Summary All of the ingredients are in place for a major rain event to affect the Mid-Atlantic region including the DC, Philly and New York City metro regions from tonight into early Thursday and it might include some nasty thunderstorms along the way. Occasional rain or drizzle will fall this afternoon and evening in the Mid-Atlantic region and some of it will be heavy at times. Additional periods of rain on Wednesday and Wednesday night will fall heavily at times and strong-to-severe thunderstorms can bring torrential downpours and damaging winds to the region. By the time the steadiest of the rain pushes off to our north and east early Thursday, it is likely that many areas in the Mid-Atlantic region will have received 3-5 inches of rain from this unfolding event.
Ingredients Ingredient number 1 is a very chilly air mass that is now well entrenched in the Mid-Atlantic region and is anchored by strong Canadian high pressure. Noontime temperatures are way below normal for this time of year with a 48 degree reading at Philly Airport and 51 degrees at Reagan National Airport in DC (normal highs are 68 and 71 respectively). This chilly air mass will act as an important overrunning surface on Wednesday as warmer air tries to advance northward into the region.
Ingredient number 2 is an impressive transport of tropical moisture in the lower levels of the atmosphere right from the Gulf of Mexico towards the Northeast U.S. The latest map of the 850 millibar moisture transport (above) depicts high transport values in the eastern states and a strong flow of air from the Gulf of Mexico towards our area. High values of 850 millibar moisture transport have been connected to heavy rainfall events with convective systems.
Ingredient number 3 is a large, powerful and slow-moving low pressure system currently centered over Iowa. The slow movement of this surface storm is a result of an “omega-shaped” blocking pattern in the upper atmosphere and it ensures an extended period of precipitation in our region which increases flooding potential.
[Latest nationwide NEXRAD radar image; courtesy University of Wisconsin AOSS]