849
AWUS01 KWNH 181746
FFGMPD
NYZ000-NJZ000-PAZ000-MDZ000-WVZ000-OHZ000-182345-

Mesoscale Precipitation Discussion 0459
NWS Weather Prediction Center College Park MD
145 PM EDT Tue Jun 18 2024

Areas affected...Upper OH Valley...Central Appalachians...Northern
Mid-Atlantic

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

Valid 181745Z - 182345Z

SUMMARY...Scattered areas of showers and thunderstorms are
expected to develop this afternoon and continue into the early
evening hours. High rainfall rates and relatively slow
cell-motions may foster some isolated instances of flash flooding.

DISCUSSION...A moist and unstable airmass is in place across much
of the Upper OH Valley through the northern Mid-Atlantic region
characterized by midday MLCAPE values that are on the order of
1500 to 2500 J/kg and PWs of 1.5 to 1.75 inches. The boundary
layer in particular is quite moist with surface dewpoints in the
low 70s across large areas of eastern OH through western and
northern PA, and also across southern NY.

Additional strong diurnal heating going through mid-afternoon will
only contribute to a further destabilization of the boundary layer
and should set the stage for scattered areas of showers and
thunderstorms to develop. A combination of localized differential
heating boundaries and orographics involving the higher terrain of
the Appalachians will be key drivers of this.

However, a rather strong subtropical ridge is currently centered
over the Mid-Atlantic states and will be strengthening a bit
further this afternoon. This suggests poor mid-level lapse rates,
and with a lack of shear, the convective mode going through the
afternoon hours should generally be pulse in nature. However, as
smaller-scale convection develops, there may be sufficient outflow
boundary activity to promote additional convective growth/activity.

Areas also farther west across northeast OH, northwest PA and
southwest NY in close proximity to Lake Erie will have the
consideration of a lake-breeze boundary which will help to provide
some low-level forcing/convergence for convective initiation this
afternoon. Can`t rule out some locally focused areas of convection
across these areas with slow cell-motions in the vicinity of this
boundary.

The 12Z HREF guidance suggests rainfall rates should easily reach
1 to 2 inches/hour with the stronger storms, with the potential
for a lot of this to fall in as little as 30 minutes given the
high CAPE environment. Some localized storm totals of 2 to 3+
inches will be possible, and this may result in some isolated
instances of flash flooding. This will include potentially some
urban impacts as well for locations such as Pittsburgh and
Cleveland.

Orrison

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

ATTN...WFO...ALY...BGM...BUF...CLE...CTP...LWX...OKX...PBZ...
PHI...RLX...

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

LAT...LON   42757731 42427483 41877418 41377449 40947537
            40877657 40537795 39767864 38488014 38828100
            39938126 40918246 41638226 42088089 42687898