SPC AC 061251

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0751 AM CDT Mon Jul 06 2020

   Valid 061300Z - 071200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   NORTHERN HIGH PLAINS TO UPPER MIDWEST...

   ...SUMMARY...
   Severe thunderstorms are most probable over parts of the northern
   High Plains to Upper Midwest this afternoon and evening.

   ...Synopsis...
   A progressive mid/upper pattern is forecast across the northern
   CONUS, from the Pacific to New England.  The strongest feature will
   be a shortwave trough -- now indicated in moisture-channel imagery
   west of the Pacific Northwest and northern CA between 130W-133W. 
   This feature should move ashore around 00Z, reaching the
   northwestern Great Basin and interior Northwest by the end of the
   period.  In response, heights are expected to rise across parts of
   the northern High Plains.  

   Downstream, a convectively generated/enhanced perturbation over the
   Dakotas is expected to move east-northeastward across central/
   northern MN, reaching Lake Superior by 00Z.  A compact mid/upper
   cyclone -- initially located over NB with shortwave trough trailing
   across New England -- will progress southeastward over Atlantic
   waters through the period.

   The surface analysis at 11Z showed a weak low between MTP-ACK, which
   is forecast to move offshore today in step with the mid/upper
   perturbation.  A wave frontal zone trailed roughly westward across
   northern NJ and PA, which should reorient more toward northwest/
   southeast with time.  Farther west, an occluded/cold front was drawn
   from far northwestern ON across northwestern MN and north-central
   ND, becoming quasistationary to a weak low near BIL.  This boundary
   should be quasistationary through most of the day, with some
   southward sag possible on the mesoscale.  Meanwhile, low-level
   cyclogenesis is forecast across the northern Great Basin ahead of
   the Pacific shortwave, with the resulting low reaching southeastern
   ID by 12Z.  Warm frontogenesis should occur this evening and tonight
   from the low across northern WY and near the NE/SD border.

   ...Northern High Plains to upper Midwest...
   Four principal processes appear to drive the combined severe threat
   today and this evening, as isolated to scattered thunderstorms
   develop and move predominantly eastward to southeastward.  Some
   spatial overlap will exist between these seemingly disparate
   regimes, due to the potential for upscale growth and forward
   propagation from the genesis zones of regimes 2-4 downshear.

   1.  Scattered, perhaps numerous thunderstorms are possible in an 
   area of subtle but persistent, deep ascent preceding the shortwave
   trough and embedded MCV, from the upper Mississippi Valley region
   across northern WI and the U.P. of MI. Activity may be focused
   further by boundaries such as outflows, differential-heating zones
   and lake breezes, contributing to locally augmented convective vigor
   and severe potential.  Weak low/middle-level winds will restrict
   vertical shear, with the dominant mode expected to be multicellular,
   in a mix of discrete and clustered forms.  Rich low-level moisture
   and strong surface heating will support 1500-2500 J/kg peak MLCAPE,
   atop well-mixed subcloud layers.  

   2.  Afternoon and early evening thunderstorms may develop over
   portions of eastern SD and southwestern MN near and north of a
   composite outflow boundary analyzed from northwestern IA to east-
   central and south-central SD.  A pocket of relatively undisturbed,
   warm-sector, boundary-layer air, with surface dew points on the mid
   60s to low 70s F, will be available to convection forming along the
   boundary, as strong heating weakens inhibition.  Such rich moisture
   under steep midlevel lapse rates yields 2500-3000 J/kg MLCAPE
   (locally higher) in forecast soundings, though vertical shear will
   be modest.  Rapid convective formation/growth is possible in such an
   environment, and the potential exists for mesobeta-scale cold-pool
   growth and resultant forward propagation across portions of
   northwestern IA and/or southern MN. 

   3.  Development over the higher terrain of northern WY, southern MT
   and the Black Hills.  This is the most uncertain regime, given
   concerns over quality of airmass recovery behind yesterday's western
   SD convection and the lack of substantial upper support, with
   shortwave ridging and associated mid/upper height rises forecast
   over this area through most of the period.  Unconditional
   probabilities have been trimmed northward/lowered over parts of this
   region given these uncertainties, and in deference to more-recent
   progs accordingly showing rather meager coverage of any sustained/
   strong convection.  However, given the decidedly supercell-favorable
   CAPE/shear parameter space still apparent, and strong surface
   heating likely under the building heights -- any convection that can
   develop and mature downshear from the higher terrain may offer
   severe gusts, a tornado threat, and/or significant/damaging hail.

   4.  Thunderstorm development, with some potential for supercells and
   eventual/upscale cold-pool growth, appears increasingly probable
   over parts of southwestern ND/northwestern SD and perhaps adjoining
   eastern MT.  All severe hazards are possible with this activity. 
   The frontal zone and a weak surface trough should be the primary
   foci, with strong surface heating augmenting boundary lift to erode
   MLCINH.  A moist axis, characterized by 60s F surface dew points,
   will contribute to 1000-2000 J/kg over the area, with 35-45-kt
   effective-shear magnitudes and strong veering of winds with height. 

   ...Northeast...
   Widely scattered to scattered thunderstorms should form through the
   afternoon along and southwest of a weak frontal zone left in the
   wake of the departing low, across parts of NJ, southern NY and PA. 
   Other, isolated to widely scattered convection should develop in an
   increasingly unstable, favorably moist air mass with weak MLCINH
   across the remainder of the outlook area.  Isolated, marginally
   severe hail may occur, along with damaging gusts, with a few gusts
   near severe limits possible. 

   Height rises are expected across NY and New England today in the
   wake of the departing cyclone/trough, though the isallohypsic field
   will become more neutral with southward extent into VA.  As such,
   the main processes driving convective potential today will involve
   boundary-layer thermodynamics and mesoscale/boundary processes in
   low levels, including the frontal zone, sea-breeze boundaries, and
   preferential weakening of CINH through heating of higher terrain in
   western areas.  Weak low/middle-level winds will limit hodograph
   size and vertical shear.  However, well-mixed subcloud layers with
   steep low-level lapse rates will support maintenance of hail/gusts
   to the surface, and some mesobeta-scale clustering/cold-pool
   formation cannot be ruled out.  The threat should wane considerably
   this evening with diabatic surface cooling.

   ..Edwards/Broyles.. 07/06/2020

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