Mesoscale Precipitation Discussion 1061
NWS Weather Prediction Center College Park MD
322 AM EST Fri Dec 21 2018

Areas affected...Southern NY...Eastern PA...NJ...CT...RI...Western
MA

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

Valid 210830Z - 211430Z

SUMMARY...Atmospheric River of record moisture and strong flux,
suggest moderate rainfall rates over saturated grounds supporting
flooding through early morning.

DISCUSSION...GOES-16 WV suite shows strong shortwave/vort center
crossing SC, with modest diffluence ahead of it.  The strong DPVA
and diffluent exit region of approaching 110+kt 3H jet is
providing solid large scale ascent.  The approach of the shortwave
and jet is favorably orienting deep layer unidirectional flow as
well as as strengthening/accelerating the flow.  As such, blended
TPW/CIRA Layer PW analysis show a tightening of the moisture plume
originating SW of Cuba, with TPW magnitudes in excess of 1.75"
nosing up into the southern Delmarva peninsula.  The nose is being
enhanced by the return branch of southeasterly flow off the
Central Atlantic as well as the main warm conveyor from the due
south. The focused surge northward is angled along the NJ coast
into the Lower Hudson River valley and NYC metro with LLJ/850mb
flow expected to increase to 60-70kts by 09z, and given the
depth/strength of the conveyor, IVT transport levels are at record
values and over 1250 kg/ms.  HREF MUCAPE means support even modest
instability up to 250 J/kg across NJ/Long Island and SE New
England from 09-12z, suggesting the possibility of rates nearing
.5"/hr in parts, especially southward facing terrain in the
Catskills and Berkshire ranges.   Recent 10.3um IR channel shows
transverse banding starting to vertically enhance in the DPVA
region across MD and E PA, supporting the notion of rainfall
generation depth to support said rates at times.

Rainfall totals of 1.5-2.5 through 15z from Hi-Res guidance also
suggests 1-2 year ARI across the Lower Hudson Valley, given the
limited instability this is quite impressive and also suggests
flooding is going to be likely possibly with flashy conditions in
terrain especially given saturated less permeable grounds in this
late autumn time period. 

Gallina

ATTN...WFO...ALY...BGM...BOX...OKX...PHI...

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

LAT...LON   42777384 42697266 42287194 41507189 40957219
            40737290 40457376 39967398 39467453 39947514
            40617535 41527548 42387492