The National Weather Service has posted a Flash Flood Watch for Baltimore, Washington and their suburban counties, beginning at 5 p.m. Thursday as an approaching cold front threatens the region with heavy, slow-moving thunderstorms.

UPDATE: The NWS has issued a Severe Thunderstorm Watch until 9 p.m. for all of Maryland between Allegany County and the Bay, plus the northern portions of the Eastern Shore.  

Storms could begin in the Baltimore region earlier this afternoon as the bay breeze picks up and moves over hotter air to the west. The more severe weather is likely late this afternoon as the cold front approaches. Those storms will be capable of producing strong winds and large hail, forecasters warned.

Worse, from a flooding standpoint, will be the slow movement of the storms, allowing heavy rain accumulations in some locations, especially in the Baltimore-Washington area. Hence, the flash flood worry. Rain totals of a half- to three-quarters of an inch are possible.

Behind the front things will calm down, although showers may continue off and on through the night and Friday, forecasters said. But the hot weather we've had for the past two days will ease. Highs Friday should hold in the low 70s, rising to the low 80s by Sunday.