SPC AC 261230

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0730 AM CDT Tue Sep 26 2023

   Valid 261300Z - 271200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON AND EVENING ACROSS PARTS OF NORTHERN AND EASTERN ILLINOIS
   THROUGH WESTERN AND SOUTHERN INDIANA AND ACROSS SOUTHERN NEW MEXICO
   INTO WEST TEXAS...

   ...SUMMARY...
   Widely scattered strong thunderstorms are possible today across
   parts of the Midwest into lower Ohio Valley vicinity, and across
   parts of southern New Mexico into west Texas.  Some of these may be
   accompanied by hail and gusty winds.

   ...Synopsis...
   As an initially broad and deep cyclone undergoes considerable
   further weakening near the northern British Columbia coast,
   associated large-scale mid-level troughing may gradually lose some
   amplitude while continuing to progress inland across the Pacific
   Coast and Canadian/northern U.S. Rockies today through tonight.  It
   does appear that one vigorous embedded short wave impulse will
   accelerate toward the Pacific Northwest coast, accompanied by a more
   modest and compact surface cyclone late this afternoon into early
   Wednesday.  This short wave has been preceded inland by at least a
   couple of other significant perturbations.  One of these has already
   slowly begun to suppress a prominent downstream blocking high,
   initially centered near southern Hudson Bay, though the elongating
   center of higher heights may maintain considerable strength across
   southern Hudson/James Bays into the St. Lawrence Valley through this
   period.

   Beneath the mid-level high, a sprawling surface ridge (centered over
   Quebec) will maintain a considerable influence across much of the
   nation to the east of the Rockies.  Seasonably high moisture content
   will generally remain confined to areas along and south of a weak
   surface front across southern Atlantic and eastern Gulf coastal
   areas, downstream of an elongating cyclonic perturbation migrating
   north-northeast of the Yucatan Peninsula, and merging into weak
   larger-scale troughing over the Gulf of Mexico.

   ...Southern Rockies into Texas Big Bend...
   Within the weaker southern branch of westerlies, and in advance of
   the significant inland migrating northern branch troughing, a weak
   short wave trough is forecast to slowly turn across and southeast of
   the Four Corners vicinity during this period.  As it does,
   associated forcing for ascent may augment convective development,
   particularly storms initiating in response to heating across the
   mountains of southern 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 evening.

   ...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.

   ...Florida Peninsula...
   Inland destabilization may support fairly widely thunderstorm
   development this afternoon and evening, with high precipitable water
   content supporting heavy precipitation loading and some potential
   for locally strong surface gusts.  However, given the initial large
   degree of saturation in the thermodynamic profiles and the weakness
   of the ambient mean flow, peak gusts seem likely to remain mostly
   below severe limits.

   ...Indiana/Illinois vicinity...
   To the southwest of the lingering blocking mid-level ridging, a weak
   mid-level low is forecast to continue slowly digging from parts of
   the Upper Midwest toward the lower Ohio Valley.  It appears that
   this will maintain at least broad, weak inverted surface troughing
   on the southwestern periphery of the Quebec surface high, which is
   forecast to become a focus for scattered thunderstorm development
   today beneath modestly cool mid-level temperatures (around -14 C
   near 500 mb).  Beneath an evolving mid-level dry slot, mixed-layer
   CAPE may exceed 1000 J/kg by peak afternoon heating, which may be
   sufficient to support small to, perhaps, marginally severe hail in
   stronger cells.  Due to the generally modest to weak nature of the
   deep-layer mean flow and shear, the potential for severe wind gusts
   appears low.  However, an isolated, brief tornado might not be
   entirely out of the question, where surface convergence can become
   locally enhanced in close proximity/beneath the mid-level low.

   ..Kerr/Squitieri.. 09/26/2023

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z