AWUS01 KWNH 071128
FFGMPD
VTZ000-NYZ000-NJZ000-PAZ000-071700-

Mesoscale Precipitation Discussion 1137
NWS Weather Prediction Center College Park MD
728 AM EDT Sat Oct 07 2023

Areas affected...The Poconos and northern NJ through eastern
Upstate NY

Concerning...Heavy rainfall...Flash flooding likely

Valid 071127Z - 071700Z

Summary...Showers and thunderstorms ahead of a cold front will
expand in coverage and train northward through the morning.
Rainfall rates at times may reach 3"/hr, resulting in bands of
total rainfall of 2-4" with locally higher amounts. Flash flooding
is possible.

Discussion...The regional radar mosaic this morning shows
widespread showers with isolated thunderstorms stretched from the
St. Lawrence Valley southward through the Catskills, with more
scattered activity noted along the I-95 corridor from New York
City to Baltimore, MD. This convection is blossoming immediately
ahead of a cold front analyzed by WPC draped from near Lake
Ontario through eastern PA, and this front is expected to advect
eastward into New England later today. Downstream of this
boundary, moisture advection is increasing into the front noted by
regionally backed 850mb winds of 15-25 kts driving PWs of 1.2 to
1.5 inches northwestward, above the 90th percentile for the date.
Additionally, a ribbon of MUCAPE above 250 J/kg is beginning to
surge northward from NJ, further enhancing the thermodynamics to
support heavy rain. Rainfall rates have been estimated via local
radars above 2"/hr already this morning, with the low reflectivity
centroids of these cells indicating efficient warm rain processes.

The most recent runs of the HRRR are initializing pretty well this
morning, but in general all of the high-res is in pretty good
agreement with radar evolution the next several hours. As the
front continues to push eastward, it will combine with the tail of
an intensifying jet streak pivoting poleward over Canada and
convergence along the nose of the 850mb inflow to drive more
intense ascent. This will act upon a thermodynamic environment
that will remain ripe for heavy rainfall, characterized by PWs
nearing 1.5" and modest 250-500 J/kg SBCAPE, to produce rainfall
rates for which the HREF indicates has a 10-20% chance of reaching
2"/hr, with 15-min rainfall from the HRRR exceeding 0.75" at times
(brief 3"+/hr rates). Although Corfidi vectors indicate the entire
line ahead of the front will advect progressively eastward,
increasing convective coverage and 0-6km mean winds aligned to the
front will still permit enhanced training from south to north.
This could produce 2-4" of rainfall, and the HREF suggests a
10-15% chance of more than 5" in isolated areas.

Antecedent conditions are extremely favorable for rapid runoff
from the Capital District southward through the Hudson Valley and
into the Poconos where 14-day rainfall has been 150-300% of normal
resulting in 40cm soil moisture around the 95th percentile. This
has compromised FFG to 1.5"/1hr and 2-2.5"/3hrs, and the HREF FFG
exceedance probabilities peak around 40% late this morning into
the early afternoon. This region has the greatest risk for flash
flood impacts today. Farther north into the Adirondacks, isolated
runoff leading to flash flooding is also possible where training
can move atop sensitive terrain features.


Weiss

ATTN...WFO...ALY...BGM...BTV...OKX...PHI...

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

LAT...LON 45057390 45047343 44787332 43857339 42387350
41747379 41497421 41177459 40677487 40837531
41337551 41917549 42527531 43877482 44797456


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