A highly unusual week-long heat wave is building over much of the U.S., and promises to bring the warmest temperatures ever seen so early in year to a large portion of the Midwest. The exceptional heat will also be exceptionally long-lasting: record-breaking temperatures 20 – 30 degrees F above normal are expected today through next Wednesday for much of the Midwest and Northeast U.S. The weather system responsible is a large upper-level ridge of high pressure that is “stuck” in place–a phenomenon known as a “blocking pattern.” The jet stream is bending far to the south over the Western U.S., then bending far to the north over the Rockies and into Canada, and lies far to the north of the eastern U.S. Since the jet stream acts as the boundary between cold air to the north and warm air to the south, the current looping pattern is bringing colder than normal temperatures and snow to the mountains of the West, and summer-like warmth to the Eastern U.S. It is common for the jet stream to get stuck in a blocking pattern for a period of a week or more in summer, but not in March. If the current model forecasts prove correct, a high pressure ridge over the U.S. bringing heat this intense and long-lasting in March will be unprecedented in the historical record, going back to 1872.

Figure 1. Is this March or June? Predicted high temperatures for Wednesday, March 14, 2012 over much of the Midwest are more typical of June than March.

Here are the hottest all-time recorded temperatures measured prior to March 20 for some selected U.S. cities. All of these records will be seriously threatened during the coming week:

Madison, WI: 77°F on March 7, 2000
Milwaukee, WI: 77°F on March 7 – 8, 2000
Minneapolis, MN: 73°F on March 7, 2000
Des Moines, IA: 82°F on March 11, 1972
Chicago, IL: 81°F on March 12, 1990
Detroit, MI: 80°F on March 8, 2000
Lansing, MI: 79°F on March 8, 2000
Muskegon, MI: 73°F on March 8, 2000
Grand Rapids, MI: 78°F on March 8, 2000
Flint, MI: 80°F on March 8, 2000

As you can see, the expected warm temperatures during the coming week will rival those recorded on March 8, 2000, when most of the Upper Midwest set all-time records for the warmest temperature ever measured so early in the year. That warm surge was caused by a ridge of high pressure that was not as strong as the one expected to build in during the coming week, and the March 8, 2000 ridge did not stick around long enough to generate more than two days of record-breaking high temperatures. A powerful low pressure system moved through Northern Wisconsin on March 8, 2000, dragging a cold front through the state that triggered a thunderstorm that spawned the earliest tornado ever recorded in Milwaukee County.

Figure 2. New daily high temperature records were set at 71 locations yesterday, according to our new Extremes web page, with data from NOAA’s National Climatic Data Center.

Hot days in Minneapolis
In Minneapolis, Minnesota yesterday, the high temperature reached 67°F, which is the 7th warmest temperature measured so early in the year, and 27°F above the normal high of 40°F for the date. Since weather records in the city go back to 1872, we can expect that Minneapolis will experience a temperature of 67°F or higher once every 20 years. What’s really remarkable is that the forecast for Minneapolis calls for a high temperature of 70 – 75° every day for the next seven days. Since 1872, there have only been nine days that the temperature has gotten to 70°F prior to March 20, with 73°F on March 7, 2000 being the hottest day. So, over the course of the next week, we are likely to break the all-time high for so early in the year, and add nearly double the number of 70°F-plus days. The situation is similar for much of Wisconsin, Michigan, Illinois, and surrounding states. With temperatures already averaging at least 5°F over much of the Midwest this month, it’s very likely that this month will be the warmest March on record for at least seven states.

Unusual snows on the Oregon coast
As is often the case, record heat in one part of the country means that another part is experiencing unusual cold, due to a kink in the jet stream. On Monday, 6.0″ of snow fell at Newport, OR, and 8.5″ at Tillamook (about halfway up the coast between Newport and Astoria). According to statistics at the Western Regional Climate Center, the Newport snowfall was their greatest March snowfall on record (previous was 2.0″ in March 1906) and the their 3rd greatest snowfall of any month since records began in 1893. The latest-greater snowfall at Newport was 11.0″ on Dec. 3-4, 1972. This tied with another 11.0″ snow in January 1943 as their greatest snowfalls on record. For Tillamook it was the biggest snow since 9.0″ in January 1971 (but well short of their all-time snowfall of 19.0″ in March 1951).

Jeff Masters