SPC AC 221250

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0750 AM CDT Sun Oct 22 2017

   Valid 221300Z - 231200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM
   SOUTHEASTERN LOUISIANA TO PARTS OF THE FLORIDA PANHANDLE...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   THE NORTHERN HIGH PLAINS...

   ...SUMMARY...
   A few severe storms will be possible this afternoon through tonight
   near the central Gulf Coast.

   ...Synopsis...
   The main upper-air feature for this forecast will be a
   full-latitude, synoptic-scale trough -- currently arching from
   southern MB across western MN, eastern KS, central OK, and southwest
   TX.  Approximately the northern half of this trough will break
   northeastward across the upper Great Lakes and ON through the
   period, with a cyclone forming on its northern extremity over
   northern Hudson Bay.  Meanwhile, the southern portion will shift
   eastward from the southern Plains across the lower/mid Mississippi
   Valley.  An embedded/500-mb low should develop this evening over the
   west-central Ozarks and move generally eastward near the MO/AR
   border toward the Mississippi River.  By the end of the period, the
   trailing shortwave trough should become slightly negatively tilted,
   extending south-southeastward from the low across MS to near BIX and
   over the adjoining northern Gulf.

   Northern-stream shortwave perturbations -- now evident in
   moisture-channel imagery near the coastal Pacific Northwest and
   coastal BC -- will cross the northern Rockies today.  These features
   phase into a strengthening, high-amplitude shortwave trough over MB,
   the Dakotas, and NE by 12Z, contributing to some acceleration of the
   southern trough/low near the end of the period and into day 2.

   At the surface, the 11Z analysis showed a cold front from western
   Upper MI across western IL, then southwestward in more diffuse form,
   temporarily hung up in one convectively related wake low over
   southwestern/south-central MO and another near DAL.  The front then
   became better-defined again southwestward across the southern Hill
   Country to near DRT.  A warm/marine frontal zone was evident from
   southeastern AR across southwestern and coastal MS,
   east-southeastward over shelf waters to near AAF, then in diffuse
   form over the north-central FL Peninsula.  The warm front should
   move slowly east-northeastward across the north-central Gulf coastal
   plain through the period.  By 00Z, the cold front should reach
   western portions of lower MI, IN, KY, and MS, southwestward over the
   northwestern Gulf.  By 12Z, the cold front should become more
   meridional from eastern lower MI through an area of low pressure
   over eastern portions of KY/TN, southward across the western FL
   Panhandle.

   ...Central Gulf Coast...
   An extensive area of scattered convection is ongoing across portions
   of southeastern LA and southern MS, over and southwest of the
   effective warm/marine front.  While deep shear is not strong over
   this region, expect backed flow and well-curved hodographs near the
   warm front, along with rich low-level moisture and related low LCL. 
   This will foster a conditional risk for a brief/small tornado or two
   and isolated damaging gusts from the most heavily water-loaded
   downdrafts, until the main/pre-cold-frontal convective band
   overtakes it from northwest-southeast late morning through early
   afternoon.

   Areas of diurnal destabilization are likely ahead of that convection
   today, augmented by low-level theta-e advection; however, the amount
   and pace of that destabilization is in question given the cloud
   cover associated with the ongoing convection.  Regardless, surface-
   based effective-inflow parcels should be available across most or
   all of the risk area as the marine air shifts slowly inland,
   displacing a recycled continental/polar air mass from the frontal
   passage a few days ago.  Meanwhile, with the approach of the basal
   trough aloft, overlying mid/upper winds should contribute to an
   increase in deep shear over the warm sector and the potential for
   sporadic organized convection, with damaging gusts the main concern;
   a tornado cannot be ruled out.

   Overnight, as the mid/upper trough and associated vorticity banner
   become more negatively tilted, the resulting adjustment of kinematic
   geometry will result in an eastward acceleration of the southern
   part of the corridor of most favorable lift near the front.  That,
   in turn, should reorient the main convective plume more north-south
   with some acceleration possible the last 3-6 hours of the period. 
   Spreading of large-scale ascent (from both DCVA and warm advection)
   over more of the warm sector also is possible, supporting
   development of surface-based or nearly surface-based convection over
   the Gulf, moving inland with some merger into the main line
   possible.  All of these factors support an eastward expansion of the
   existing outlook area at least to near the Apalachicola/
   Chattahoochee River system for tonight.  Some subset of the "MRGL"
   area may need an upgrade in succeeding outlooks, once uncertainties
   are better-resolved regarding the impact of the large area of
   ongoing convection on favorable destabilization to its east.

   ...Northern High Plains...
   As the northwestern shortwave trough consolidates/intensifies and
   approaches, both large-scale lift and deep-layer flow will intensify
   markedly.  Accordingly, high-wind warnings are in effect for much of
   this region.  Isolated thunderstorms may supplement the already
   strong-severe gradient flow at the surface late this afternoon,
   resulting in damaging/severe gusts in convective downdrafts. 
   Forecast soundings, modified for high temps in the lower 60s F,
   yield steep low/middle-level lapse rates and well-mixed boundary
   layers conducive to convective downward transfer of momentum through
   the short vertical distance from a field of 50-60-kt flow in the
   850-700-mb layer.  Essentially uninhibited MLCAPE of 100-200 J/kg is
   possible with low CAPE density, but also, buoyancy extending into
   the icing layer suitable for at least isolated lightning production.
    The threat should diminish markedly after dark with surface
   diabatic stabilization and resultant loss of both CAPE and DCAPE.

   ..Edwards/Dean.. 10/22/2017

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