SPC AC 171623

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1023 AM CST Fri Dec 17 2021

   Valid 171630Z - 181200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON AND EVENING ACROSS PARTS OF SOUTHEASTERN OKLAHOMA...

   ...SUMMARY...
   Generally isolated severe thunderstorms are possible beginning this
   afternoon across parts of the southern Great Plains.

   ...Synopsis...
   In the wake of the broad and deep, but now weakening, cyclone
   forecast to continue migrating across and northeast of northern
   Quebec, a trailing frontal zone is stalling or has stalled across
   the Mid Atlantic coast vicinity into the Ohio Valley and southern
   Great Plains.  This is generally on the northwestern periphery of
   prominent lower/mid tropospheric ridging centered to the east of the
   Bahamas, but relatively cold air in at least a shallow surface-based
   layer has advanced southward beneath the mid-level ridging, near the
   Red River vicinity of the southern Great Plains into areas near or
   just south of the Ozark Plateau.

   Southerly low-level moisture return above the shallow cold air has
   contributed to weak destabilization sufficient to support
   considerable convection and weak thunderstorm development across
   southeastern Oklahoma into the Ozark Plateau and Tennessee Valley. 
   This is likely being aided by forcing associated with weak
   perturbations within a belt of mid/upper westerlies emanating from
   the subtropical Pacific, and focused to the north of warmer and more
   strongly capping warm layers aloft, which will likely be maintained
   across much of the Southeast through this period.

   A digging short wave trough within the mid-latitude westerlies may
   increasingly come in phase with the subtropical stream later today
   through tonight.  The strongest impulse within consolidating
   troughing may still be west of Baja California Sur at 12Z Saturday,
   but it appears that it will be preceded by at least one weaker
   perturbation across parts of the northern Mexican Plateau and Rio
   Grande Valley (to the east of the Texas Big Bend) by late tonight.

   Meanwhile, consolidating mid-level troughing, within a separate
   branch of westerlies to the north, is forecast to turn east of the
   Rockies and progress toward the Upper Midwest today through tonight.
    This may contribute to mid-level height falls as far south as the
   Red River/Ark-La-Tex by 12Z Saturday, and support weak surface
   frontal wave development across the Ozark Plateau into lower Ohio
   Valley, while also being accompanied by a significant reinforcing
   cold intrusion into portions of the Ozark Plateau through central
   and southwestern Texas by the end of the period.

   ...Southern Great Plains into Ozark Plateau...
   Relatively steep mid-level lapse rates associated with capping
   elevated mixed-layer air in a plume across the lower Rio Grande
   Valley toward the Ark-La-Tex, above seasonably moist boundary-layer
   air, may allow for the development of moderately large CAPE
   (1000-2000 J/kg) with daytime heating.  However, through this
   evening, appreciable forcing for ascent, particularly in mid/upper
   levels, likely will remain focused to the cool side of the surface
   frontal zone.  Furthermore, in response to the northern branch
   mid-level troughing, moderately strong southerly low-level flow
   (including 30+ kt around 850 mb) is forecast to weaken across much
   of southern through northeastern Texas, while shifting northeast of
   the Ark-La-Tex through the lower Ohio Valley.

   It is possible that there may be sufficient erosion of the cool
   surface-based air along the low-level jet axis, across parts of
   southeastern Oklahoma (also to the north of the stronger mid-level
   capping), to support boundary-layer destabilization late this
   afternoon and evening.  If this occurs, it may coincide with
   modestly large and clockwise curved low-level hodographs (in
   addition to strong deeper layer shear) to support the development of
   isolated supercell structures posing a risk for severe weather.

   Isolated boundary-layer based, or near boundary-layer based storms,
   might also not be completely out of the question late tonight, just
   ahead of the southward surging cold front, perhaps aided by forcing
   associated with the mid/upper subtropical perturbation.  However,
   the bulk of the convective development through this period probably
   will remain confined to areas north of the surface front, or along
   the portion of the front once it begins to surge southward through
   the southern Great Plains, where severe weather potential will be
   limited as cold air tends to undercut it.

   ..Kerr/Wendt.. 12/17/2021

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