look at the high-resolution satellite imagery at 12:30 PM..the arrow is pointing to the convective cell (white spot)…..you would not write home to your mother about this.

So the only explanation of this is that there was enough horizontal shear (change of winds in the horizontal) to produce some rotation (we call it vorticity in the business).  A weak updraft spun this weak rotation up, like an ice skater rotating faster as she pulls her arms in.  In my NW weather book I go into this mechanism in some depth.

As an aside…quite a blustery April day today as a fairly strong low pressure passed to our north, setting up a large north-south pressure difference (see map).

Some folks got gusts to 30-40 mph, particularly near the water–branches and leaves were all over the ground as I biked home today from the UW.  Take a look at the winds on the Evergreen Point Floating Bridge…bet there was some serious spray on the eastbound roadway!

Figure 1. Hurricane Wilma’s pinhole eye as seen at 8:22 a.m. CDT Wednesday, Oct. 19, 2005, by the crew aboard NASA’s international space station as the complex flew 222 miles above the storm. At the time, Wilma was the strongest Atlantic hurricane in history, with a central pressure of 882 mb and sustained surface winds estimated at 185 mph. The storm was located in the Caribbean Sea, 340 miles southeast of Cozumel, Mexico. Image source: NASA’s Space Photo Gallery.

Figure 2. Damage in Joplin, Missouri after the EF-5 tornado of May 22, 2011. Image credit: wunderphotographer thebige.

Official all-time strongest winds in an Atlantic hurricane: 190 mph
The official record for strongest winds in an Atlantic hurricane were for Hurricane Camille of 1969, which was estimated to have 190 mph winds when it made landfall in Pass Christian, Mississippi. In Dr. Bob Sheets’ and Jack Williams’ book, Hurricane Watch, they recount: On Sunday afternoon, August 17, and Air Force C-130 piloted by Marvin Little penetrated Camille’s eye and measured a pressure of 26.62 inches of mercury. “Just as we were nearing the eyewall cloud we suddenly broke into a clear area and could see the sea surface below,” the copilot, Robert Lee Clark, wrote in 1982. “What a sight! Although everyone on the crew was experienced except me, no one had seen the wind whip the sea like that before…Instead of the green and white splotches normally found in a storm, the sea surface was in deep furrows running along the wind direction….The velocity was beyond the descriptions used in our training and far beyond anything we had ever seen.” So, the 190 mph winds of Camille were an estimate that was off the scale from anything that had ever been observed in the past. The books that the Hurricane Hunters carried, filled with photos of the sea state at various wind speeds, only goes up to 150 mph (Figure 2). I still used this book to estimate surface winds when I flew with the Hurricane Hunters in the late 1980s, and the books are still carried on the planes today. In the two Category 5 hurricanes I flew into, Hugo and Gilbert, I never observed the furrowing effect referred to above. Gilbert had surface winds estimated at 175 mph based on what we measured at flight level, so I believe the 190 mph wind estimate in Camille may be reasonable.

Figure 3. Appearance of the sea surface in winds of 130 knots (150 mph). Image credit: Wind Estimations from Aerial Observations of Sea Conditions (1954), by Charlie Neumann.

Figure 4. Radar image of Hurricane Camille taken at 22:15 UTC August 17, 1969, a few hours before landfall in Mississippi. At the time, Camille had the highest sustained winds of any Atlantic hurricane in history–190 mph.

The infamous hurricane hunter flight into Wilma during its rapid intensification
While I was at last week’s conference, I had a conversation with Rich Henning, a flight meteorologist for NOAA’s Hurricane Hunters, who served for many years as a Air Reconnaissance Weather Officer (ARWO) for the Air Force Hurricane Hunters. Rich told me the story of the Air Force Hurricane Hunter mission into Hurricane Wilma in the early morning hours of October 19, 2005, as Wilma entered its explosive deepening phase. The previous airplane, which had departed Category 1 Wilma six hours previously, flew through Wilma at an altitude of 5,000 feet. They measured a central pressure of 954 mb when they departed the eye at 23:10 UTC. The crew of the new plane assumed that the hurricane, though intensifying, was probably not a major hurricane, and decided that they would also go in at 5,000 feet. Winds outside the eyewall were less than hurricane force, so this seemed like a reasonable assumption. Once the airplane hit the eyewall, they realized their mistake. Flight level winds quickly rose to 186 mph, far in excess of Category 5 strength, and severe turbulence rocked the aircraft. The aircraft was keeping a constant pressure altitude to maintain their height above the ocean during the penetration, but the area of low pressure at Wilma’s center was so intense that the airplane descended at over 1,000 feet per minute during the penetration in order to maintain a constant pressure altitude. By they time they punched into the incredibly tiny 4-mile wide eye, which had a central pressure of just 901 mb at 04:32 UTC, the plane was at a dangerously low altitude of 1,500 feet–not a good idea in a Category 5 hurricane. The pilot ordered an immediate climb, and the plane exited the other side of Wilma’s eyewall at an altitude of 10,000 feet. They maintained this altitude for the remainder of the flight. During their next pass through the eye at 06:11 UTC, the diameter of the eye had shrunk to an incredibly tiny two miles–the smallest hurricane eye ever measured. During their third and final pass through the eye at 0801 UTC, a dropsonde found a central pressure of 882 mb–the lowest pressure ever observed in an Atlantic hurricane. In the span of just 24 hours, Wilma had intensified from a 70 mph tropical storm to a 175 mph category 5 hurricane–an unprecedented event for an Atlantic hurricane. Since the pressure was still falling, it is likely that Wilma became even stronger after the mission departed.

I’ll have a new post by Tuesday at the latest.

Jeff Masters

Amarillo, TX: 99° (old April record 98° on 4/22/1989 and 4/22/1965)
Lubbock, TX: 101° (old April record 100° on 4/16/1925 and /22/1989)
Dalhart, TX: 96° (old April record 94° on 4/22/1989)
Borger, TX: 99° (tied April record set on 4/22/1965)
Midland, TX: 104° (old April record 101° on 4/21/1989)
Abilene, TX: 104° (old April record 102° on 4/16/1925)
Childress, TX: 106° (old April record 102° on three occasions, most recently on 4/3/2011)

According to wunderground’s weather historian Christopher C. Burt, both Texas and Oklahoma came within 2°F of their all-time April state high temperature record yesterday. Altus, Oklahoma hit 104°, falling 2° short of the April state record of 106° set at the Magnum Research Station in 1972. In the Texas Mesonet, it hit 111° at Knox City 3NW, which is just 2° short of the Texas April state record of 113° set at Catarina in 1984. According to Mr. Burt, What is amazing is that Knox City is in the north-central part of the state, not down in the Rio Grande region like Catarina. The 111° would probably be pretty close to whatever the all-time hottest temp for ANY month might be in that location (probably around 115°). On Sunday this week, Nevada just missed setting their April state high temperature record, when the mercury hit 105° in Laughlin (April state record: 106° in 1989.)

Figure 1. At least 36 of the roughly 400 major U.S. cities that maintain automated weather sensors at their local airports (8%) have set or tied all-time April high temperature records so far this month. The records set yesterday in Texas are not yet in the database, and are not included on this map. Image taken from our new Record Extremes page.

Earlier this week, all-time record April heat hit large portions of Arizona, California, Nevada, Utah, Colorado, and Wyoming. At least 36 of the roughly 400 major U.S. cities that maintain automated weather sensors at their local airports (8%) have set or tied all-time April high temperature records so far this month; no all-time April cold records have been set. The U.S. has been on an extraordinary pace of setting high temperature records so far in 2012. During March 2012, an astonishing 32% of all the major airports in the U.S. set all-time March high temperature records. For the year-to-date, there have been 184 new all-time monthly high temperature records set at the major airports, and 6 all-time monthly low temperature records. Not surprisingly, the period January – March this year has been the warmest such period in the U.S. since record keeping began in 1895.

Figure 2. Total precipitable water (in mm) for this morning shows a surge of moisture moving westwards though the Caribbean. Precipitable water values in excess of 51 mm (2 inches, orange colors) are capable of generating heavy flooding rains. Image credit: University of Wisconsin CIMSS.

Heavy rains kill nine in Haiti
The rainy season has begun on the Caribbean island of Hispaniola, where heavy rains that began on Monday have triggered mudslides and floods that killed nine people. Nearly 500,000 people are still homeless in Haiti from the January 2010 earthquake, making the country highly vulnerable to flooding disasters. Heavy flooding was also a problem this week in the neighboring Dominican Republic, where 11,000 people were evacuated; no deaths were reported there, however. Precipitation forecasts from the GFS model suggest that the worst is over for Hispaniola, with the axis of greatest moisture expected to move west of the island today. This surge of moisture will bring heavy rains to Jamaica, Cuba, the Bahamas, Cayman Islands, and South Florida during the remainder of the week.

Jeff Masters

Figure 1. Departure of temperature from average for March 2012. The U.S. and Canada experienced the most extreme warmth of anywhere in the globe during March. Image credit: National Climatic Data Center (NCDC) .

La Niña conditions no longer present
La Niña conditions are no longer present in the equatorial Pacific, where sea surface temperatures were approximately 0.4°C below average during March and the the first half of April. The threshold for a La Niña is for these temperatures to be 0.5°C below average or cooler. NOAA’s Climate Prediction Center forecasts that La Niña will be gone by the end of April. The majority of the El Niño computer models (48%) predict neutral conditions for this fall, during the August – September – October peak of hurricane season, though 35% of the models predict an El Niño will develop. El Niño conditions tend to decrease Atlantic hurricane activity, by increasing wind shear over the tropical Atlantic.

Figure 2. Ice age data show that first-year ice made up 75% of the Arctic sea ice cover this March. Thicker multi-year ice used to make up around a quarter of the Arctic sea ice cover. Now it constitutes only 2%. Image credit: National Snow and Ice Data Center (NSIDC).

March Arctic sea ice extent ninth lowest on record
Arctic sea ice extent was at its ninth lowest extent on record in March, according to the National Snow and Ice Data Center (NSIDC). This was the highest since 2008 and one of the highest March extents in the past decade. Ice extent as of April 23 was close to average, one of the few times during the past decade that has occurred. However, ice in the Arctic is increasingly young, thin ice, which will make it easy for this year’s ice to melt away to near-record low levels this summer, if warmer than average weather occurs in the Arctic. During the 1980s, more than 20% of the Arctic ice was more than 4 years old; this March, that fraction was just 2%. Satellite sea ice records date back to 1979.

I’ll have a new post by Friday.

Jeff Masters

The Pendleton, Oregon radar got a nice view of the passing storms….here are a few examples an hour apart.  The red and blue areas are either very heavy rain or hail. (click on images to enlarge)

These thunderstorm were associated with a log of lightning…here are some samples from the national lightning detection network at 7 PM and 9:30 PM.

Lets take a look at the rainfall totals for the storm, which ranged as high as 1.29 inches at Heppner, Oregon (see graphic).  Do you know what happened at Heppner?  It was the site of the most deadly convective flash flood in U.S. history in which 247 individuals lost their lives.

Here is a picture of the aftermath of the storm on June 14, 1903.   There is a

Heppner, Oregon in 1903

comprehensive book on the subject published by UW Press called Calamity (info here).
Heppner is on the slopes of the Blue Mountains and located on a normally minor creek…thunderstorms in the Blues turned it into a torrent.

Strong winds accompanied these powerful storms–take a look at the observations at Walla Walla.  Winds gusted to 47 knots (54 mph), with higher gusts at nearby Hermiston (60 mph).  Before the storm the temperature was near 80F with a dew point of 62F.  Folks,that is a really high dew point for these parts….more like the eastern U.S.

How good were the forecasts?   Well, I wish I could cite this as another success for the WRF and other models, but it wasn’t.  The morning WRF model run (initialized at 5 AM Monday) showed very little, just a few light showers moving through (see graphic of the 24h hour precipitation ending 5 AM this morning)

24-h precipitation from UW WRF ending 5 AM

This failure was also evident in the NWS model (NAM) and the UW ensembles.  The interesting thing is that the models and soundings showed great instability (high CAPE–convective available potential energy);they just didn’t have any forcing (front, upper level short wave) to trigger them off.   The NWS folks in Spokane discussed this situation during the afternoon and were not sure what to predict, so they just noted the potential for thunderstorms, with a lot of uncertainty.  Here is what they said:

"Tonight and Tuesday...Upper level ridge axis remains fixed over   the PacNW...with deep southerly flow continuing to pump moisture   and instability northward. Model soundings and LAPS analysis show   CAPE values exceeding 1000 j/kg over most of our forecast area   east of a line from Republic to the Tri-Cities. These values are   very impressive for April...let alone any time of year...but that`s   what happens when surface temps rise into the 80s...with dewpoints   in the mid 40s to mid 50s. Despite the potential   instability...tapping into it is no sure bet...especially given   the lack of a trigger or focusing mechanism. Looking at the latest   satellite imagery...there really is nothing downstream of us which   will trigger convection in the immediate future. Looks like better   chances will arrive as the convection over southern OR/ID move   northward overnight. There is a small chance some of this will   reach the WA/OR border toward sunset...and spread gradually   northward through the overnight hours." 

This was a very reasonable analysis.  The NWS staff in Pendleton were even more emphatic about the potential  for thunderstorm…and they were correct.    Shows you the importance of having human forecasters for such difficult events.

Clearly, there was some forcing feature\s that the models missed…it will be interesting to explore this case further.  The runs initialized at 5 PM were much better, but they weren’t available until after the action got started.

Figure 1. All-time heat records for the month of April were set at 56 stations April 21 – 23, including at seven major cities. Image taken from wunderground’s new extremes page.

As is often the case when a major Nor’easter is affecting the Eastern U.S., the record-breaking heat is due to a contortion of the jet stream that has created a strong ridge of high pressure over the Western U.S. Wunderground’s extremes page lists 56 stations in the West in the past four days that have tied or broken all-time heat records for the month of April, including:

Phoenix, Arizona: 105°F (previous 105° April temperatures occurred on 4/20/1989 and 4/29/1992)
Las Vegas, Nevada: 99°F (tying old record set 4/30/1981)
Reno, NV: 90° (old record 89° 4/30/1981)
Elko, NV: 87° (old record 86° 4/30/1981). This also beat the previous so-warm-so-early-in-the-season record by 4°
Ely, NV: 84° (old record 82° 4/28/1992)
Winnemucca, NV: 90° (tying old record set 4/30/1981)
Grand Junction, CO: 89° (tying all-time April record also set on 4/29 and 4/30, 1992)

Boise, ID (91°) and Salt Lake City (88°) both came within 1°F of their record April max.

Figure 2. A late-season Nor’easter on April 23, 2012 leaves heavy snow on a farm in Penfield, NY. Image credit: wunderphotographer tvsportsguy.

Late-season Nor’easter winding down
The powerful late-season Nor’easter that brought snow, high winds, and heavy rains to the Northeast yesterday is winding down as it moves northwestwards into Canada. The storm brought an unusual amount of snow for so late in the season to western Pennsylvania, western New York, and the higher elevations in West Virginia. Laurel Summit in the Allegany Mountains of Western Pennsylvania recorded 23.2″ on snow from the storm, according to the latest NWS Storm Summary, and many other higher-elevation locations saw snowfall amounts in the 6 – 11 inch range. Snow amounts were considerably lower in the major cities of the region; Buffalo, New York got 0.9″, Rochester, New York, 2.8″, and Erie, Pennsylvania, 0.5″. The wet, heavy snow fell on regions where trees had already come into leaf, thanks to the surprise “Summer in March” heat wave that brought 80° temperatures to the Northeast over a month ago. High winds that accompanied the heavy snow caused extensive tree damage and power outages to at least 75,000 people in the region. However, the storm may have done more good than harm–widespread rainfall amounts of 2 – 4 inches occurred across Massachusetts, Rhode Island, New Jersey, Connecticut, and Southeast New York, which is under moderate to severe drought. Rainfall deficits in the region were generally 5 – 10 inches, so the Nor’easter’s rains will make a significant dent in the drought. Wunderground’s weather historian Christopher C. Burt made a post last year on Record Late Season Snowfalls, and will updating that post later today with the statistics from this new storm.

Jeff Masters

Figure 1. A late-season Nor’easter takes aim at the Northeast U.S. in this satellite image taken at 9:31 am EDT 4/23/12. Image credit: NASA/GSFC.

Heavy snow belting PA, NY, and WV
The big story with this Nor’easter is the heavy snow falling in Western Pennsylvania, Western New York, and the higher elevations in West Virginia. Wet, heavy snow of 6 – 12 inches will be common, particularly at elevations higher than 2,000 feet. Over six inches of snow had already fallen in the Allegany Mountains of Western Pennsylvania as of 8 am EDT this morning. The wet, heavy snow is falling on regions where trees have already come into leaf, thanks to the surprise “Summer in March” heat wave that brought 80° temperatures to the Northeast over a month ago. High winds will accompany today’s snow, and extensive tree damage and power outages can be expected. Winds have gusted as high as 39 mph this morning in Rochester, NY. Sustained north to northeast winds blowing off of Lake Ontario are expected to rise to 40 mph later today, creating waves up to 14 feet high, causing lake shore flooding problems.

History of late season snowfalls
Wunderground’s weather historian Christopher C. Burt made a post last year on Record Late Season Snowfalls. He documents that the latest measurable snow in Buffalo, NY was 0.1″ on May 20, 1907. Not including today, Buffalo has had ten calendar day snow events of an inch or more after the date of April 22nd. Recent events include May 7th, 1989 when 7.9 inches fell, and April 24th, 2005 when 1.2 inches of snow fell. In Rochester, NY, such events are slightly more common, with 18 such events of one inch of snow or greater after April 22nd. Recent events include April 25th, 1983, when 3.5 inches of snow fell, May 7th, 1989 when 10.7 inches of snow fell, and May 12th 1996, when 1.1 inches fell.

Two major Nor’easters this season: one in October, one in April
What’s crazy about this Nor’easter is that it is only the second significant Nor’easter of the 2011 – 2012 snow season. The other major Nor’easter occurred October 30 – 31. It’s pretty bizarre to have your only two significant Nor’easters of the season occur in October and April–and none in November, December, January, February, and March. I talked to a weather disaster expert in the insurance industry last week, who told me that NOAA’s National Climatic Data Center will probably end up classifying last year’s October 30 – 31 Nor’easter as 2011′s fifteenth billion-dollar weather disaster.

Record April heat in Phoenix and Las Vegas
As is often the case when a major Nor’easter is affecting the Eastern U.S., the jet stream is contorted to bring a strong ridge of high pressure over the Western U.S., accompanied by record-breaking heat. Phoenix, Arizona hit 105°F yesterday, its tying its record for hottest April temperature (previous 105° April temperatures occurred on 4/20/1989 and 4/29/1992.) Las Vegas, Nevada hit 99°F yesterday, the hottest temperature on record for so early in the year, and tied for the hottest April temperature on record. The mercury climbed to a scorching 113° in Death Valley yesterday, a record for the date, and the hottest temperature measured in the U.S. so far in 2012.

Jeff Masters

But what is so disturbing about all this is that there is very little evidence that these claims are true….that the extreme events of late are the result of greenhouse gas increases caused by humans.

Take the recent amazing heat wave in the eastern and central U.S.:  canary in the coal mine for global warming?   No evidence of this.  In fact, an in-depth analysis by Dr. Martin Hoerling of  NOAA Earth Systems Research Lab (ESRL), found here, suggest that the heat wave was the result of natural variability and an unusual, but not unprecedented, change in the upper level flow pattern that pushed tropical air northward over the eastern U.S..  A recent discussion of the March warming by UW Professor Michael Wallace, one of the nation’s leading climate scientists and a member of the U.S. National Academy of Sciences, found here, reaches a similar conclusion.

Well, what about the extensive tornado outbreaks of 2011 over the southeast U.S. and the early tornadoes of 2012.    Unusual extreme weather connected with global warming?  There is no reason to believe this is true.   Backing for this statement comes from a comprehensive report of the Intergovernmental Panel on Climate Change (IPCC) on extreme weather events, found here.  To quote the IPCC report: “There is low confidence in observed trends in small spatial-scale phenomena such as tornadoes and hail.”

What about hurricanes?  Have we seen an upward trend in those?   The IPCC conclusions:
“There is low confidence in any observed long-term (i.e., 40 years or more) increases in tropical cyclone activity (i.e., intensity, frequency, duration), after accounting for past changes in observing capabilities.”

What about extreme temperatures, heavy precipitation, and drought?  To aid us in evaluating the trends in these extreme climate events, the National Weather Service has developed a Climate Extremes Index that you can access and plot online. 
I tried it out and here is the information for these parameters for 1910-2010:

A lot of variability and the tendency for higher amounts of extremes during early and later parts of the record, with less extremes in the 1950s-1970s.   There is no clear evidence of a long-term trend.

An important issue that is rarely discussed  is that changes in extremes…either natural or from human-forced global warming will not be spatially uniform.  So even a strong global warming signal will result in some places getting more extreme weather, while others will get less extreme weather.  An obvious example is temperature….if temperatures warm there will be a tendency for more extreme highs and more heat waves.  But that also implies that the cold waves will be weaker and less extreme.   Ever wonder what is the biggest weather killer in the Northwest U.S.?  Not hurricanes or tornadoes, not heat waves or droughts, not windstorms and floods.  I am convinced from that statistics I have collected that roadway icing kills and injures more people around here than anything else.   And warming should help reduce those deaths and injuries.

And consider that most of the climate models suggest the jet stream will move north under global warming.  Big storms and floods are associated with the jet stream.  So some folks (on the north side of the current jet stream location) may experience more extreme storminess, but those on the south side could well experience less.   There will certainly be losers due to changed extremes under global warming, but there will be a lot of winners as well.  Never seem to hear about that.

Two final points:

(1) Even if there are changes in the frequency of extremes, that does not necessarily mean human influences are behind them.  For example, the earth has been warming for roughly 100-150 years as the planet exited the “Little Ice Age”.   Much of this warming has undoubtedly been natural, with human-forced warming only really significant during the past 30 years or so.  Glaciers have been melting back over the past century and thus some of this loss is undoubtedly due to natural causes.

(2) If we haven’t seen trends in extremes that does not mean that we won’t see them in the future when the impact of anthropogenic greenhouse warming increases substantially.  The earth is only starting to warm up due to mankind’s influence on greenhouse gases.  The big action…including changes in extremes…is AHEAD of us.  Activist types have made a huge mistake in thinking they need to point to observed changes in extremes to make their case for dealing with GW.  They are particularly making a mistake when they make claims that have no scientific basis.   Global warming skeptics and deniers have made the huge mistake of assuming that a lack of clear changes in the atmosphere during the past decades says something about what will happen in the future, since most of the GW impacts have not yet occurred .  Ironically, the activist types are providing the deniers with a potent weapon, since it is pretty easy to disprove many of the activist claims of human-induced global warming enhancing past and current extreme weather.

I believe the science is fairly clear…the impacts of global warming due to human-enhanced greenhouse gases will be be very significant, that the effects will increase gradually at first, but then accelerate later in the century.  There will be substantial impacts on extremes, but the magnitudes and spatial distributions will be complex, and we don’t necessarily have a good handle on it at present. 

The latest radar imagery (Friday 9:30 PM) indicates a few shower over SE Washington and that they are dissipating and moving out rapidly.

Tomorrow, as the ridge builds, temperatures will warm up considerably from the temperatures we experienced today: mid to upper 50s in the west and the upper 60s over the basin of eastern Washington.

Lets take a look what we expect.  First, lets start with the 4-km run of the UW WRF for 5 PM Saturday…near the time of max temperatures:

 Amazing.  Temperatures reach well into the 70s across the Willamette Valley and the basin of eastern Washington, and upper 60s extend as far north as Seattle.  Twenty four hours later, on Sunday afternoon, the temperatures surge upward, with upper 70s and lower 80s spreading across eastern Washington, the SW WA interior, and the Willamette Valley.  Even the Puget Sound region hits the lower 70s away from the water (see graphic)

Now this is of course, from a single model run.  Lets take a look at output from PROBCAST, which is based on statistically postprocessing an ensemble of forecasts.   PROBCAST is very good with warm temperature situations.  Below is Probcast’s max temps for Saturday and Sunday.  Very similar idea to the WRF runs.

 Now the only reason to get nervous about this forecast is that there will be some clouds along the coast that will stretch into southern B.C.  Here is the cloud forecast from WRF for 5 PM Sunday:

You see what I mean–coastal cloud that extend into BC….and here is the probability of precipitation on Sunday from Probcast.  It looks like nearly all of WA, except for the far NW coast is safe.  Don’t go up Vancouver Island or north of Vancouver—it looks wet there.  Personally, I plan to experience some real heat and mountain bike around the Ancient Lakes area near Quincy.  Enjoy the weekend….this is surely not like last spring.

PS:  the Lyrid Meteor Shower is this weekend…if you get up before dawn on Saturday or Sunday, take a look…

Impact of Tropical Cyclones on drought alleviation in the Atlantic and Gulf Coasts
Dr. Pat Fitzpatrick of the Stennis Space Center in Mississippi discussed how landfalling tropical storms and hurricanes can alleviate drought. The biggest winner tends to be the Southeast U.S. states of Florida, Georgia, South Carolina, and North Carolina, where about 20% – 50% of all droughts between 1960 – 2009 were busted by a landfalling tropical storm or hurricane. It is uncommon for Texas to see a drought busted; less than 10% of all Texas droughts have been ended by a hurricane or tropical storm. This occurs because the Southeast U.S. can receive heavy rains from hurricanes moving up the East Coast, or moving through the Gulf of Mexico, while relatively few storms track over Texas. Over the course of a year, hurricanes and tropical storms contribute 15 – 20% of rain along the Gulf Coast, and 3 – 16% along the East Coast. The length of a drought does not seem to affect whether a drought can be ended by a hurricane or not. Hurricanes have been able to end both short (< 3 month) and long (> 12 month droughts) equally well.

Figure 1. Example of a drought-busting tropical storm. Moderate drought (Palmer Drought Severity Index, PDSI, ≤ –2.0) was present in 52 percent of the Florida, Georgia, South Carolina, and North Carolina climate divisions in May 2006. The percentage decreased to 29 percent after Tropical Storm Alberto passed through on June 11 – 15, 2006. Image credit: U.S. Drought Monitor.

Figure 2. Rainfall in inches from the passage of Tropical Storm Alberto in 2006. Image credit: NOAA/HPC.

According to the U.S. drought monitor, over 90% of the area of Florida, Georgia, South Carolina, and North Carolina are currently in moderate to exceptional drought. There is 1 – 2 inches of rain coming to much of the region over the next few days, but that will not be enough to bust the drought. Based on Dr. Fitzpatrick’s research, there is a 20% – 50% chance that the drought will be broken by a tropical storm or hurricane. The first storm on the list in 2012 is Alberto again; let’s hope we get another Alberto this year that imitates the 2006 version of Alberto.

Patterns of rapid intensification
Peter Yaukey of the University of New Orleans studied patterns of hurricane rapid intensification in the Atlantic from 1950 – 2009. The Gulf of Mexico and Caribbean saw the most rapid intensification events, and the Northeast Atlantic the fewest. Interestingly, he found that rapid intensification events did not peak in September, but tended to be more common in June and July. Hurricane are less likely to intensify in the late afternoon and early evening (near 00 UTC), and more likely to intensify just after midnight, at 06 UTC.

Jeff Masters

This last month has been very different form the sorry, soggy, chilly spring of 2011…April is roughly an inch drier than normal so far…and there is no sign that this pattern will change.

To begin, here is an interesting comparison between this year and last, showing you the actual temperatures at Sea-Tac and the average highs and lows.  Last year, we only reached normal highs a few times in March and never during the first 18 days in April.  This year (second plot) we have seen normal or higher temperatures several time in April…with a big high of 70F.  May seem subtle…but it makes a big difference.

Tomorrow should start out dry, but that will change during the afternoon when a warm front approaches western Oregon and then western Washington (see graphic of low level temperatures and winds).  In the graphic the warm front is indicated by the change from cold (green) to warm (yellow) and a wind shift from southeasterly to southwesterly.  Warm fronts are actually pretty rare around here…most of the time we experience old, weak occluded fronts.

Ok, we get some rain and the temperatures warm up for a while, and then a cold front moves through Friday morning, with more rain.   But by Friday afternoon things should start drying out as a ridge builds over the western U.S.  The issue is that the ridge line is a bit too far east to insure a completely dry weekend for the western side of the NW (see upper level map for Saturday morning), but except for the coast, we should see above normal temperatures and very little precipitation.  Eastern Washington and Oregon are sure bets.  The coast of Vancouver Island could be wet.

Here are the predicted temperatures from the UW model for 5 PM on Saturday…lower 60s in western Washington, while eastern WA and the Willamette Valley could get into the 70s.  Partly cloudy skies, with sky cover lessening during the day.

 And Sunday it gets even warmer, particularly over eastern Washington, where some folks will see the 80s.  Southern Puget Sound and the Willamette could reach near 70F.  But keep in mind that clouds will be moving in along the coast, and a modest error could ruin things for the west, particular during the afternoon.  Eastern WA should be solid…that is where I am headed.

 My next blog will go to extremes….I will talk about the recent national poll on people’s perceptions of trends in extreme weather.

Figure 1. Morning satellite image of Invest 91L. The island of Bermuda is seen at the left side of the image.

Global tropical cyclones and climate: current signal
Now, I’ll summarize a few of the excellent talks given at this week’s AMS hurricane conference. Dr. Greg Holland of the National Center for Atmospheric Research talked about the impact of global warming on hurricane intensities. Using data beginning in 1975, the beginning of the satellite era, he showed that while the total number of hurricanes globally has decreased in recent years, the proportion of hurricanes that are of Category 4 – 5 intensity has increased by 40%. He showed that this change could be related to a 0.8°C increase in global temperature during the period. He concluded that when hurricanes form, they are finding that it is easier for them to reach higher intensities.

Sensitivity of the strongest hurricanes to ocean surface warmth
Dr. Jim Elsner of Florida State University showed that observations indicate a sensitivity of hurricane winds of 8.2 m/s +/- 1.19 per degree Centigrade of ocean warmth, using data in the Atlantic from 1981 – 2010 (for oceans areas warmer than 25°C.) Using a high resolution model (HiRAM) with 50 km resolution, a sensitivity of only 1.5 +/- .6 m/s was found, calling into question the usefulness of current models for assessing future hurricane activity.

How will climate change affect hurricane tracks?
Angela Colbert of Florida State University used 17 global climate models, the BAM hurricane tracking model, and the Atlantic historical HURDAT data base to see how hurricane tracks might change in the future. She classified storms as either straight moving (which tend to hit the Caribbean or U.S. Gulf Coast), recurving landfalling (U.S. East Coast impacts), or recurving ocean storms that miss landfall. She projected a 6% increase in recurving ocean storms and an 8% decrease in straight-moving storms by the end of the century, due to climate change. A decrease of 1- 2 storms per decade is predicted for the Caribbean and Gulf of Mexico, and an increase of 1 – 2 storms per decade in the waters of the mid-Atlantic, and along the East Coast of Florida. This occurs primarily because of an increase in westerly winds over the Central Atlantic, and to a lesser degree, an eastward change in genesis location closer to the coast of Africa. Both of these factors would tend to increase the number of recurving storms.

Jeff Masters

Figure 1. Dr. Robert Simpson addresses the 30th Conference on Hurricanes and Tropical Meteorology of the American Meteorological Society on April 15, 2012, assisted by session chair Dr. Greg Holland.

Hurricane Andrew: 20 years later: What have we learned?
This year marks the 20th anniversary of the incredible devastation wrought in South Florida by Category 5 Hurricane Andrew. Hurricane Andrew was a wake-up call for how poorly buildings were constructed in hurricane-prone areas, and Dr. Tim Marshall of Haag Engineering discussed what we learned from the hurricane. Interestingly, much of Andrew’s damage occurred in the storm’s outer bands, before the peak winds arrived. The heaviest damage occurred in subdivisions that had poor building codes. Removal of asphalt shingles was a big problem. A lot of shingles were fastened with staples that ripped out, due to poor location, orientation, and depth. Often the secondary felt barrier below the shingles was not glued on, and was ripped away once the shingles ripped away. Once you lose your shingles, you often lose your house, since rain can then get into the house and destroy the interior. Andrew led to a complete revision of the building codes in South Florida, which are now the strongest in the nation. The new building codes, however, still allow for some dubious practices–like stapling shingles to roofs, and the placement of loose gravel on roof tops. Marshall concluded the talk by emphasizing that taping windows doesn’t work. Board up your windows, or better yet, use steel shutters.

Figure 2. Hurricane Andrew as it closed in on South Florida 20 years ago. Image credit: NOAA.

Jeff Masters

In most places there IS a time when precipitation is most likely, but in some locations the diurnal (daily) modulation is weak.   The daily precipitation cycle not only varies geographically, but also seasonally.

So what causes rainfall to vary around the day?  Some reasons:

1.   As air cools at night, the relative humidity tends to rise, leading to more drizzly fog and stratus during the morning hours with a peak near or just after sunrise.

2.  As the surface heats during the summer the temperature variation with height (the lapse rate) increases, leading to more convection (thunderstorms and convective showers) during the afternoon.

3.  Sea breezes develop during the morning and push inland around lunchtime.  Showers can occur at the leading edge of the cool marine air:  the sea breeze front.

In general, diurnal variation of precipitation is largest during the summer months, when the sun’s heating is strongest and thunderstorms are most frequent.

Well,  what about western Washington?  I did a paper on diurnal weather variations around here and below is a graph showing the typical mid-summer diurnal variation in sky cover and precipitation.  Both tend to be most frequent around 3-8 AM.  This makes a lot of sense.  We don’t get many thunderstorms around here and low clouds and drizzle sometimes occur when the region is flooded by marine air.  Often the clouds burn off or lift by late morning.  So plan your outdoor activities in the afternoon or early evening!   But take a look at the actual numbers…the variation is only from 4 to 5.5 % chance of rain during any three-hour period.   Not a strong signal.

If you want a bigger signal you need to look eastward to places that have thunderstorms.  Here is a neat figure produced by some folks at NASA showing summertime (July-August)  precipitation timing across the U.S.  The first figure shows the amplitude of the daily precipitation variations:  over the for western U.S., where there is little summer convection, the diurnal amplitude is very small….just the morning drizzle.   You want a big daily variation head for Florida–the thunderstorm capital of the U.S.! 

The second figure tells you the time when precipitation is most frequent in Local Standard Time. For the Rockies, Southeast and Eastern U.S., precipitation is most frequent between noon and 6 PM (greens to yellows), but over the Great Plains, there is an evening and early morning maximum.  Why?  It turns out that a lot of convection builds over the Rockies and progressively moves out into the {lains over time.  There is also an influence of a low-level jet that I won’t get into here.

 The last figure shows the time of minimum precipitation; for the Rockies the the eastern U.S., it is overnight…and during mid-day over the High Plains.

During the winter the precipitation variations are much less.  There is no color graphic that I know of, but Mike Wallace of my department prepared the next figure.  The amplitude of the daily precipitation variation is indicated using wind barbs…just like the wind plots I have shown you.  More barbs, the stronger the cycle.  Not very strong anywhere.  The time of maximum precipitation is shown by the direction of the barbs.  Pointing from the east:  6 AM max.  Point from the south:  noon max., etc.  Thus the Northwest max precipitation mainly occurs between 6 AM to noon.  Much of the rest of the country around 6 AM.   Thus, it appears that early morning is the WORST time to be up and about if you want to avoid rain during the cool season.  Any teenager or college student can tell you that those are times to avoid.

SPC Storm Reports from Saturday

The SPC storm reports show the typical track of these storms, with Kansas being the hardest hit. Early in the afternoon, most of the supercell thunderstorms were consistently producing weak tornadoes that skipped across western and central Kansas. As the low-level jet kicked in late in the afternoon and into the evening, these thunderstorm strengthened considerably. Probably the most remarkable supercell thunderstorm began in western Oklahoma near Woodward as a stunning multi-vortex tornado. The tornado actually was several tornadoes that danced into southern Kansas and eventually threatened Wichita. While it did pass south and east of the city, it did produce an 84 mph wind gust at the Wichita Airport. It also did extensive damage at the airport. In addition, the Oaklawn area of Wichita was declared a disaster area very soon after the tornado passed. The Sedgwick County commissioner declared the county a disaster area with a couple hours of the devastating tornado’s passage. This wedge tornado eventually moved along the Kansas Turnpike northeast of Wichita before it finally dissipated.

Double Tornadoes in Oklahoma close to the Kansas border

This tornado sparked a fire in Oklahoma as it passed through the area northeast of Woodward

The scariest tornado struck Woodward, OK early Sunday morning as the main squall line moved through western Oklahoma. This tornado was very large and particularly dangerous since it occurred at night when most residents assumed the threat had ended. There were multiple reports of not hearing the city’s siren, so it is entirely possible that the siren was either struck by lightning or hit by the tornado itself. Probably the scariest video of a tornado I have ever seen was taken of this Woodward tornado as it moved into town. Note you can see the scale of the tornado as between power flashes. There was reported widespread damage in Woodward along with several injuries.

Perhaps the most telling feature of this severe weather outbreak was its repetitive nature. Salina, KS was tornado warned three times by three separate severe thunderstorms. Similarily, areas from Medicine Lodge to Kingman, Kansas were warned for two separate thunderstorms, while Woodward itself was warned very early in the day before a different tornado moved through the city late in the night.

The good news is that as the as the dry line and associated thunderstorms continue moving east, the severe threat will lessen.

Shaun Tanner