A severe-weather setup migrated toward the U.S. East Coast on Thursday after leaving less damage than had been feared across the Great Lakes states on Wednesday. An upper-level impulse embedded in the jet stream was racing on Thursday morning toward the mid-Atlantic. Ahead of the impulse, a line of storms was marching from northern Pennsylvania across central New England at midday.

As new storms develop toward Thursday evening, localized wind gusts could reach severe limits (58 mph) from eastern Pennsylvania to the Delmarva region and New Jersey.

At 12:30 p.m. CDT Thursday, July 29, the NOAA/NWS Storm Prediction Center was forecasting an enhanced risk of severe weather (the third highest of five tiers) for Thursday afternoon and evening over parts of the mid-Atlantic. (Image credit: SPC)

The powerful thunderstorm complex that barreled from Wisconsin across northern Illinois and Indiana and southern Michigan on Wednesday night was amply predicted (see Wednesday post).

Fortunately, the possibility seen in computer forecast models of a high-end derecho – a corridor of prolonged, destructive thunderstorm winds – failed to materialize. The most intense thunderstorm cores of the evening tended to pulse rather than reaching a steady state, perhaps because of limited surface instability as a result of extensive clouds earlier in the day.

As of midday Thursday, the Storm Prediction Center had logged more than 130 reports of severe wind (58 mph or greater), including more than 100 from the storm complex that raced through Wisconsin into Illinois, Indiana, and Michigan overnight. But there was only a single report in this area of wind gusts reaching 75 mph. The standard definition of a derecho requires at least three such reports and/or wind damage of at least EF1 strength, along with a concentrated corridor of severe wind gusts extending at least 250 miles.

A proposed enhanced definition is even more stringent: a nearly continuous damage swath that must cover a corridor at least 100 km (60 mi) wide and 650 km (400 mi) long.

One of the strongest thunderstorm cores on Wednesday night narrowly missed striking the EAA AirVenture show in Oshkosh – thought to be the world’s biggest aviation show, with thousands of aircraft on hand and tens of thousands of spectators camping onsite. Many aircraft had left the site by late afternoon, and buses ferried many spectators to a nearby shelter well ahead of the storms, which generated a fearsome amount of lightning.

Derechos hinge on a delicate balance of weather ingredients that makes them tough to predict, especially more than a few hours in advance. Even when conditions appear favorable in the morning, only a subset of those situations will end up producing a derecho.

Most parts of the central and eastern U.S. experience a derecho only about once every one to four years. (Image credit: Dennis Cain, NOAA/NWS/SPC)

Thus far there is no conclusive research showing trends in derechos that might be linked to climate change. In theory, as a warming atmosphere tends to shift the jet stream poleward, the locations and times of year most favorable for derechos could also shift northward, as noted by SPC in its FAQ page on derechos.

The outlook for severe and tropical weather: steady as it goes

After Thursday’s round of severe weather, strong thunderstorms will become more scattershot into the first week of August. Seasonably mild air will grace the Northeast for several days, while the northern Great Plains continue to roast in readings topping 100°F.

There’s also no sign of tropical activity brewing in the Atlantic into the first week of August. However, the Eastern Pacific is starting to bubble. As of early Thursday, July 29, the National Hurricane Center was giving each of two systems well southwest of Mexico a 60 percent chance of development by Sunday and an 80 percent chance by Tuesday. Both are expected to remain far out to sea.

Both climatology and the Madden-Julian Oscillation (MJO) suggest that activity will likely ramp up in the Atlantic as we move through August. On average, tropical cyclones become considerably more likely as the climatological peak of activity approaches in early September. In addition, the MJO – a large-scale zone of rising air that circumnavigates the global tropics – may be moving into a position by the latter half of August that favors rising motion in the Main Development Region of the Atlantic. 

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Bob Henson

Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...