SPC AC 211252

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0752 AM CDT Tue Jul 21 2020

   Valid 211300Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE CENTRAL HIGH PLAINS...AND MIDDLE AND UPPER MISSISSIPPI VALLEY
   REGIONS...

   ...SUMMARY...
   Severe thunderstorms appear most probable today across parts of the
   central High Plains, and Middle and Upper Mississippi Valley
   regions.

   ...Synopsis...
   A rather chaotic mid/upper-level pattern is forecast across the
   CONUS through the period, dominated by a belt of westerlies across
   the northern tier of states that will contain numerous vorticity
   maxima and shortwaves of varying amplitude.  The area of ridging to
   its south -- extending from northern Baja across the southern Plains
   to the Carolinas and beyond -- is not particularly intense, and will
   be interspersed with several slow-moving vorticity lobes and MCVs as
   well. 

   The two most crucial upper-air features for this forecast are, and
   will remain, nearly in phase:
   1.  A northern-stream shortwave trough, now evident in moisture-
   channel imagery near the western border of MN.  A precursory MCV is
   apparent in radar animations over north-central MN, and should move
   eastward today ahead of the main shortwave trough, which will reach
   Lake Superior and eastern WI by the end of the period.
   2.  A trough from southeastern NE southwestward to east-central NM,
   with embedded MCVs evident over southeastern NE, northeastern KS,
   the eastern OK Panhandle, and between CVS-HOB in southeastern NM. 
   The northern part will split away and shift eastward today, reaching
   northeastern MO and northern IL by 00Z, then portions of OH and
   southern IN by 12Z.

   At the surface, 11Z analysis showed a low over MN between RWF-AXN,
   with cold front southwestward across southern SD to southeastern WY.
   A warm front was drawn from the low southeastward across
   southeastern MN to northern IL.  The low and fronts are forecast to
   move eastward across MN and portions of WI by 00Z, while the
   southern segment of the cold front stalls across northern/central
   NE.  An older, more diffuse frontal zone was analyzed across east-
   central NE and west-central KS, and should weaken through the day.

   ...Front/Laramie Ranges and adjoining High Plains...
   Widely scattered thunderstorms should develop this afternoon in
   areas of well-heated higher terrain, mainly following the Front and
   Laramie Ranges and nearby foothills.  Damaging wind and sporadic
   large hail are the main concerns.

   Strong directional shear will exist area-wide, and sufficient
   deep/speed shear will occur across the northern parts of the outlook
   area (mainly WY) for a supercell threat.  Steep low/middle-level
   lapse rates are forecast beneath 1000-2000 J/kg MLCAPE, with
   well-mixed subcloud layers supporting severe-gust potential. 
   Isolated significant/2+ inch hail is possible, especially in about
   the northern half of the outlook where supercell parameters appear
   more favorable.  However, concerns over supercell/threat coverage
   preclude an unconditional 15%/hatched significant-hail area for this
   cycle.  Some potential exists for any sustained supercell(s) to
   persist into the evening across the southern NE Panhandle/
   northeastern CO region, with upscale growth and limited cold-pool/
   forward-propagational processes possible.  If this occurs, a narrow
   corridor of greater severe-wind threat may develop before cooling/
   stable boundary-layer air contributes to a diminishing convective/
   severe potential overnight. 

   ...Upper Mississippi Valley...
   Isolated severe cannot be ruled out through midday with ongoing
   convection, ahead of the MCV.  This activity is occurring in a
   roughly southwest/northeast-aligned corridor of enhanced deep-layer
   ascent south through southeast of the midlevel vorticity max. 
   However, the main threat should occur with the next episode, which
   will have better access to favorably unstable, surface-based inflow
   air.

   Scattered thunderstorms should develop from midday through the
   afternoon across portions of eastern MN and western WI, atop a
   diabatically heating/destabilizing boundary layer behind and south
   of the earlier convection.  Damaging gusts, tornadoes and isolated
   large hail are possible.  Some of this convection may become
   supercellular while moving generally eastward across a narrow
   corridor of favorable buoyancy/shear parameter space.  Diurnal
   heating and moistening of the boundary layer will offset modest
   midlevel lapse rates enough to yield areas of 1000-1500 J/kg MLCAPE.
   Meanwhile, strong mid/upper flow and localized backing of surface
   winds will contribute to 35-45-kt effective-shear magnitudes.  Some
   enhancement to low-level SRH/vorticity is possible near the warm
   front and boundaries from morning convection.  Low-level convergence
   should be maximized near the surface low and adjoining cold-frontal
   segment.  This lift, with minimal MLCINH during the afternoon,
   should be sufficient to support storm development/organization,
   despite the weak nature of the surface low itself. 

   ...Mid Mississippi Valley and vicinity...
   Scattered thunderstorms in bands and clusters are expected to form
   episodically through this evening and move eastward to northeastward
   across this region.  Some of this activity should produce damaging
   to severe gusts, as well as isolated large hail.  A tornado cannot
   be ruled out, but this threat appears very conditional and isolated,
   dependent on mesobeta- to storm-scale factors yet to be resolved.

   The air mass across the region will become favorably unstable, with
   a deep troposphere, surface dew points commonly near 70 F and areas
   of sustained heating contributing to peak MLCAPE commonly in the
   2000-3000 J/kg range (locally higher).  Near-surface flow should
   remain weak, substantially limiting low-level shear and hodograph
   size.  However, the perturbation aloft will tighten midlevel height
   gradients enough to boost deep shear (effective-shear magnitudes of
   30-40 kt) over the destabilizing boundary layer, as well as deep
   lift in a regime of weak MLCINH.  This will support potentially
   well-organized multicell clusters.  Embedded, transient supercell
   structures are possible as well, though the likely dense and messy
   convective mode and lack of stronger low-level SRH should limit such
   storm-scale organization.  The threat should diminish with eastward
   extent and time this evening as nocturnal cooling gradually
   stabilizes the boundary layer, though strong/isolated severe storms
   may persist as far east as OH overnight.

   ..Edwards/Dean.. 07/21/2020

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