Mesoscale Precipitation Discussion 0918
NWS Weather Prediction Center College Park MD
638 PM EDT Tue Oct 02 2018

Areas affected...Portions of the Mid Atlantic from PA into CT

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

Valid 022237Z - 030400Z

Summary...Thunderstorms will continue to move across portions of
the Mid Atlantic, and several waves of storms may repeatedly
affect some areas. This could lead to locally heavy rainfall and
flash flooding. Rain rates of 1-2 in/hr will be possible with the
stronger and more persistent storms.

Discussion...Convection across the Mid Atlantic region continues
to be organized predominately in west-east oriented bands, with a
broken line extending from DUJ-AVP-HFD. This orientation of the
convective bands was roughly parallel with the deep layer mean
flow, and was setting up some cell training across the region.
Locally heavy swaths of rainfall have been observed, with rain
rates in the strongest storms peaking in the 1-2 in/hr range per
WSR-88D dual pol and MRMS estimates. The environment should
continue to support these sort of rain rates into the evening
hours, with around 1000 j/kg of MLCAPE in the inflow region to the
south of the most significant convective bands, and precipitable
water values around 1.5 inches. Although neither of these values
is exceptionally high, they are in ranges that support organized
convection with heavy rain rates. Another negating factor will be
low-level flow (850mb winds around 25-30 knots) weaker than the
mean wind speeds (closer to 40 knots), which does not favor as
much backbuilding. Therefore, individual bands and clusters of
convection should continue to be relatively progressive. Flash
flood potential would emanate mainly from repeated rounds of heavy
rain due to cell training. This is occurring in enough abundance
across the region for flash flooding to remain possible as long as
vigorous convection continues. Hi-res models show strong
convection being maintained ahead of the progression of a +PV
anomaly, now reaching northwest Pennsylvania (2230Z) per GOES-16
Air Mass RGB. Linear extrapolation of this feature would take it
to NE PA near Wilkes-Barre and Scranton by 02Z, and into CT by
04Z. This should be the primary controlling factor on when
convection should diminish. Therefore, central and eastern PA
should have another few hours of training convection. It could
last longer for points further east (N NJ, NYC Metro, S CT), but
the convective line was also showing signs of gradually sinking
south, so this may reduce the size of the area impacted to the
east of Pennsylvania. Nevertheless, the flash flood threat should
extend as far east as CT, and include the NYC Metro Area if
convection can build far enough south prior to the arrival of the
+PV anomaly.

Lamers

ATTN...WFO...ALY...BGM...BOX...CTP...LWX...OKX...PBZ...PHI...
RLX...

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

LAT...LON   41847474 41817187 41157253 40677418 40347670
            39687855 39008093 40868031 41607891 41827697