SPC AC 171243

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0643 AM CST Wed Nov 17 2021

   Valid 171300Z - 181200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   Severe thunderstorms are unlikely across the contiguous United
   States today.

   ...Synopsis...
   A progressive, fairly high-amplitude upper-air pattern will feature
   a synoptic-scale trough trailing southward from a mid/upper cyclone
   now located over southern MB.  The cyclone should pivot eastward to
   northeastward across northwestern and far northern ON through the
   period.  Embedded shortwaves will reinforce the synoptic trough
   which should extend from northern MN to eastern CO by 00Z.  By 12Z,
   the trough should extend from the cyclone across Lake Superior, WI,
   IA, MO, and eastern OK.

   At the surface, 11Z analysis showed a related cold front from an
   occlusion triple point near GRB southwestward near MSN, IRK, and CNU
   to a weak frontal-wave low near END, then to near LBB and ROW.  By
   00Z the front should extend from southeastern MI to western IN,
   southern IL, southeastern MO, northern AR, extreme southeastern OK,
   central and southwest TX, and northwestward over southeastern to
   north-central NM.  By 12Z the front should reach western portions of
   NY/PA, eastern KY, middle TN, northern parts of MS/LA, the middle TX
   Coast, and deep south TX, while shifting southwestward erratically
   through the rugged terrain of Chihuahua, southwestern NM and eastern
   AZ. 

   ...Mid/lower Mississippi Valley, western Gulf Coast...
   Low-level return flow ahead of the cold front will be characterized
   by a northward-narrowing sector of partially modified Gulf-
   transiting trajectories and surface dew points ranging from the 60s
   south to the 50s north. Modest modest midlevel lapse rates atop
   marginal theta-e will yield weak CAPE with small CAPE density over
   the general-thunderstorm area.  Despite favorable low-level shear
   over parts of the warm sector in the mid/lower Mississippi Valley,
   the anafrontal nature of the convective-scale forcing and weak
   buoyancy (MLCAPE and MUCAPE generally less than 700 J/kg) should
   preclude an organized severe threat. 

   ...South FL/Keys...
   An old frontal zone now drawn across the central/southern Bahamas
   and Cuba should become more diffuse and move slowly northward
   through the period, allowing a gradual increase in low-level
   moisture/theta-e across south FL and the Keys.  As this occurs,
   isolated thunderstorms may become possible especially overnight
   across the Straits, some of which may move inland and affect parts
   of south FL.

   ..Edwards.. 11/17/2021

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