NewsJune 30, 2012

Heat Wave Spawns Deadly Severe Thunderstorms

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Andrew Freedman

By Andrew Freedman

An intense heat wave broke records in more than a dozen states on Friday, and spawned one of the most damaging severe thunderstorm events seen in years. At least 3 million Americans, mainly in areas from Indiana Southeast to Virginia, were facing the prospect of another day of triple-digit heat heat without the benefit of air conditioning on Saturday, after the storms knocked out power. The severe weather also killed at least 13 people, including two young boys who were killed by a falling tree as they camped with their family in southern New Jersey.

A shelf cloud at the leading edge of the severe thunderstorms, taken in Illinois as the storms gathered strength. Click on image for a larger version. Credit: Twitpic via Angela Fritz.

The severe thunderstorms took shape over Iowa and Illinois during the early afternoon on Friday, and by evening they had roared all the way to the Atlantic Ocean, rippling along the boundary between the extremely hot and humid conditions to the south, and cooler and drier air to the north.

The storm complex, known as a “derecho,” knocked out power to two-thirds of Ohio residents, and caused large-scale tree damage in that state. Columbus, home to Ohio State University, was particularly hard hit. The thunderstorms then raced southeastward, delivering a punishing blow to the Washington, D.C. metro area, where more than a million customers were without power as of noon on Saturday. The storms even affected the Internet, as Amazon’s cloud computing service experienced power outages, taking out popular online services such as Instagram and Pinterest.


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Derechos typically occur during late June or early July, when a large dome of High Pressure sets up across the South Central states, pumping warm, humid air northward, and setting up a sharp boundary between the hot conditions and cooler air to the north. They have been known to bring winds as strong as some tornadoes, and that occurred on Friday when numerous locations reported wind gusts to 80 miles per hour or higher.

With the heat continuing through the weekend, conditions will be favorable for more damaging thunderstorms, including derecho events.

The derecho event was similar to storms that took place during the deadly Chicago heat wave of 1995, during which heat took the lives of about 750 people in the Windy City. The heat helped fuel the storms by making the atmosphere extraordinarily unstable, and providing the energy needed to sustain thunderstorms for long periods of time. 

There were numerous daily, monthly, and all-time record high temperatures established on Friday. In total, there were 276 record daily high temperatures set or tied on Friday, along with 70 record warm overnight low temperature records. There were also 96 monthly record high temperatures, and 37 all-time high temperature records set or tied. 

Satellite image showing the towering thunderstorms as they raced southeast toward Ohio on Friday afternoon. Click on image for a larger version. Credit: SSEC/CIMSS satellite blog.

For the year-to-date, warm temperature records are outpacing cold temperature records by a ration of 7-to-1. Since January 1 there have been 21,402 daily high temperature records set, compared to just 3,338 daily records for cold high temperatures, according to a database maintained by the National Climatic Data Center.

In a long-term trend that demonstrates the effects of a warming climate, daily record-high temperatures have recently been outpacing daily record-lows by an average of 2-to-1, and this imbalance is expected to grow as the climate continues to warm. According to a 2009 study, if the climate were not warming, this ratio would be expected to be even. Other studies have shown that climate change increases the odds of extreme heat events and may make them warmer and longer lasting.

Since climate change can boost the odds of major heat waves such as this one, and the extreme heat contributed to the severe weather, it's plausible — albeit rather speculative at this point — that climate change played some sort of role in the derecho event. However, it will require rigorous scientific analysis to determine whether this may have been the case.

Severe storm reports for Friday, June 29. The blue dots represent wind damage. Credit: NWS/SPC.

In Washington, both Reagan National Airport and Dulles Airport had their warmest June days on record. At Reagan National Airport the high temperature was 104°F, beating the old June record of 102°F. According to the Capital Weather Gang blog, June 29 was one of just 10 times on record that Washington has reached 104°F or higher in any month. 

Charlotte, N.C. also reached 104°F, breaking the old daily record and tying the all-time record high temperature for that location.

Columbia, S.C. experienced searing heat of 109°F, breaking their previous all-time high temperature record of 107°F. Daily temperature records in Columbia date back to 1887.

Nashville, TN also hit 109°F, breaking their all-time high of 107°F. In Paducah, KY, the high temperature of 108 deg°F tied that city’s all-time high temperature record, which was set in 1942.

In addition to severe weather, the combination of the heat and drought conditions in the Midwest is causing corn prices to rapidly increase due to growing fears of a poor harvest.