Thunderstorm watch

A severe thunderstorm watch is in effect until 8 p.m. Thursday in parts of Maryland. (Storm Prediction Center / May 15, 2014)

A severe thunderstorm watch is in effect for Frederick, Montgomery and Washington counties until 8 p.m. as a cold front moves through the region, bringing periods of heavy rain Thursday afternoon through Friday afternoon.

A flash flood watch is in effect across Central Maryland from late Thursday through Friday afternoon. Rainfall amounts of 2-3 inches, with possibly higher localized totals, are forecast.

The deluge threatens to spoil the Black-Eyed Susan Stakes at Pimlico on Friday, the prelude to Saturday's Preakness Stakes. And the mess of mud it leaves behind could meanwhile affect the races, Preakness organizers said.

Radar images showed a line of storms moving into western Maryland about 2 p.m., expected to reach the Baltimore area in the afternoon and evening hours.

The storms bring risks of strong winds and heavy downpours, but risks of hail and tornadoes are low, according to the Storm Prediction Center.