Forecasters warn of potential derecho for Midwest, mid-Atlantic

Severe thunderstorms are forecast to rumble through an area of the United States that covers hundreds of miles from Monday night into early Tuesday, and AccuWeather meteorologists are raising the alarm that the storms could merge into a powerful weather phenomenon known as a derecho.

The potentially damaging storms will pack high winds and torrential rain and are forecast to erupt and move swiftly from southeastern Wisconsin and western Michigan to Virginia and Maryland spanning late Monday to Tuesday.

The storms will tend to fire on the northern and northeastern rim of intense heat and high humidity that is roasting the Plains and Mississippi Valley.

The atmosphere is primed for a significant damaging wind risk through at least Monday night with a sharp temperature contrast over the Great Lakes between expanding, record-setting heat to the south and much cooler and less humid air to the north, AccuWeather's chief meteorologist Jonathan Porter said. Thunderstorm clusters tend to intensify and thrive moving along such a boundary.

One of the unique aspects of this type of severe thunderstorm complex is that the threat of wind damage is much more widespread than what typically occurs with most severe thunderstorms. Typically, severe thunderstorm wind damage tends to be much more localized to a part of a community, whereas this cluster of thunderstorms may create widespread damage over a much larger area and raises the risk for significant power outages, some of which could be long-lasting in the hardest-hit communities, according to Porter.

"This is especially concerning because some of the areas at greatest risk for damage from this thunderstorm cluster will be impacted by high temperatures in the coming days. For example, AccuWeather RealFeel® Temperatures will be around 100 in lower Michigan, Ohio and parts of Indiana on Tuesday and Wednesday," Porter said.

On this radar snapshot captured at about 1 p.m. CDT on Monday, a complex of heavy, gusty thunderstorms was organizing over Wisconsin, while locally severe thunderstorms were erupting farther south over Illinois and Indiana.

There is the potential for thunderstorms to organize into a large complex and then reach the threshold of a derecho, a high-powered severe weather event in which wind gusts reach or exceed 58 mph along an area of 240 miles or greater.

A derecho is sometimes called an "inland hurricane," where significant damage and widespread power outages can occur over thousands of square miles. The National Weather Service makes the determination as to whether or not a long-lived thunderstorm complex reached the criteria for a derecho, usually after the event has concluded.

Locations along a corridor from central Wisconsin through northern West Virginia are expected to be at the greatest risk of severe weather from this system, with widespread gusts over 70 mph possible. This is the area where AccuWeather meteorologists feel there is the greatest threat of a derecho.

The storms will be firing up and are likely to first hit Milwaukee and the Chicago metro area late Monday afternoon before moving on to part of the Detroit metro area and perhaps blasting right through Columbus, Ohio, Monday night.

There are some questions as to the path, longevity and extent of the thunderstorm complex, but there is no doubt of the likelihood of at least isolated severe storms over hundreds over hundreds of miles, according to AccuWeather chief on-air meteorologist Bernie Rayno.

"It is possible that instead of one intense complex of severe thunderstorms that multiple, less extreme complexes form," Rayno said.

In situations such as this, the thunderstorm complex tends to travel in a slightly curved line that bends to the southeast.

In extreme cases, thunderstorms can turn so much that they end up traveling more to the south, rather than to the east. For this reason. severe thunderstorms are not expected to reach Boston and are unlikely to hit New York City and possibly avoid Philadelphia. However, it is possible for the storm to turn toward cities such as Cincinnati on Monday night and perhaps Nashville and Charlotte, N.C., on Tuesday.

Regardless of an official designation of a derecho or not, severe thunderstorms are likely to occur along a swath of 700 miles from the Great Lakes region during late Monday and Monday night to portions of the central Appalachians early Tuesday. One or more lines of storms may extend all the way to the Chesapeake Bay region and perhaps into portions of the Carolinas during the day Tuesday.

This means that many communities will be at risk for potentially dangerous thunderstorms. Power outages, property damage and dangerous travel conditions are likely in the path of the storms. Ground stops are likely at area airports as the storms approach.

In addition to powerful wind gusts, there is the risk of flash flooding, frequent lightning strikes, large hail and perhaps a couple of isolated tornadoes.

Another risk not to be overlooked will be flash flooding.

"This thunderstorm complex can produce heavy rainfall on the order of 2-3 inches in a short period of time, elevating the risk for rapidly rising water, including flooding of creeks and streams, as well as roadways," Porter said. "Of particular concern is if heavy rainfall persists over parts of Ohio, West Virginia, western Pennsylvania and surrounding areas, where the ground is already saturated from recent heavy rainfall."

Prior to daybreak Tuesday, the storms may roll through Pittsburgh before potentially reaching areas from Washington, to Norfolk, Va., during Tuesday morning and midday.

Once the thunderstorm complex develops, it could begin to move at a forward speed of 40-60 mph. Storms tracking at this speed could allow just a few minutes for residents to take shelter after the first rumbles of thunder and flashes of lightning are observed.

People should be prepared for the storms in advance, forecasters said. Precautionary measures include bringing pets indoors and securing loose items in the yard such as trash cans, tents, trampolines and deck furniture.

Ten years ago this month, a historic derecho raced from Chicago to the mid-Atlantic on June 30, 2012. More than 20 people were killed and the high winds caused $2.9 billion in damage, knocking out power for 4.2 million customers. Damage to trees and power lines was so extensive in some communities that electricity was out for days. Cellphone towers were toppled in the strongest storms.

A derecho also moved from Rochester, Minn., to Baltimore on June 12, 2013, and was followed by a second derecho that swept through North Carolina and the mid-Atlantic the next day.

More recently, on Aug. 10-11, 2020, a derecho caused multiple fatalities and an enormous swath of damage and crop destruction from Nebraska and Iowa to Wisconsin, Illinois and Indiana.

Four people were killed in the violent storms and damage reached a reported $11 billion. In the case of the August 2020 derecho, wind gusts frequented 100 mph with the maximum wind gusts estimated to be near 140 mph, according to the NWS. Many locations experienced sustained high winds and damaging gusts for 30 to 60 minutes, compared to 10 to 20 minutes, which is more common for derechos.

Beyond the derecho threat into Tuesday, more thunderstorms are likely to erupt along the boundary between intense heat in the Midwest and cooler air in the mid-Atlantic, the Appalachians, the upper Great Lakes and the High Plains from Tuesday to Thursday. While many of these storms may be spotty in nature, there is the ever-present risk of a large severe thunderstorm complex forming.

During the middle and latter part of the week, cool air is likely to produce showers, thunderstorms and locally severe weather over the central and eastern United States.