Tornado watch issued as severe storms threaten major cities in the Midwest

 Large hail, damaging winds and tornadoes were threatening states in the Midwest on May 15, and cities such as Chicago, Minneapolis and Milwaukee were in the crosshairs, forecasters said.

A tornado watch – meaning conditions are ripe for tornadoes to form – was posted by the National Weather Service for portions of Wisconsin and Minnesota, including the Twin Cities of Minneapolis/St. Paul.

The thunderstorm threat will focus on the upper Mississippi Valley to the Great Lakes, according to the weather service. The storm is expected to progress relatively quickly, so chances of flooding from heavy, lingering rains are limited. However, as the storm spreads to the Ohio Valley and middle Mississippi Valley on May 16, its motion could slow and bring scattered flash flooding.

In some areas, whether or not storms form is still up in the air. There was a 30 to 40% chance of formation in the Chicago area. The primary risks were coming from destructive hail, damaging winds and scattered tornadoes, forecasters said.

"If storms form, they will likely become severe," the weather service in Chicago said.

Meanwhile, flooding remains a concern across parts of the rain-soaked Mid-Atlantic, where "any additional rainfall could be problematic," the National Weather Service said. An atmospheric river has already dumped several inches of rain in the region, where water rescues had to be performed at a Maryland school and a 12-year-old child in Virginia died after being swept away by floodwaters.

See the national forecast map.

Tornado threat grows through end of the week

Severe weather impacting the Midwest on May 15 was expected to intensify as the week comes to an end, AccuWeather forecasters said.

"As the week progresses, the threat will advance south and eastward, impacting major cities such as Chicago, Detroit and Indianapolis on Thursday before reaching locations like St. Louis, Nashville, Memphis, Tennessee, on eastward to Washington, D.C., and Richmond, Virginia, on Friday," AccuWeather said in an advisory.

Damaging winds during that time could cause power outages, fallen trees and damage to roofs.

Long-track tornadoes, which remain on the ground for several miles, are especially a threat in the Mississippi and Tennessee valleys, AccuWeather said. Over the weekend, the storms could focus on southern states and continue through early next week.

Flooding concerns in Mid-Atlantic

Risk for additional flooding in the Mid-Atlantic, where soaking rain and flash flooding have already turned deadly, remained on May 15 as forecasters warned about areas around rivers and streams.

The storm system behind the flooding dumped up to a foot of rain over some areas of the Southeast before moving into the Mid-Atlantic region on May 13. That day, 4 to 6 inches of rain were recorded in parts of Virginia. Pennsylvania, Maryland, Virginia and West Virginia all saw flooding.

Widespread flood warnings were lifted by May 15, but river flood warnings were in effect in parts of the Potomac, Shenandoah and Rappahannock basins, the weather service in Baltimore, Maryland, said. Thunderstorms with large hail and damaging winds were also a threat south of Interstate 66 and Route 48, forecasters said.

Search crews near Charlottesville, Virginia, on May 14 located remains believed to be those of 12-year-old Jordan Sims, who went missing after being swept away by floodwaters the day before. Authorities said rushing water in a nearby creek overwhelmed the area.