Day 1 Severe Weather Outlook

Valid 231200Z – 241200Z


Thunderstorms with wind damage and isolated large hail will be
possible today across parts of Arizona. Storms with a few strong
wind gusts will be possible in parts of the Northeast.

…Desert Southwest…
An upper-level low will dig southward across the Desert Southwest
today. At the surface, a low will deepen across far southeastern
California as a moist airmass advects slowly northward across
southwestern Arizona. Surface dewpoints will reach the upper 60s and
lower 70s F across parts of southwest Arizona where moderate
instability should be in place by midday. Model forecasts suggest
scattered thunderstorms will develop across south-central Arizona
during the afternoon with convection spreading east-northeastward
into north-central Arizona by early evening. RAP forecast soundings
in the 21Z to 00Z timeframe across south-central Arizona show MLCAPE
values reaching the 1500 to 2500 J/kg range. Wind profiles show
substantial speed shear in the low to mid-levels with 850 to 500 mb
lapse rates around 7.0 c/km. This should support the development of
supercells with isolated large hail and wind damage. If cells can
congeal into a bowing line segment, then the more dominant threat
could transition toward wind damage by early evening.

An upper-level trough will move across the lower Great Lakes region
today as southwest mid-level flow remains over much of the
Northeast. At the surface, a low will develop across southern Quebec
as a corridor of maximized low-level moisture sets up ahead of a
cold front moving southeastward into the Northeast. Scattered
thunderstorms are forecast to develop this afternoon along and ahead
of the front across western Pennsylvania, New York and western New
England. Surface dewpoints in the mid 60s F will contribute to weak
destabilization ahead of the front. In addition, strong deep-layer
shear will be present making condition supportive of an isolated
severe threat. Short multicell line segments should be able to
produce a few marginally severe wind gusts this afternoon especially
as low-level lapse rates become maximized.

..Broyles/Elliott.. 09/23/2019


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