SPC AC 261636

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1136 AM CDT Tue Sep 26 2023

   Valid 261630Z - 271200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
   MIDWEST...NEW MEXICO/WEST TEXAS...AND NORTHERN FLORIDA...

   ...SUMMARY...
   Isolated severe thunderstorms are possible today across parts of the
   Midwest, across parts of New Mexico into west Texas, and possibly
   northern Florida.

   ...Midwest including Illinois/Indiana/Missouri...
   An upper low (-16C at 500mb) centered over eastern Iowa this morning
   will continue to drift east-southeastward toward and across northern
   Illinois through tonight. A semi-moist environment, with surface
   dewpoints in the upper 50s/low 60s F, will diurnally persist
   near/northeast of a weak surface low and occluded front/wind shift
   that will generally extend southeastward from the surface low. While
   deep-layer wind profiles will generally be weak (15-20 kt effective
   shear), ample CAPE with respect to low-topped storms will support
   the possibility of hail with the stronger pulse-type cells. Funnels
   appear quite plausible across areas including eastern Iowa, southern
   Wisconsin and northern/central Illinois, and sufficient low-level
   CAPE/ambient vorticity near the boundary could yield a brief tornado
   risk.

   Farther to the southwest, there is some potential that some strong
   to severe thunderstorms could develop near the front across
   central/eastern Missouri late this afternoon into tonight. MLCAPE
   could reach 1000-1500 J/kg MLCAPE near/south of the front, and wind
   profiles will be moderately strong, influenced by increasingly
   strong cyclonically curved westerlies in the base of the Upper
   Midwest shortwave trough. Thermodynamic/kinematic profiles could
   conditionally support some high-based supercells aside from
   multicellular clusters if/where storms develop. While near-frontal
   convergence will be modest, and the likelihood and extent of
   sustained deep convection may remain limited, there will be
   additional late night opportunities for convective development as
   warm advection/moisture transport increase, and some of these storms
   could also be strong. This region will be closely reevaluated in
   subsequent outlooks for a potential reintroduction of severe
   probabilities.

   ...New Mexico/West Texas...
   Within the weaker southern branch of westerlies, a weak shortwave
   trough is expected to move southeastward across the Four Corners
   vicinity through tonight. As it does, associated forcing for ascent
   should augment convective development, particularly storms
   initiating in response to heating across the mountains of
   southern/central New Mexico by early afternoon. It appears
   that moisture across this region will be sufficient to support
   initially sizable CAPE, beneath modest westerly to northwesterly
   flow, at least in upper levels. Stronger storms may become capable
   of producing marginally severe hail, then a few strong to severe
   gusts while propagating southeastward off the higher terrain into an
   unstable and more deeply mixed boundary-layer through early this
   evening.

   ...Florida Peninsula...
   Low severe probabilities have been introduced for the northern
   Florida Peninsula where storms are expected to increase/develop
   east-northeastward across the region, all while deep-layer shear
   modestly increases. High precipitable water content will support
   heavy precipitation loading and some potential for locally strong
   surface gusts.

   ...Coastal Oregon...
   Forcing for ascent within the exit region of a strong jet streak
   (including 70-90 kt around 500 mb) may contribute to an organized,
   vigorous pre-frontal convective band, associated with the surface
   cyclone approaching the Pacific Northwest. However, with the cyclone
   forecast to occlude prior to reaching the coast near/after 27/06Z,
   appreciable inland boundary-layer destabilization appears unlikely,
   and stable near-surface lapse rates may tend to mitigate the
   potential for downward mixing of severe surface gusts as the
   convective line progresses inland.

   ..Guyer/Squitieri.. 09/26/2023

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