SPC AC 190544

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1244 AM CDT Sun Jul 19 2020

   Valid 191200Z - 201200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM INDIANA
   THROUGH WESTERN NEW YORK STATE AND ACROSS PORTIONS OF THE
   CENTRAL/NORTHERN HIGH PLAINS...

   ...SUMMARY...
   Isolated strong to severe storms may occur Sunday from portions of
   the Great Lakes/Ohio Valley as well as the central Plains. Damaging
   wind and large hail will be the main threats.

   ...Synopsis...
   Quasi-zonal flow will hold across the northern tier of the CONUS
   throughout the forecast period.  A shortwave trough initially
   centered over Ontario will shift eastward into Quebec during the day
   while providing a subtle influence for convective activity south of
   a front over the Great Lakes.  Mid-level flow drops off
   substantially around 40N and points south.

   At the surface, a weak front will shift southeastward across
   Michigan and northern Illinois during the day, and will probably be
   augmented by any ongoing convective complexes in the morning hours. 
   This front will extend westward through southern Nebraska and into
   eastern Wyoming.  South of the front, high boundary-layer moisture
   will contribute to abundant surface-based instability in areas
   undisturbed by convection, leading to areas of strong to severe
   thunderstorms.

   ...Illinois northeastward to New York State...
   A few CAMs indicate potential for ongoing convection in Michigan and
   northern Illinois, although any lingering storms may be weakening in
   response to the diurnal minimum in surface-based instability. 
   Throughout the day, surface heating amidst 70s F dewpoints will
   yield moderate to strong buoyancy (despite only 6-7 deg C/km
   mid-level lapse rates) and minimal inhibition, leading to
   redevelopment of convection along/ahead of any surface boundaries. 
   30-kt westerly flow aloft will be enough for loose organization -
   with a few forward-propagating linear segments expected especially
   in the Slight risk area.  Areas of wind damage and isolated hail are
   the most likely threats, with storms weakening after dark due to
   nocturnal boundary layer stabilization.

   ...High Plains eastward to Kansas/Missouri...
   Surface heating will foster gradual deepening of convection in the
   afternoon and evening within a couple of regimes - 1) near higher
   terrain in Colorado/New Mexico and 2) within an upslope flow regime
   in eastern Wyoming and western Nebraska.  As storms initiate, weak
   low-level flow across much of the Plains should result in
   organization into linear segments and forward-propagating clusters,
   - especially in western Nebraska and vicinity where low-level
   easterlies will veer and strengthen to westerly aloft.  These
   clusters of convection should grow upscale and maintain themselves
   as the move eastward across Kansas - partially due to a subtle
   increase in low-level flow in the evening and also due to moderate
   to strong instability within the pre-convective airmass.  Any threat
   of strong wind gusts and/or hail is expected to be maximized in the
   23-03Z timeframe across the Slight risk area, where storms are
   expected to be slightly more organized and upscale growth appears
   most likely.  A tornado cannot be ruled out completely in the
   afternoon/early evening given the vertically veering surface winds
   in western Nebraska and vicinity.

   ..Cook.. 07/19/2020

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