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   ACUS01 KWNS 182002   
   SWODY1   
   SPC AC 182000   
      
   Day 1 Convective Outlook   
   NWS Storm Prediction Center Norman OK   
   0200 PM CST Thu Dec 18 2025   
      
   Valid 182000Z - 191200Z   
      
   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF   
   THE MS/TN/OH VALLEYS INTO THE SOUTHEAST...AND ALSO ACROSS COASTAL   
   NC...   
      
   ...SUMMARY...   
   Isolated strong to severe thunderstorms capable of producing   
   occasional damaging winds and perhaps a tornado or two may occur   
   through this evening across parts of the lower/mid Mississippi   
   Valley into the Tennessee and Ohio Valleys and Southeast, and late   
   tonight across portions of coastal North Carolina.   
      
   ...MS/OH/TN Valleys into the Southeast...   
   The primary changes to the Marginal Risk have been to clear areas   
   behind the low-topped convective line and cold front, and to expand   
   northeastward somewhat based on where recent HRRR/RRFS guidance   
   maintains a low-topped but well-defined frontal band with potential   
   for localized strong/damaging gusts.   
      
   A line of low-topped convection has developed along a cold front and   
   is moving eastward across parts of IL and southeast MO this   
   afternoon. Despite very weak to negligible buoyancy, very strong   
   low-level flow/shear (with 50+ kt at 1 km AGL per regional VWPs) has   
   supported occasional gusts of 40-55 mph and isolated wind-damage   
   reports along this line. Forcing related to a strong mid/upper-level   
   shortwave trough moving quickly eastward across the region will help   
   to sustain a low-topped frontal band into this evening, which may be   
   capable of producing sporadic strong/damaging gusts into parts of   
   the Ohio Valley.   
      
   Recent HRRR/RRFS runs suggest that gusty winds associated with the   
   frontal band may spread across parts of the Allegheny Plateau   
   overnight, but with essentially nil buoyancy expected in this area,   
   confidence is currently too low to expand wind probabilities into   
   this area.   
      
   Farther south into parts of TN/MS/AL, somewhat greater (though still   
   weak) buoyancy will be in place later this afternoon into the early   
   evening, as 60s F dewpoints spread northward to near the TN/MS/AL   
   border region. A broken band of storms may develop in advance of the   
   front, with potential for at least marginal supercell structures and   
   an attendant threat of isolated damaging wind and possibly a   
   tornado. Any longer-lived cells may tend to merge into the   
   eastward-moving frontal band with time, with at least an isolated   
   severe threat spreading east across TN through the evening.   
      
   ...Coastal NC...   
   No major changes have been made to the Marginal Risk across coastal   
   NC. See the previous discussion below for more information.   
      
   ...FL Panhandle vicinity...   
   Cells with occasional rotation have been noted offshore of the FL   
   Panhandle this afternoon, though temperatures remain relatively cool   
   with widespread cloudiness inland. There is some potential for a   
   stronger cell or two to eventually approach the coast or develop   
   slightly inland, but lingering near-surface stability may tend to   
   limit the severe threat across this area.   
      
   ..Dean.. 12/18/2025   
      
   .PREV DISCUSSION... /ISSUED 1031 AM CST Thu Dec 18 2025/   
      
   ...Tennessee Valley/Lower Ohio Valley into the Southeast...   
   A pronounced mid/upper-level shortwave trough with attendant 70-90   
   kt mid-level jet will advance quickly eastward today across the   
   Upper Midwest into the MS/OH/TN Valleys. An associated deep surface   
   low over northern MN this morning is forecast to continue developing   
   northeastward into Ontario/Quebec through tonight, with a trailing   
   cold front forecast to sweep east-southeastward over much of the   
   mid/lower MS Valley into the OH/TN Valleys and Southeast. Low-level   
   moisture return ahead of the cold front remains quite limited,   
   except for parts of MS/AL where low to mid 60s surface dewpoints are   
   beginning to spread inland. Even so, wind fields remain quite   
   favorable for thunderstorm organization, with ample low-level and   
   deep-layer shear present. Current expectations are for a low-topped   
   line of convection to further develop along the cold front through   
   this afternoon/evening. Although instability is expected to remain   
   minimal with northward extent into the OH/TN Valleys, there is still   
   potential for strong to severe gusts that will be convectively   
   enhanced beyond the already strong background gradient wind field.   
   Some chance for a tornado may also exist across parts of MS/AL where   
   greater instability/low-level moisture will reside. Given latest   
   observational and guidance trends, a Marginal Risk has been included   
   where the best potential for occasional strong to damaging winds is   
   apparent.   
      
   ...Coastal North Carolina...   
   Isolated strong to locally severe convection may occur late tonight   
   through early Friday morning as an increasingly negative-tilt   
   mid-level trough and a related robust strengthening of the   
   deep-layer wind field approach eastern NC. The gradual modification   
   and erosion of continental/cold air is a key uncertainty due to   
   persistent surface high pressure along the East Coast into western   
   Atlantic. Some guidance such as the 12Z NAM may be a bit too   
   aggressive regarding late-night near-surface-based destabilization.   
   Still, the main area of concern is for the immediate NC coastal   
   areas/Outer Banks, where somewhat greater airmass   
   modification/low-level moistening should occur amid strong low-level   
   and deep-layer shear. If surface-based thunderstorms can form and   
   spread northeastward across this area, they could pose a threat for   
   isolated severe/damaging winds and perhaps a tornado. But, it is   
   also possible that convectively enhanced wind gusts materialize   
   inland (including Piedmont areas) in the predawn hours, although   
   confidence in this occurring remains low.   
      
   $$   
      
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   --- SBBSecho 3.28-Linux   
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