The watch is part of a forecast that includes the potential for severe weather all week.
A Flood Warning is in effect for Frederick County until early Wednesday morning.
WJZ's Marty Bass says the weather will be monitored throughout the day for possibility of severe weather.
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Lee said hail is a "highly localized event" that is hard to predict. Heavy rain is expected to occur over a short period of time.
The temperature was about 63 degrees when the hail began falling, which Lee said is representative of the "unstable atmosphere" that's resulting in stormy weather.
If the sky darkens, it's best to get inside or off the road, he said.
The National Weather Service has issued a hazardous weather outlook for Monday afternoon and evening throughout Will County, warning of "hail up to ping pong ball size" and possible flooding.
Numerous water rescues had been made in and near Frederick, the weather service said, adding that water had been reported inside buildings.
Police in the city of Frederick are calling many roads not passable and are asking drivers to stay off them. "Do not underestimate the power of water".
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