Mesoscale Precipitation Discussion 0655
NWS Weather Prediction Center College Park MD
1207 AM EDT Tue Aug 14 2018

Areas affected...central/western NY into central PA

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

Valid 140406Z - 141000Z

SUMMARY...Continued flooding with possible faster response flash
flooding will remain possible across portions of central PA into
central NY through 10Z. The potential for 3 hour rainfall of 1.5
to 3+ inches will exist across the region through early morning.

DISCUSSION...GOES 16 mid-level water vapor imagery showed numerous
small scale vorticity maxima revolving about the main mid-upper
low centered over southwestern PA, and local radars showed a
number of slow moving mesoscale circulations from northeastern PA
into south-central NY, each tied to locally higher rainfall rates
among largely stratiform light to moderate rainfall across the
region. A somewhat larger scale vorticity max was located over the
lower Hudson Valley at 04Z with an associated region of lift
producing steady rainfall to its northwest. MLCAPE values of 500
J/kg or less (via the latest SPC mesoanalysis page) have been
limiting maximum rainfall rates as of 03Z to 0.5 to 1.0 in/hr, but
Flash Flood Guidance is rather low across northeastern PA into
central NY due to recent heavy rainfall and flooding is ongoing in
several locations.

Given little movement to the synoptic scale low over the next 3-6
hours, small scale vorticity maxima over northeastern PA will
continue to be responsible for local flare ups of rain rates
between 0.5 and 1.0 in/hr on top of saturated soils, exacerbating
ongoing flooding concerns. Weak instability is expected to be the
greatest limiting factor to more large scale heavier rainfall
rates.

Farther north, the low to mid-level circulation over the lower
Hudson Valley is forecast to track toward the west-northwest
through 10Z into a region of locally higher instability seen on
RAP analyses and the SPC mesoanalysis page. Slow movement of heavy
rainfall cores are expected to develop as this occurs with
potential for 1-3 inches in 3 hours. While Flash Flood Guidance
values are a bit higher across central NY (1.5 to 2.5 inches in 3
hours), these rates could still produce flash flooding in a few
locations. The added atmospheric ingredients of upper level
divergence within the left exit region of a forecast 50-70+ kt jet
streak and pronounced upper level diffluence are expected to
contribute to increased ascent.

Otto

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

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

LAT...LON   43917627 43797493 42687411 42057407 41677426
            41367475 41017560 40657697 40717783 41047814
            41567860 42387883 42947834 43307719