082
AWUS01 KWNH 300230
FFGMPD
NJZ000-PAZ000-DEZ000-MDZ000-VAZ000-DCZ000-WVZ000-300830-

Mesoscale Precipitation Discussion 0518
NWS Weather Prediction Center College Park MD
1029 PM EDT Sat Jun 29 2024

Areas affected...northern MD and DE into surrounding portions of
PA/WV/VA/DC

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

Valid 300230Z - 300830Z

Summary...Hourly totals of 1-2" may locally train/repeat across
vulnerable urbanized terrain. Isolated instances of flash flooding
are possible.

Discussion...A relatively small line of thunderstorms has gained
organization over portions of western MD into adjacent portions of
WV/VA, taking on a training orientation in the past hour or so
with hourly totals up to ~2" (per MRMS estimates). The mesoscale
environment is highly favorable for continued heavy rainfall, with
ample instability (SB CAPE 1500-2000 J/kg), highly anomalous
tropospheric moisture (PWATs 2.0-2.3", near or above the max
moving average per IAD sounding climatology), and impressive
effective bulk shear (30-40 kts) for convective organization.
Along with moderate low-level (925-850mb) southerly moisture
transport and frontogenesis (moist pronounced at 1000-925 mb)
indicated by RAP mesoanalysis, upwind propagation vectors of only
10-15 kts towards the southeast should allow for some backbuilding
and training/repeating along the southwestern flank of the
organizing cluster of storms, increasing concerns for short term
accumulations of 2-4" (and possible subsequent flash flooding,
particularly in more vulnerable urban areas).

While the 12z/18z hi-res CAMs haven`t done a great job handling
the convective evolution (and particularly so with more recent
runs of the HRR), there is still a meaningful signal (10-20%) for
3" exceedance (per 40-km neighborhood probabilities of exceedance
from the 18z HREF) though 09z, favoring portions of the populated
I-70/95 corridor (encompasing the Baltimore and Wilmington metro
areas, and possibly getting as far south as Washington D.C. and
surroundings). While Flash Flood Guidance (FFG) is as high as 4-5"
for much of the area, more vulnerable urbanized terrain has FFGs
of 3" or less. As a result, localized/isolated instances of flash
flooding are possible (and could be quite impactful for low-lying,
vulnerable localities).

Churchill

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

ATTN...WFO...CTP...LWX...PHI...

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

LAT...LON   39877579 39617552 39277581 38937636 38807712
            39027822 39337799 39577747 39767674