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AWUS01 KWNH 221807
FFGMPD
MAZ000-NHZ000-RIZ000-VTZ000-CTZ000-NYZ000-NJZ000-PAZ000-DEZ000-MDZ000-VAZ000-DC
Z000-WVZ000-OHZ000-TNZ000-KYZ000-230005-

Mesoscale Precipitation Discussion 0659
NWS Weather Prediction Center College Park MD
206 PM EDT Mon Jul 22 2019

Areas affected...Central Appalachians...Northern
Mid-Atlantic...Southeast NY and Southern New England

Concerning...Heavy rainfall...Flash flooding likely

Valid 221805Z - 230005Z

SUMMARY...Showers and thunderstorms initially focusing over the
Appalachians and Blue Ridge over the next 1 to 2 hours will
gradually expand in coverage off to the east across the Piedmont
and toward the coastal plain including the I-95 corridor from
Washington D.C. and Baltimore northeast through Philadelphia and
New York City.  Heavy rainfall rates falling on locally sensitive
soil conditions, and over the more prone urbanized metropolitan
areas will likely result in areas of flash flooding through the
afternoon and evening hours.

DISCUSSION...The latest GOES-16 visible satellite imagery shows
strong diurnal heating/solar insolation working to rapidly
destabilize the boundary layer across the northern Mid-Atlantic
and far southern New England. Meanwhile, a cold front is gradually
settling southeast from the Great Lakes region as an upper trough
pivots across the region. One area of low pressure is noted across
northwest PA, but there is a boundary that extends east from this
in a wavy fashion across northern PA, northern NJ and then
northeast up along coastal areas of southern New England. Multiple
waves of low pressure are noted along this portion of the front.
To the south of the boundary in the warm sector, there is already
a nose of 1000 to 1500 j/kg of MLCAPE that extends from along and
east of the Blue Ridge.

PWATs across the region are quite high with values of 1.75 to 2
inches. The latest CIRA-LPW data set analysis shows somewhat
better moisture pooled west of the Appalachians, but over the next
several hours, it is expected that areas east of the Blue Ridge
will see deeper moisture return which will in part be related to
advection of more concentrated mid-level moisture from the TN
Valley. An additional component will be a well-defined southerly
low-level fetch of moisture lifting north across the Mid-Atlantic
coastal plain.

Radar imagery already shows an expansive area of showers and a few
thunderstorms across the Appalachians which is beginning to
advance east of the terrain. Over the next several hours, this
activity will build east into the aforementioned instability pool
and will expand in coverage as favorable thermodynamics work in
tandem with local orographics and improving large scale ascent via
250 mb right-entrance jet dynamics in association with the upper
trough pivoting across the Great Lakes region.

Rainfall rates are expected to reach 2 to 3 inches/hr within the
stronger and more organized convective cores, which will likely
include multiple linear convective segments through the afternoon
and evening hours. The 12Z HREF hires model suite, and the 16Z-17Z
runs of the experimental WoFS model output supports as much 3 to 5
inches of rain locally, with some of the stronger signals
appearing across eastern PA, northern NJ, southeast NY and
portions of southern New England where moisture and instability
pooling along the front and with stronger orographics and surface
convergence will support more organized heavy rainfall with
numerous convective clusters.

Given the expected short-term rainfall rates, and storm totals
through the afternoon and early evening hours, areas of flash
flooding will be likely. This will include the highly urbanized
I-95 corridor from New York City and Philadelphia down to
Baltimore and Washington D.C., and include all adjacent suburbs.
Additional areas of sensitivity are noted across southwest to
south-central PA, and over WV where heavy rains from last evening
have helped to locally saturate the ground. Thus any additional
rain for these areas will enhanced runoff and flash flooding
concerns.

Orrison

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...AKQ...ALY...BGM...BOX...CTP...GYX...ILN...JKL...
LWX...MRX...OKX...PBZ...PHI...RLX...RNK...

ATTN...RFC...LMRFC...MARFC...NERFC...OHRFC...SERFC...

LAT...LON   43057279 43007192 42667148 41917179 41057369
            40067487 38957625 37987806 36908002 36608130
            36598232 36808289 37288333 38058342 39098284
            40188061 41307824 42187654 42857390


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