Mesoscale Precipitation Discussion 0659
NWS Weather Prediction Center College Park MD
1253 PM EDT Thu Jul 29 2021
Areas affected...Southern New England...Northeast
Concerning...Heavy rainfall...Flash flooding possible
Valid 291651Z - 292251Z
Summary...Thunderstorms expected to develop this afternoon could
pose a localized flash flood risk with some hourly totals between
1-2" at times.
Discussion...An approaching line of storms from western/central
Pennsylvania may begin to grow upscale and strengthen as it
approaches the urban corridor of southern NY, NJ, and eastern PA
where some breaks in cloud cover and daytime heating will
contribute to an axis of sufficient instability within an axis of
higher moisture transport. Current visible satellite shows some
breaks in cloud cover this hour while regional radar had a
weakened line of showers and thunderstorms over western/central
PA. That activity is struggling with lack of instability but to
the east, the area is becoming increasingly unstable. This could
allow for the line to strengthen with an increase in rain rates.
The 15Z HRRR shows potential for isolated 2-3" totals in and
around the NYC metro and this lines up with the 12Z HREF which
suggested potential for totals to exceed the 5 year ARI. While the
storms should be relatively progressive and will limit widespread
flash flood threat, some localized instances of flash flooding
will be possible, especially for the highly urbanized corridor.