SPC AC 231244

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0744 AM CDT Fri Oct 23 2020

   Valid 231300Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER SOUTHEASTERN
   LOWER MICHIGAN...PARTS OF WESTERN/NORTHERN OHIO...AND NORTHEASTERN
   INDIANA...

   ...SUMMARY...
   Threat for a few severe storms capable of damaging wind, hail and
   perhaps a tornado or two will exist this afternoon into the early
   evening over a portion of the Great Lakes and Ohio Valley.

   ...Synopsis...
   In mid/upper levels, broadly cyclonic flow will persist across
   approximately the northern 1/2 of the CONUS, with two substantial/
   embedded shortwave troughs this period:
   1.  A perturbation now located over portions of SD and NE, forecast
   to deamplify somewhat as it ejects northeastward across WI toward
   eastern Upper/northern Lower MI through the daylight hours.  The
   trough should reach western QC by 12Z tomorrow.
   2.  A trough currently evident in moisture-channel imagery over
   northern BC and parts of the AK Panhandle, which will move
   southeastward down the length of BC and the Canadian Rockies to the
   interior Northwest near the international border.

   At the surface, 11Z analysis depicted a low near MBL with cold front
   southwestward near ORD, SGF, and MLC, to north-central TX, the
   Permian Basin, and south-central/central NM.  The low should move
   northeastward across Lake Huron, the neck of ON, and southwestern to
   northeastern QC through the period, ahead of the aforementioned
   eastern shortwave trough.  By 00Z, the trailing cold front should
   reach southern ON, western OH, western KY, northwestern MS, ad the
   TX coastal plain.  By 12Z the front should extend from northeastern
   NY or northern VT across southern PA, eastern TN, and AL, to the
   west-central Gulf. 

   ...Southeastern MI/northwestern OH to Ohio Valley...
   Scattered to numerous thunderstorms are forecast to form mainly
   along the cold front during midday to early afternoon from Lower MI
   at least as far southwestward as the Arklatex region, moving
   eastward over the adjoining Great Lakes and Ohio Valley States and
   Mid-South to lower Mississippi Valley in the afternoon.  While a
   strong gust cannot be ruled out south of the existing outlook areas,
   the most favorable parameter space for severe wind/hail and a threat
   of a tornado or two still appears to be near the northern part of
   this frontal passage, across southeastern MI, eastern IN and
   western/northern OH.  Only minor expansive adjustments were made
   accordingly to account for observational and model trends seen the
   past several hours, and no substantial alternations to the prior
   area nor reasoning are required at this stage. 

   By the time the front and its relative max in lift reach suitably
   destabilizing, minimally capped boundary-layer air in the 17-20Z
   time frame, diurnal heating and moist advection will contribute to
   around 1500-2000 J/kg prefrontal/preconvective MLCAPE from the
   Mid-South to Lower MI.  Deep-layer wind fields and shear generally
   will increase with northward extent over the cold front and
   adjoining warm sector.  Forecast soundings show effective-shear
   magnitudes ranging from only around 15-20 kt near MEM to around 30
   kt near IND and 40-45 kt near DTW.  SRH will be limited to some
   extent by a westerly surface wind component, but still may reach
   150-300 J/kg over the "slight risk" area in association with a 45-
   55-kt LLJ.  Rather messy, quasi-linear modes will predominate,
   though any sustained, discrete to semi-discrete convection in the
   same area may become supercellular, with concordant local
   enhancement of hail/tornado threat.  Convection should weaken this
   evening and with eastward extent as it encounters progressively
   less-favorable inflow-layer theta-e.

   ..Edwards/Mosier.. 10/23/2020

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