Pop-up storms threaten to soak Lollapalooza crowds

CHICAGO -- Hail, lightning and brief heavy downpours are all possible as part of a storm system heading toward downtown Chicago and the Lollapalooza music festival Friday afternoon.

TRACK THE STORM: Click the Interactive Radar Map at right

Video taken of one these storms along the Stevenson Expressway near 1st Avenue shows extremely heavy rain and darkened skies.

Grant Park, Navy Pier and Northerly Island could all be hit with half-inch diameter hail, 40 mph winds and occasional lightning, according to the National Weather Service.

Thunderstorms appeared rapidly Friday afternoon on the I-80 corridor in Indiana, as well as near suburbs Barrington and Crystal Lake in Illinois.

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The weather service advises people to seek shelter until the storm has passed. Gusty winds may cause objects such as trash bins to blow around.

The Sun-Times Media Wire contributed to this report.