SPC AC 081957

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0257 PM CDT Wed Jul 08 2020

   Valid 082000Z - 091200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS EVENING
   INTO THE OVERNIGHT HOURS ACROSS PARTS OF CENTRAL NEBRASKA INTO
   NORTHERN KANSAS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON INTO THIS EVENING ACROSS PARTS OF THE UPPER MIDWEST AND
   PARTS OF NEW ENGLAND...

   ...SUMMARY...
   Severe thunderstorms are likely late this afternoon into tonight
   across parts of the central Great Plains into the Upper Midwest,
   with other severe thunderstorms possible across parts of New England
   through early evening.

   ...20Z Outlook Update...
   Categorical and probabilistic lines have been adjusted in an attempt
   to better account for the progression of synoptic and sub-synoptic
   features, and latest trends concerning destabilization, which will
   impact convective potential through the remainder of the period.

   The most substantive convective potential still appears focused in
   association with strongest ongoing destabilization beneath a plume
   of warm and initially capping elevated mixed-layer air extending
   across the central high plains through the middle Missouri Valley
   and Upper Midwest.  This is on the southern periphery of broad
   mid-level troughing gradually shifting east-northeastward within the
   mid-latitude westerlies, near/north of the Montana/North Dakota
   international border area.

   Within surface troughing, just to the southeast of a weak cold
   front, the axis of strongest ongoing surface heating extends from
   roughly west/southwest of North Platte NE through areas southeast of
   Valentine NE, to a weak surface low migrating northeastward into
   west central Minnesota.  The remnants of an outflow boundary, 
   northeast of the low into northwest Wisconsin, appear to be shifting
   northeastward.  Stable air to the northeast of the outflow boundary
   will probably minimize severe potential, while storms initiate
   within the axis of stronger heating.

   Forcing for ascent associated with the primary mid-level short wave
   trough may remain displaced to the northwest of the frontal zone. 
   However, a more subtle impulse is currently progressing across
   southeastern Wyoming and probably will aid the initiation of
   scattered storms by 22-23Z.  By 02-03Z, nocturnal southerly
   low-level jet strengthening across the southern through central high
   plains is expected to contribute to an upscale growing,
   southeastward propagating cluster of storms across parts of central
   Nebraska into northern Kansas.

   ..Kerr.. 07/08/2020

   .PREV DISCUSSION... /ISSUED 1150 AM CDT Wed Jul 08 2020/

   ...Central Plains...
   Widely scattered to scattered thunderstorms are expected to develop
   this afternoon along and ahead of the cold front across northern
   Nebraska, and also within the low-level upslope-flow regime across
   the central High Plains including southeast Wyoming/Nebraska
   Panhandle. Large hail and damaging gusts will be possible. 

   A weakness in mid-level flow should lead to clustering of convection
   rather quickly, though a few initial are possible beneath initially
   strong anvil-level winds. With steep low/mid-level lapse rates, peak
   MLCAPE values of 2500-3500 J/kg are expected, decreasing gradually
   with southward extent and time, in proximity to stronger ridging and
   warmer temperatures aloft. With time, a pronounced/well-organized
   quasi-linear convective system (QLCS) is expected to aggregate and
   organize a cold pool, propelling it southeastward across Nebraska
   into Kansas, with potentially widespread severe wind and some hail
   likely continuing tonight. This complex of storms may remain well
   organized through much of the late-night hours as it spreads
   generally southeastward.

   ...Upper Midwest/Upper Great Lakes...
   The remnants of a long-lived MCS continue to slowly weaken across
   the western Lake Superior vicinity/northwest Wisconsin at late
   morning. The exact impacts and details of recovery in the wake of
   this MCS are still a bit uncertain, but outflow across southern
   Minnesota should gradually decay/shift northward later today, with
   strong destabilization plausible as far north as north-central
   Minnesota, aided by a diurnally strong low-level jet/warm advection
   preceding a slow-southeastward-moving cold front. 

   Initial thunderstorm development this afternoon will be most
   probable in vicinity of a weak surface wave/nearby warm front
   extending across north-central/northeast Minnesota into western
   Wisconsin. Pending the details of destabilization,
   severe-weather/supercell potential should be maximized within the
   corridor with deep-layer/low-level shear also expected to maximize
   in these areas. All severe facets may occur including severe hail,
   damaging winds, and a tornado risk.

   Other more isolated severe thunderstorms should develop near the
   cold front as extends southwestward across west-central/southwest
   Minnesota into southwest South Dakota within a very unstable/modest
   shear environment.

   ...New England/Northeast States...
   The region will be influenced by a low-amplitude mid-level shortwave
   trough/speed max. The strongest combination of deep-layer shear
   (30-40 kt) and moderate destabilization (possibly 1500-2000 J/kg
   MLCAPE) is expected across Vermont/New Hampshire and western Maine
   this afternoon, particularly in vicinity of a warm
   front/differential heating zone. As surface-based storms
   develop/intensify this afternoon, this corridor should yield
   sustained multicells and possibly some supercells, with damaging
   winds/severe hail potential. Other more episodic severe
   thunderstorms, with wind damage as the primary hazard, will be
   possible across additional portions of the Northeast/southern New
   England.

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