Violent severe thunderstorms swept through Eastern North Carolina Sunday afternoon and evening, killing three people–one in a collapsed building, and two due to a falling tree. The deadly thunderstorms were fueled by the extreme heat affecting the Southeast, coupled with unusually high levels of moisture. The extraordinary heat and moisture caused high levels of atmospheric instability rarely seen. For those of you familiar with atmospheric stability indicies, the Convective Available Potential Energy (CAPE) in Eastern North Carolina and South Carolina at 2 pm EDT Sunday was 5000 – 6000, with a lifted index of -14. The Morehead City NWS office analyzed CAPE levels in excess of 7000 in the region, which is a truly rare occurrence. Fortunately, there was very little wind shear Sunday, so the upper-level winds were not able to induce the kind of twisting force needed to generate tornadoes. Sunday’s powerful storms brought more power outages and damage to a region still suffering the impacts of Friday’s rare derecho event, which killed 14 people and left power outages that still affect at least 2 million people. According to insurance broker Aon Benfield, the storm initially knocked out power to 2.5 million people in Virginia, the largest non-hurricane related power outage in state history. A derecho is a fast-moving, long-lived, violent thunderstorm complex that usually develops along the northern edge of a very hot air mass, in conjunction with an active jet stream. Friday’s derecho was one of the largest and most destructive in U.S. history, and compares to the May 8, 2009 derecho that swept across the nation from Kansas to Kentucky, killing six and causing $500 million in damage.

Figure 1. Radar image of Sunday’s violent thunderstorms that killed three people in North Carolina.

Dangerous extreme heat continues in the Southeast
Temperatures near all-time record levels continued for much of the Southeast on Sunday. The temperature in Atlanta, GA hit 105°F, which would have been the hottest temperature in city history, had the mercury not hit 106° the day before. On Sunday, Columbia, South Carolina hit 106°, the 5th highest temperature ever recorded there. Columbia hit 109° on Friday and 108° on Saturday, the two hottest days since record keeping began in 1887.

People in the Southeast and Mid-Atlantic without power must endure more extreme heat the next few days, with temperatures in the 100s and upper 90s expected over much of the region. While the temperatures are generally expected to be a few degrees cooler than what was seen Friday and Saturday, it is not the extremity of the temperatures in a heat wave that result in the highest heat stress to vulnerable people, but rather the length of time very high temperatures last. A multi-day period of exceptionally hot weather often causes high mortality. Yesterday’s airmass was exceptionally humid, which greatly increases heat stress, since the body cools less efficiently in humid conditions. Aberdeen, Mississippi recorded a temperature of 104° with a dewpoint of 84° at 3 pm EDT Sunday, resulting in ridiculously high heat index of 136. Goldsboro, NC had a dewpoint of 87° at 11 am Sunday, the highest dewpoint I can recall seeing in the U.S., and something more typical of what is seen in Saudi Arabia along the shores of the Red Sea. According to wunderground’s weather historian, Christopher C. Burt, the highest dew point temperature in world history is probably the 95°F (35°C) recorded at Dhahran, Saudi Arabia at 3 p.m. on July 8, 2003. The dry bulb temperature stood at 108°F at the time, so theoretically the heat index was 176°F. Sharjah in the United Arab Emirates (UAE) apparently once recorded a dew point of 93.2° (date unknown) according to ‘Weather Climate Extremes’ Army Corps of Engineers TEC-0099 report.

Mr. Burt has a new post called The Amazing June Heat Wave of 2012 Part 2: The Midwest and Southeast June 28-30 summarizing the later portion of the historic heat wave currently affecting the U.S. He comments:

What was truly astonishing was the number of all-time any month records that were broken or tied. This is especially extraordinary since they have occurred in June rather than July or August when 95% of the previous all-time heat records have been set for this part of the country (unlike the Southwest where June is often the month that all-time heat extremes are recorded). Eighteen of the 298 locations I follow closely (because of their long Period of Record and representation of U.S. climate) have already broken or tied their all-time heat records. The only year with more all-time heat records is 1936, when 61 cities set all-time heat records.

It is just July the 1st and the summer has just begun.

Quiet in the Atlantic
There are no threat areas to discuss in the Atlantic, and none of the reliable computer models are developing a tropical cyclone over the next seven days.

You won’t want to miss my next post, scheduled for 2:30 pm EDT Monday.

Jeff Masters