Mesoscale Precipitation Discussion 0480
NWS Weather Prediction Center College Park MD
1212 PM EDT Fri Jun 21 2024

Areas affected...Portions of the Upper Ohio Valey...Northern
Mid-Atlantic...New York...Southern New England

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

Valid 211611Z - 212211Z

SUMMARY...Scattered to broken areas of showers and thunderstorms
are expected this afternoon across areas of the Upper Ohio Valley,
northern Mid-Atlantic, New York and New England. Heavy rainfall
rates and potentially some localized instances of cell-training
may support some instances of flash flooding.

DISCUSSION...GOES-E visible satellite imagery is showing rapidly
expanding areas of CU/TCU across large areas of northwest PA
through NY state. The airmass is already moderately unstable with
a belt of MLCAPE values of 1500+ J/kg across much of northwest to
north-central PA through southern NY, and with this nose of
instability also becoming increasingly focused across southern New
England to the south of a west/east oriented frontal zone.

Differential heating boundaries in close proximity this front
along with some terrain-induced circulations/forcing involving
areas of higher terrain will support increasing coverage of
showers and thunderstorms this afternoon as additional strong
solar insolation/boundary layer destabilization occurs.

The low-level wind fields are also generally rather convergent and
especially across south-central NY through southern New England,
and there will be some modest effective bulk shear traversing this
corridor for some locally organization swaths of multi-cell
convection. More separately farther to the west, areas away from
Lake Erie involving northeast OH and northwest PA may have focus
for convection from not only close proximity of the front, but
some lake-breeze convergence/interactions.

The 12Z HREF guidance favors some of the heaviest rainfall
potential across areas of southern New England where some of the
higher PWs and better overall forcing will be in place. PWs across
these areas are already locally over 1.75 inches which is a solid
2+ standard deviations above normal. Also, there are
considerations of a separate lee-side trough axis noted over
southern New England down near CT/RI which should tend to provide
additional small-scale forcing.

Rainfall rates of 1 to 2 inches/hour are expected with the
stronger storms, and with some localized potential for
repeating/training convective cells, there may be some pockets of
2 to 4 inch rainfall totals. Some instances of flash flooding will
be possible as a result of this.

Orrison

ATTN...WFO...ALY...BGM...BOX...BUF...CLE...CTP...GYX...OKX...
PBZ...PHI...

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

LAT...LON   43737405 43377368 42927322 42957238 42737170
            42367126 41957101 41517132 41307274 40877385
            40717462 40887673 40797911 40228136 40398251
            41018276 41398239 41798151 42218011 42567925
            42837823 42997731 43007645 43237554 43697478