Through yesterday, the region has received just 0.81" of rain during the month of September, 2.5" below average. Though the immediate metro region is not officially in "drought", the U.S. Drought Monitor does state the region is "abnormally dry". Parts of western Virginia and western Maryland are officially designated drought areas, with isolated locations in severe drought.
Much of the region may make up at least the September portion of the current rainfall deficit in the next 24 hours, with 1-3" of rain forecast.
The cause of the rain is an expansive area of low pressure over the Tennessee Valley which is being fed by a steady flow of moisture from the Gulf of Mexico and Atlantic Ocean.
The highest rainfall totals during the day today should be just west of I-95 with the axis of the heaviest rain shifting east later tonight and tomorrow morning.
Though forecast models are consistently producing generous totals from this storm system, NOAA's Hydrometeorological Prediction Center cautions:
Putting it simply..model [precipitation forecasts] have not been doing very well handling the rainfall with this system to this point in time.
Nonetheless, confidence is high appreciable rain will fall. My thinking is that the region will probably see totals averaging 1.5-2", but would not be surprised to see some spots seeing as little as around 1", with isolated 3+" totals as well.
Today's rainfall records at the three local airports occurred in 1985 when Hurricane Gloria came up the coast. They are as follows:
Reagan National (DCA): 3.17" (1985)
Dulles Airport (IAD): 1.55" (1985)
BWI: 5.00" (1985)
The 1985 DCA and BWI records are probably safe; Dulles could challenge its record.