[12Z NAM computer forecast model total rainfall amounts for the upcoming event where the red region from DC-to-Philly-to-NYC represents 3+ inches of rainfall]
A rainy stretch of weather will get underway this evening in the Mid-Atlantic region and the period of biggest concern for flooding rainfall will be from late tomorrow night through early Thursday with more than three inches possible in DC, Philly and New York City by the time the steadiest rain moves away from the area later Thursday morning. The combination of a blocking pattern in the upper atmosphere, persistent low-level moist onshore flow, and copious amounts of moisture streaming this way from the Gulf of Mexico will likely cause “flood watches” to be posted by the National Weather Service in the I-95 corridor for the late Tuesday night/Wednesday/Wednesday night time period. There will be periods of rain or drizzle around here from tonight into early tomorrow night, but the heaviest rainfall is likely to develop late tomorrow night and early Wednesday when warmer air begins to push northward into well-established cool ocean air. Thunderstorms will likely break out on Wednesday and any storm that does form can produce torrential downpours for brief periods of time. While the steadiest rain exits the I-95 corridor to the north and east early Thursday, there still can be shower activity later in the day along with a threat for more thunderstorms.