SPC AC 260506
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1206 AM CDT Tue Sep 26 2023
Valid 261200Z - 271200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
MIDWEST AND FROM CENTRAL NEW MEXICO TO SOUTHWEST TEXAS...
Thunderstorms capable of producing isolated hail and strong wind
gusts will be possible this afternoon and tonight across parts of
Illinois and Indiana, and also across parts of New Mexico into west
Latest water-vapor imagery depicts an amplified upper ridge shifting
east from the Intermountain West into the Plains over the past 12
hours. An occluded low over the upper MS River Valley is beginning
to shift to the southeast within the mean northwesterly flow regime,
and is expected to reside across central IL by peak heating.
Filtered heating of a moist airmass currently in place across the
Midwest, combined with weak ascent in the vicinity of the low, will
support scattered thunderstorm development this afternoon and
evening. To the south, a weak surface high over OK is fostering
modest moisture return flow into NM. Thunderstorm development is
expected by mid to late afternoon within the upslope flow regime,
and may feature a few strong/severe storms.
Thunderstorms will likely be ongoing across northern IL and adjacent
areas of IA and WI at the start of the period to the east of the
surface low within a weak warm advection regime. A second round of
thunderstorms is probable along the surface trough during the
afternoon hours across IL as ascent associated with the left-exit
region of a weak upper jet overspreads a weakly capped environment.
Cold temperatures aloft should support SBCAPE values on the order of
500-1000 J/kg, but values up to 1500 J/kg are possible depending on
the degree of daytime heating and residual cloud cover. Regardless,
deep-layer shear is forecast to be adequate (around 20-25 knots) to
support a few stronger cells capable of large hail and damaging
gusts. A brief/weak tornado is possible with initial cells along the
surface trough and in the vicinity of the surface low where forecast
soundings show some low-level veering and effective SRH values on
the order of 100-150 m2/s2.
...New Mexico and Southwest Texas...
04 UTC surface observations show a plume of 50-60 F dewpoints
advecting to the west/northwest into southeast NM. With no
appreciable strengthening of the surface high expected over the next
12 hours, further moistening into the 50s and low 60s across much of
southern NM appears probable by mid-afternoon with MLCAPE values
around 1500-2000 J/kg. Temperatures climbing into the 80s and low
90s should mix through the nocturnal inversion, allowing for
convective initiation within the upslope flow regime around peak
heating. Despite fairly weak northwesterly flow aloft, southeasterly
low-level winds will support straight hodographs with sufficient
effective bulk shear (around 30 knots) for organized cells with an
attendant large hail and severe wind risk.
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