Mesoscale Precipitation Discussion 0917
NWS Weather Prediction Center College Park MD
210 PM EDT Tue Oct 02 2018

Areas affected...Southern New York, Northern Pennsylvania

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

Valid 021809Z - 022315Z

Summary...Several waves of heavy rain and thunderstorms are likely
to affect areas near the New York and Pennsylvania border region
this afternoon. 1-2 in/hr rain rates will be possible, which could
lead to flash flooding.

Discussion...12Z soundings from New York and Pennsylvania showed
flow aligned in a westerly direction through most of the
troposphere, and this has largely continued into the early
afternoon per RAP initialization soundings. Therefore, it is not
surprising to see that some of the convective bands in S NY and N
PA have aligned with the flow and are set up in a favorable
training pattern. This should continue through the afternoon
hours. There are two important trends to consider. First,
instability is projected to increase with MLCAPE exceeding 1000
j/kg in the inflow region across most of Pennsylvania, and this
appears likely given the filtered sunshine evident on GOES-16
visible satellite channels. Second, continued redevelopment of
convection appears possible as far west as Lake Erie, still well
within the warm sector, given the upstream +PV anomaly is still
over Michigan. Therefore, convective bands should continue to be
channeled into the PA-NY border region along the northern
periphery of a building reservoir of moderate instability. The
increasing CAPE should favor increasing rain rates by the 21-00Z
time frame, and given PW values around 1.5 inches, 1-2 in/hr rain
rates appear possible. With renewed upstream development of
convection expected, the training pattern could last sufficiently
long to produce flash flooding in narrow swaths that get
repeatedly affected by heavy rain. However, the low-level inflow
(850mb winds around 25-30 knots) is generally weaker than the deep
layer mean flow (850-300mb winds around 40 knots) and this does
not favor significant backbuilding or flash flood potential;
rather, it favors relatively progressive convective bands.
However, given the orientation of the convective bands to the mean
flow, this should still create some chance of flash flooding.

Lamers

ATTN...WFO...ALY...BGM...BUF...CTP...OKX...PBZ...PHI...

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

LAT...LON   42737574 42487341 41197434 40947649 41227852
            41647951 42427788