AWUS01 KWNH 121407
FFGMPD
NYZ000-NJZ000-PAZ000-DEZ000-MDZ000-VAZ000-DCZ000-122000-

Mesoscale Precipitation Discussion 0610
NWS Weather Prediction Center College Park MD
1006 AM EDT Fri Jul 12 2024

Areas affected...eastern Mid-Atlantic states

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

Valid 121405Z - 122000Z

Summary...Showers and thunderstorms will expand and train within
an extremely moist environment through the afternoon. Rainfall
rates of 2"/hr are likely, which could produce 2-3" of rain with
locally higher amounts. Flash flooding is possible.

Discussion...The regional radar mosaic this morning indicates an
expanding area of showers with embedded thunderstorms from
northern South Carolina through central New Jersey, encompassing
much of the Mid-Atlantic states. This precipitation is occurring
in the vicinity of a stationary front analyzed by WPC, aided by a
weak shortwave impulse which will lift north from Virginia, and
from the distant but still influential RRQ of a jet streak over
the Ohio Valley. The environment across the region is extremely
supportive of heavy rainfall, with PWs measured by morning U/A
sounding reaching 2 to 2.3 inches (2.39 inches, a daily record at
OKX) with coincident MLCAPE of 1000-2000 J/kg. The most intense
instability remains just east of the coastline, but low-level S/SE
flow will continue to draw that CAPE northward to enhance the
thermodynamics through the afternoon.

The CAMs are in good agreement that waves of precipitation will
continue to surge onshore in response to increasing IVT (peaking
above 500 kg/m/s in VA) as onshore flow persists. This will likely
push the stationary front at least a bit to the west, with a sharp
instability and moisture gradient remaining along that boundary.
This suggests that the heaviest rain will occur along and just
east of the front, which is supported by HREF and REFS
probabilities reaching 40-50% for 3" in the next 6 hours, and it
is possible a few areas approach 4-5" of rainfall by this evening
across the DelMarVa. These impressive rainfall amounts will occur
in response to intense rainfall rates which have a 60% (15%)
chance of reaching 1"/hr (2"/hr), with short duration rates even
higher than that reflected by HRRR 15-min accumulations as much as
0.75". While mean cloud-layer winds are likely to remain
progressive at 15-20 kts, this flow will be nearly aligned to the
front, and as Corfidi vectors become increasingly aligned against
the mean flow this will support backbuilding and regeneration of
echoes to support enhanced training.

NASA SPoRT 0-10cm soil moisture is less than 40% in most areas,
which is resulting in FFG that is quite elevated at 3-4"/3hrs.
This suggests that in most areas, flash flooding will be isolated,
and HREF exceedance probabilities peak at only 15%. However, the
intense rain rates supported by record PWs and collocated weak
lapse rates could overwhelm soils in the more vulnerable
locations, especially urban areas, and where any training of these
rain rates occur, instances of flash flooding could result.


Weiss


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

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

LAT...LON 41107480 41107424 40907386 40577374 39997389
39737405 39147444 38897472 38677492 38487525
38437569 38447621 38537663 38757720 39157748
39897676 40757563
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