Severe thunderstorms erupt along a swath 1,500 miles long, killing 2

A huge outbreak of severe thunderstorms hit the nation on Thursday, causing damage in fifteen states and knocking out power to over 250,000 customers. Two people were killed, one by a falling tree in Pennsylvania, and one due to a lightning strike in New York. Two possible tornadoes touched down: one in Elmira, New York and another in Brookville, PA. The severe storms covered an unusually large area, erupting along a 1,500-mile long swath of the country from Texas to Connecticut. The intensity of the thunderstorms was increased by a very hot and moist airmass; temperatures in the mid to upper 90s were common across the region Thursday. A number of record highs for the date were set, including a 98° reading at Washington D.C.'s Dulles Airport. The threat of severe weather continues for Friday afternoon over portions of the Ohio Valley and mid-Atlantic, as NOAA's Storm Prediction Center has placed these regions in their "Slight Risk" area for severe thunderstorms.


Figure 1. A rare sight: at 7 pm EDT on July 26, 2012, severe thunderstorm warnings were in effect for 132 counties in 15 states along a swath 1,500 miles long.


Figure 2. Visible satellite image taken at 7:02 pm EDT July 26, 2012, of the line of severe thunderstorms that extended from Texas to Connecticut. Image credit: NASA/GSFC.


Figure 3. An imposing sight: A squall line of severe thunderstorms with two bowing segments takes aim at New York, Connecticut, and New Jersey on July 26, 2012. The tip of a bow echo typically has the most violent winds in a severe thunderstorm.

Was Thursday's outbreak a derecho?
Thursday's outbreak of severe thunderstorms was not nearly as violent as the June 29 - 30 Mid-Atlantic and Midwest derecho, since the atmosphere wasn't as unstable. The June 29 storm was one of the most destructive and deadly severe thunderstorm complexes in North American history. It killed 22 people, knocked out power to at least 3.7 million customers, and did hundreds of millions in damage. There were 871 reports of damaging winds logged by the next day, and 36 of the thunderstorms had wind gusts in excess of hurricane force--74 mph. In contrast, yesterday's event had only two thunderstorms with wind gusts in excess of 74 mph, and 383 reports of damaging winds. We can probably classify yesterday's severe thunderstorm event as a weak derecho, since it met the main criteria, as defined by NOAA's Storm Prediction Center: "A derecho is a widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms. Although a derecho can produce destruction similar to that of tornadoes, the damage typically is directed in one direction along a relatively straight swath. As a result, the term "straight-line wind damage" sometimes is used to describe derecho damage. By definition, if the wind damage swath extends more than 240 miles (about 400 kilometers) and includes wind gusts of at least 58 mph (93 km/h) or greater along most of its length, then the event may be classified as a derecho."


Figure 4. NOAA's Storm Prediction Center logged 374 reports of damaging winds from Thursday's severe thunderstorms. Two of these thunderstorms had winds in excess of hurricane force (65 knots, or 74 mph.)


Figure 5. The climatology of derecho events over the U.S. Image credit: NOAA Storm Prediction Center.

Quiet in the Atlantic
There are no tropical cyclone threat areas to discuss in the Atlantic. A strong tropical wave is expected to bring heavy rains to the Lesser Antilles Islands and Puerto Rico on Monday and Tuesday, but none of the models are developing the wave. The NOGAPS computer model is suggesting development of a second tropical wave between Africa and the Lesser Antilles Islands August 2 - 3.

Have a great Olympic weekend, everyone, and I'll be back Monday with a new post.

Jeff Masters