Day 1 Severe Weather Outlook

Valid 231300Z – 241200Z


Thunderstorms with very large hail and damaging winds will be
possible today across parts of southwest and central Arizona. Storms
with a few strong wind gusts will be possible in parts of the

…AZ through tonight…
A transition season severe weather episode is expected today into
tonight across AZ. A midlevel shortwave trough over northwest NV
this morning will dig south-southeastward toward the lower CO River
valley and evolve into a closed low by Tuesday morning. Low-level
mass response to the approaching midlevel trough will result in
northward transport of moisture from southwest/south central AZ to
the rim through the day. Some slightly elevated convection is
ongoing along the north edge of the returning moisture, and this
convection may persist through the morning and pose a marginal hail
threat. The background environment will become favorable for
additional thunderstorms through the day as ascent and midlevel
cooling develop southeastward into AZ. Daytime heating across
southwest AZ will support surface-based thunderstorm development by
late morning (17-18z) along a pseudo-dryline near the CO River, and
storms will spread eastward near the rim through the afternoon and
into this evening.

Surface dewpoints have increased to 70-76 F across southwest AZ with
a moisture surge from the Gulf of CA. After accounting for some
daytime heating/mixing, boundary-layer dewpoints of 65-70 F beneath
midlevel lapse rates near 7 C/km will result in MLCAPE of 1500-2500
J/kg and minimal convective inhibition. Effective bulk shear of
40-50 kt and relatively long/straight hodographs will favor
splitting supercells capable of producing isolated very large hail
near 2 inches in diameter, and there is some potential for
clustering of hail events west of Phoenix this afternoon. The
stronger supercells, as well as eventual upscale growth into small
clusters, will pose a threat for damaging winds. Otherwise, an
isolated tornado cannot be ruled out, but this threat will be
secondary given relatively weak low-level flow/shear. Convection
should persist into the overnight hours, with at least a low-end
threat for hail/wind continuing.

…NY and vicinity this afternoon/evening…
A midlevel shortwave trough and associated surface cold front will
move eastward across NY by late afternoon into early tonight.
Clouds and poor lapse rates in the pre-frontal warm sector will
limit buoyancy, with MLCAPE likely to remain near 500 J/kg. Still,
an increase in midlevel flow with the approach of the shortwave
trough could support isolated strong gusts with downward momentum
transfer in convection near the front this afternoon/evening.

..Thompson/Leitman.. 09/23/2019


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