Forecasters are watching an expected outbreak of severe weather from Illinois to Maryland that some are likening to last June's derecho; one meteorologist predicted it would be a "multi billion dollar storm" causing massive power outages.
Storms were developing in Illinois and Wisconsin early Wednesday evening, bringing tornado threats from there through Indiana and into Ohio. Meteorologists say conditions could be conducive for those storms to strengthen into a massive squall line packing up to 70 mph winds, large hail and heavy rain.
Some storms could cross over the Appalachians and reach Maryland overnight Wednesday into Thursday, forecasters said.
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"It's just one of those things where you get a cluster of storms, it can go severe pretty quickly," said Kevin Witt, a meteorologist in the National Weather Service's Baltimore/Washington forecast office. The forecasters planned to monitor the storms as they move toward the East Coast.
There is a chance that, like with the derecho, the squall line, or at least some remnant storms, could make it over the mountains, forecasters said. Forecasters cautioned, though, that it's unrealistic to forecast a derecho, a weather phenomenon not to be tossed about lightly when talking about storms.
"THURSDAY BEGINS WITH SOME UNCERTAINTY REGARDING THE FATE OF A MESOSCALE CONVECTIVE SYSTEM AFFECTING THE OHIO VALLEY TONIGHT. IT IS UNCLEAR WHAT THE STRENGTH OF THIS SYSTEM WILL BE AT THE BEGINNING OF THURSDAY," meteorologists wrote in a National Weather Service forecast discussion for the Baltimore/Washington region.
"By late afternoon or evening, the storms are expected to organize into a squall line and may possibly grow into a derecho," the weather service's eastern region headquarters wrote on Facebook. "The squall line will move into the central Appalachians and possibly the mid Atlantic region overnight. While the storms are expected to be weakening during the overnight hours, severe weather will still be possible."
But the main risk in Maryland comes Thursday afternoon.
The local forecast calls for an 80 percent chance of thunderstorms Thursday, potentially starting in the morning hours, and bringing more than an inch of rain. A flash flood watch is in effect for Central Maryland from midnight Thursday through Thursday evening, with the ground already saturated with recent rains from storms and the remnants of Tropical Storm Andrea.
AccuWeather.com severe weather tracker Henry Margusity, who predicted the potential billion-dollar cost of the storm, suggests risks of tornadoes, damaging winds and flash flooding on Thursday from the Carolinas to New Jersey.
"Simulated radars suggest after 2 p.m. the storms develop and start as supercells and quickly become a squall line with damaging wind gusts," he wrote. Supercells are massive thunderstorms with tall clouds that contain a rotating updraft of wind and often bring tornadoes.
Baltimore Gas and Electric Co. is mobilizing its resources into storm mode, in which all employees are on call to be devoted to storm-related outage restoration around the clock. The utility is also arranging for extra crews from other areas -- something that became an issue with customers after the derecho because the breadth of the storm's damage delayed crews' arrival from other parts of the country.
More rains may render tree roots weakened, BGE officials said, increasing the possibility of outages and damage from felled trees.
But conditions Wednesday and Thursday are not expected to be the same as those that fueled the June 29, 2012, derecho. The storm caused more than 760,000 power outages in the Baltimore area and widespread damage, blamed for at least three deaths.
Temperatures surged above 100 degrees with high humidity that day, making for a clash between hot and humid air and cool, dry air as a cold front approached. Derecho storms get their name from the powerful straight-line winds they bring and the motion the squalls take -- "al derecho" in Spanish means "straight ahead".
In this case, if some storms occur Wednesday night, cooling the air ahead of the larger storm threat Thursday, it could limit the severity of the weather system, local meteorologist "Eric the Red" suggested. The severe weather risk will also depend on how much sunshine warms the region ahead of the storms, he said.
The severe weather outbreak would be the second this week. Four tornadoes are confirmed to have touched down in Maryland on Monday, and heavy rains caused flooding across the state as well. More than 3 inches of rain fell in downtown Baltimore, nearly a month's worth of rain in the span of a few hours.
The weather service confirmed three tornadoes in Baltimore, Baltimore County and St. Mary's County on Tuesday and added a fourth to the list Wednesday afternoon. A tornado rated EF-0 on the enhanced Fujita scale touched down in Woodbine for three minutes about 7 p.m. Monday. The tornado sheared off and twisted tree trunks and branches and destroyed a two-car garage, with maximum winds of 80 mph.