Mesoscale Precipitation Discussion 1060
NWS Weather Prediction Center College Park MD
930 PM EST Thu Dec 20 2018

Areas affected...Mid-Atlantic to Lower Hudson River Valley...N
VA/N MD/N DE/E PA/NJ/S NY/E CT...

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

Valid 210229Z - 210829Z

SUMMARY...Tropical tele-connection and strong shortwave support
highly anomalous/record moisture flux into highly saturated area. 
Flooding is likely with some localize flash flooding even
possible.

DISCUSSION...GOES-16 WV suite depicts a mature deep cyclone
extending the length of the eastern CONUS with excellent T-Bar
shape and broad anticyclonic baroclinic leaf/shield capping the
system indicative of strong outflow/ideal synoptic ascent pattern.
 The right entrance region of 110-120kt 250kt jet is lifting north
but secondary jet axis is lifting north and broadening across the
area of concern throughout the next 3-6hrs providing excellent
diffluence in the region.

A surface cyclone is lifting through the southern/central
Appalachians with a warm front extending eastward through the
Piedmont of VA into the Delmarva peninsula, a cold front advances
along the eastern Gulf near a secondary developing surface low. 
In advance of this front, a highly anomalous warm conveyor branch
is lifting northward across the Coastal Carolina Plain, angling
westward slightly across N VA/MD into Central PA attm.  This
tropical branch is connected back to the Western Caribbean with
Total PWats to 1.75" detected through its length. CIRA advected
Layer PW suggest this is mainly lower atmospheric loaded with
Sfc-850 slug of near 1-1.25" advancing/pooling along the warm
front.  Of interest, this is also the layer which is providing
some return southeasterly moisture along the NW to SE frontal
boundary through the Sargasso Sea into the central Atlantic.

This combined/confluent flow supported by 20-25kt sfc flow and
50-60kt 850kt winds pool this moisture into the Mid-Atlantic and
focus it along the PA Appalachians and Pocono Mountains, but as
the shortwave and associated upper level right jet entrance region
lift north and away from the base/shortwave energy in the Eastern
Gulf, the alignment of deep moisture flux will shift from a veered
profile and orient more unidirectional and support IVT values in
the 1250-1300 kg/ms range (given record or near record TPW of 1.7"
and strong winds), this will support increasing rainfall
efficiency with potential of .5-.75"/hr rates toward 08-09z across
the Eastern Poconos/Catskills and Lower Hudson River Valley. 
Rainfall totals are generally expected in the 1.5-2.5" range
through 09z especially across Central PA and N MD with some
localized areas over 3" likely.  This year's high rainfall totals
across the Mid-Atlantic will further be pushed supporting
increased run-off and the potential for isolated flash flooding
but likely areal and main stem flooding by early morning.  

Gallina

ATTN...WFO...ALY...BGM...CTP...LWX...OKX...PHI...

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

LAT...LON   42707479 42327383 42047354 41287317 40647356
            40167388 39567431 39367482 39517560 39317618
            39117675 38827737 38497818 38867846 39337841
            40437784 41387717 42227655 42577569