Mesoscale Precipitation Discussion 0659
NWS Weather Prediction Center College Park MD
1133 AM EDT Tue Aug 14 2018

Areas affected...Central NY...North Central & Northeast PA...

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

Valid 141530Z - 142130Z

SUMMARY...Limited instability but sufficient moisture and dynamic
ascent to support weak convection over saturated soils continuing
flooding through afternoon.

DISCUSSION...Overall there is a significant downward trend/warming
in 10.3um IR imagery but small breaks in clouds build up some
surface heating for up to 500 J/kg.   The vertically stacked low
over Oneida/Madison county is providing DPVA ascent; WV suite
depicts some shearing/confluence from western approaching trof, to
help tighten the gradient and upper level divergence for continued
ascent.  Coincident deep moisture with low 70s Tds over eastern
Lake Ontario to the Sequehanna Valley and stacked saturated
moisture supports a narrow band of 1.6-1.7" TPWs from the lake to
northeast PA, starting to wrap around the base of the cyclone and
break the dry slot over the Hudson Valley.  All combined this
environment is supporting percolating shallow CU/TCU across
flooded areas with rates up to .5"/hr.  Slow cell motions may
support as much as an additional 1-1.25" over the next 3-5hrs. 
Both rates would exceed the compromised FFG values in the region
keeping flooding ongoing.   However, the weakness of the
updrafts/rates are not likely to produce flashy hydrologic
conditions per se, except in the most intense (but narrow)
downdrafts over the most compromised areas.  Still, the additional
rainfall will keep the flooding hazard high. 

Greatest risk for prolonged, strongest updrafts with some
potential back-building will be along the eastern periphery of
Lake Ontario along the western edge of the dry slot where modest
moisture remains but clearer skies will support an increase in
instability up to SBCAPE 1500 J/kg, over Lewis/Oswego to
Onondaga/Cayuga counties, where heavy rain has not fallen over the
last few days. Here the rain rates may be sufficient to exceed the
higher FFG, in the 1.5"/hr range, here the back-building/ repeat
cells will be need to produce flash flooding conditions, and that
remains a bit uncertain.  

Either area will have positives and negatives toward flash
flooding to occur, so it is considered possible given the
uncertainty or slower compiling/inundation nature to the flooding.

Gallina

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

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

LAT...LON   43877565 43257555 42617536 42197445 41717439
            41337455 41007512 41087610 41537715 42217772
            43107759 43847656