402
AWUS01 KWNH 220319
FFGMPD
PAZ000-DEZ000-MDZ000-220718-

Mesoscale Precipitation Discussion 1041
NWS Weather Prediction Center College Park MD
1119 PM EDT Sat Sep 21 2024

Areas affected...central/eastern Pennsylvania, north/northeastern
Maryland, northern Delaware

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

Valid 220318Z - 220718Z

Summary...A cluster of deep convection over central Pennsylvania
was producing spots of rainfall rates approaching 2 inches/hr.
Isolated flash flood potential will exist with this activity as
storms migrate toward the southeastern Pennsylvania/northeastern
Maryland border region through 06-07Z.

Discussion...A vigorous shortwave trough over Pennsylvania has
helped to force a relatively fast-moving convective complex that
currently extends from near Williamsport to State College.  Ahead
of this complex, cells have developed within a warm/moist axis
characterized by ~1000 J/kg MLCAPE and 1-1.5 inch PW values -
highest near Chesapeake Bay.  The cells ahead of the linear
complex were moving considerably slower (around 5-10 mph),
allowing for several cell mergers and prolonged rainfall, with
rates approaching 1.5-2 inches/hr at times generally from
Williamsport to Harrisburg.

Both models/observations and objective analyses indicate potential
for the southward-moving convective complex to continue for a few
hours, potentially reaching the MD/PA border region through 0530Z
and the I-95 corridor thereafter.  The combination of cells and
linear convective structures suggests potential for at least an
isolated flash flood risk to persist with this activity during
that timeframe - especially near cell mergers and slower-moving
storms.  FFG thresholds in much of the region are in the 2.5
inch/hr range, suggesting that the best risk of excessive runoff
should remain 1) isolated and 2) tied to low/sensitive spots and
urbanized land surfaces.

Cook

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

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

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

LAT...LON   41157682 40537581 39897560 39197581 38917653
            39237727 40497798