Just before Hurricane Irene roared up the East Coast, dropping 3 to 5 inches of rain and more across the region, the rainfall deficit at Reagan National was 2.49 inches. That all changed following Irene... now there is a surplus of precipitation for the year at DCA... we are now almost 1" above normal for the year. BWI Thurgood Marshal is more than 4 inches above for the year while Dulles International did not benefit as much from Irene... coming in with a deficit of 2.81 inches for 2011 so far.
Well, rainfall totals for the year will tack on another 1 to 3 inches this coming week as a moisture-driven cold front brings showers and a few heavy rainfall-producing storms Monday afternoon. Yes, this will come at the expense of Labor Day picnics, barbecues and pool parties. If there is any good news... most of the rain will fall after the midday, so Labor Day won't be a complete washout in the Nation's Capital. These totals will also include the rain from Tropical Storm Lee, which will move north along the Appalachian spine and bring periodic showers through much of next week.
This is a good time to reference the term "flash flooding." When very heavy rain falls in a short period of time, it tends to not get absorbed by the ground and ends up rolling off the ground into streets, creeks and streams causing rapid rises in the water level. Storm drains in urban locations could get overwhelmed, and in rural areas near creeks and streams, you should drive with caution because water could cover the roadway in these flood prone areas.
Given the heavy rain recently, it will only take about 2 inches of rain in one hour to cause flash flooding. The amount of moisture that will be available in the showers and storms is about 2.25 inches, known as the "precipitable water" in the atmosphere. It won't be uncommon to get a shower or storm late Monday or anytime Tuesday and Wednesday that drops this amount of rain in an hour or less.
Flash Flood Watches have been posted north of Tropical Storm Lee all the way to eastern Tennessee. The local weather service office could issue Flash Flood Watches for the District and surrounding areas as early as this afternoon for the Monday-Wednesday rain event coming our way.
Drive safe and stay tuned to ABC7 and WTOP Radio for the latest weather information!