SPC AC 051259

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0759 AM CDT Wed Sep 05 2018

   Valid 051300Z - 061200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   THE UPPER GREAT LAKES REGION AND THE CENTRAL GULF COAST TO NORTHERN
   MISSISSIPPI...

   ...SUMMARY...
   Damaging thunderstorm winds are possible over parts of the Upper
   Great Lakes region. The weakening remnants of Gordon may produce a
   tornado or two over the central Gulf Coast inland across
   Mississippi.

   ...Synopsis...
   In mid/upper levels, the CONUS part of the large-scale pattern will
   continue to be characterized by weak, broad troughing over the West,
   and an anticyclone slowly weakening and drifting eastward across and
   offshore from the Tidewater/Delmarva region.  The main belt of
   midlatitude westerlies will remain over or north of the Canadian
   border through the period.  That flow belt will be perturbed
   primarily by a positively tilted shortwave trough -- now evident in
   moisture-channel imagery from a cyclone over southwestern Hudson Bay
   across far northwestern ON, south-central MB, to near the SK/ND/MT
   border junction.  The southern lobe of this trough is forecast to
   pivot southeastward and eastward across the Lake Superior region
   this evening and overnight, reaching northern Lake Huron by 12Z. 

   At the surface, a cold front was analyzed from a low over eastern
   Upper MI southwestward over southwestern WI, southwestern IA,
   southwestern KS and north-central NM.  By the end of the period,
   this front should advance to northern ME, western NY, southern IN,
   and northwest TX.

   ...Upper Great Lakes...
   Scattered thunderstorms should develop primarily this afternoon near
   the cold front and move northeastward across the outlook area,
   offering the potential for isolated damaging to severe gusts.  

   In between the upper ridging and the northern-stream westerlies, a
   broad regime of confluent mid/upper-level flow will overlie a
   corridor of high-PW, low-level, prefrontal moisture transport from
   the central Plains northeastward across the outlook area.  Satellite
   imagery indicates the boundary layer will experience sustained
   insolation southeast of the front, combining with rich boundary-
   layer moisture to boost MLCAPE to around 1500 J/kg over Lower MI,
   and near 2000 J/kg over northeastern IL.  The area will reside on or
   just outside the southeast edge of appreciable midlevel winds
   associated with the shortwave trough, but beneath a departing,
   35-45-kt channel of flow near 850-mb that will boost low-level speed
   shear.  Messy modes and lack of more robust mid/upper winds (and
   therefore lack of deep shear) will limit the overall organization of
   severe potential.  The threat should diminish overnight as the
   boundary layer gradually stabilizes, concurrent with a reduction in
   low-level frontal convergence.

   ...Remnants of Gordon...
   A few small supercells and a brief tornado or two still are possible
   mainly over the eastern semicircle of the northwestward-translating
   circulation, where at least marginal juxtaposition of buoyancy and
   shear still is possible.  With time, the center of circulation will
   become more displaced from the area of most favorable instability,
   while the low-level flow and shear gradually diminish regionally. 
   However, MLCINH will be weak, with convergence and heating-related
   lift in the eastern and southeastern parts of the circulation
   remaining sufficient to support both bands and discrete cells.  

   Breaks and slots in the cloud cover may allow sufficient diurnal
   heating to offset poor low/middle-level lapse rates and boost MLCAPE
   to values ranging from less then 500 J/kg northeast of center to the
   1500-2000 J/kg range on the outer southeastern fringes of the
   vortex.  With the cyclone remaining small in size, the increasing
   buoyancy with eastward-southeastward distance will be offset
   substantially by weaker overall low-level flow in the outer
   envelope, both with distance from center and with time.

   ....Ohio Valley to northeastern international border...
   Isolated thunderstorms may develop across the area between the Ohio
   Valley, northern Appalachians and international border.  Other than
   perhaps a lake-breeze or differential-heating boundary, forcing for
   low-level ascent appears weak at best, and given the pattern,
   forecast soundings reasonably depict that low/middle-level flow will
   be modest and nearly unidirectional, with weak effective shear.  A
   damaging gust from a cell somewhere in this corridor cannot be ruled
   out.  However, given concerns about lack of greater potential
   convective coverage, duration and organization, the severe-wind
   threat now appears to be too isolated, disorganized and brief for an
   unconditional/categorical outlook.  The CAPE/shear parameter space
   may become favorable early this evening over parts of northern NY
   and northwestern New England, however, substantial convective
   forcing is expected to remain well north over QC.

   ..Edwards/Smith.. 09/05/2018

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