Mesoscale Precipitation Discussion 0859
NWS Weather Prediction Center College Park MD
1202 AM EDT Tue Sep 18 2018

Areas affected...central/northern VA, D.C., MD, northern DE,
southeastern PA, NJ

Concerning...Heavy rainfall...Flash flooding possible

Valid 180401Z - 180830Z

Summary...An axis of training heavy rain with rainfall rates of
1-3 in/hr will pose a flash flood threat across portions of the
northern Mid-Atlantic region. A quick 2-4 inches will be possible
through 09Z.

Discussion...KLWX reflectivity showed a SSW to NNE axis of
convection over central to northern VA, slowly translating toward
the northeast. Storms north of the D.C. metro were more spotty in
nature. Radar estimated rates of 2-3 in/hr were confirmed by area
Wunderground.com observations showing near 3 in/hr in Fluvanna and
Louisa counties in central VA ending 0345Z. The convective line
was occurring out ahead of a subtle shear axis observed on
mid-level water vapor imagery, located ahead of a larger scale
mid-upper level trough just west of the Appalachian spine as of
03Z. Despite some mid-level dry air above 500 mb on the 00Z LWX
sounding, the low levels were fairly moist with small dewpoint
depressions below 500 mb. MLCAPE ranged between 500-1500 J/kg
across the area and has been slowly increasing over time toward
the north over northern MD into PA given 850 mb flow from the SSW
at nearly 30 kt as observed on area VAD wind plots. The low level
flow was similar in direction and magnitude to the 850-300 mb
layer mean flow promoting periods of training.

Over the next few hours, the mid-level shear axis is forecast to
move off toward the northeast, with forcing for ascent spreading
into northern MD, southeastern PA and NJ along with increasing
MLCAPE. Rainfall rates are expected to be generally 1-2 in/hr but
locally as high as 3 in/hr will be possible given the moist
environment and the history of these storms over VA. Flash
flooding will be possible with maximum rainfall of 2-4 inches
through about 09Z, along with heavy rain over sensitive urban
locations.

Otto

ATTN...WFO...AKQ...CTP...LWX...OKX...PHI...RNK...

ATTN...RFC...MARFC...

LAT...LON   40927510 40797437 40097460 39517502 39157539
            38467611 37607700 37317784 37297851 37767858
            38397812 39047753 39897651