Subj : MESO: Heavy rain - floodi
To   : All
From : Mike Powell
Date : Sat Apr 15 2023 17:49:00

AWUS01 KWNH 152234
FFGMPD
NYZ000-NJZ000-PAZ000-160200-

Mesoscale Precipitation Discussion 0190
NWS Weather Prediction Center College Park MD
633 PM EDT Sat Apr 15 2023

Areas affected...eastern PA, northern NJ into southern NY

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

Valid 152230Z - 160200Z

Summary...A localized flash flood threat with rainfall rates near
1 inch in 15 minutes will continue until about 01-02Z for portions
of eastern PA into northern NJ and southern NY. Peak additional
rainfall of 2-3 inches will be possible.

Discussion...Thunderstorms were observed over portions of eastern
PA into NJ at 2215Z, located just east of a mid-level vorticity
max observed on water vapor imagery over southeastern PA. These
storms have had a history of generating ~1 inch of rain in 15
minutes and 2-3 inches in an hour across portions of the northern
Mid-Atlantic region this afternoon. MLCAPE has weakened over the
past 2 hours but remained at 250-750 J/kg over portions of the
lower Delaware and lower Hudson Valleys according to the 22Z SPC
mesoanalysis.

Using 850-300 mb mean layer winds as a proxy for steering flow,
wind speeds of <10 kt are likely to shift eastward as the
mid-level low centered over central PA tracks into southern NY
through 03Z. Southerly 850 mb winds of 10-15 kt in conjunction
with weak southerly steering flow will continue to support a
threat for training and slow moving thunderstorms for another few
hours. Rainfall rates of 0.5 to 1 inch in 15 minutes and peak
hourly totals over 2 inches will remain possible from portions of
eastern PA into NJ and southern NY through about 01Z. Beyond 01Z,
the loss of daytime heating and convective overturning should
reduce instability to insignificant levels, ending the flash flood
threat across the region. The HRRR has done a fairly good job with
the location and timing of this setup and has been consistent with
a significant reduction in the flash flood threat after 02Z.
Additional flash flooding through 01 or 02Z is expected to be
localized in coverage.

Otto

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

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

LAT...LON   42257467 42197397 41667363 41367364 41147378 
            40507421 40007480 39957566 40357607 40927594 
            41797526 

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