SPC AC 171300

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0800 AM CDT Fri Jul 17 2020

   Valid 171300Z - 181200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   EASTERN NORTH DAKOTA AND NORTHERN MINNESOTA...

   ...SUMMARY...
   Severe thunderstorms are expected across portions of the northern
   Plains into the Upper Midwest this afternoon into overnight, with
   gusts to hurricane force, large hail and a few tornadoes.

   ...Synopsis...
   In mid/upper levels, a belt of nearly zonal flow will prevail across
   the northern tier of the CONUS, from the Pacific Northwest across
   the Great Lakes and New England.  The main perturbation involved is
   a trough now evident in moisture-channel imagery near the coastal
   Northwest, and forecast to reach central MT by 12Z tomorrow.  Ahead
   of that, smaller-scale vorticity lobes and shortwaves are expected,
   including a subtle feature currently apparent from the western MT
   mountains southward to central/south-central ID.  The latter feature
   should reach the western border of the Dakotas around 00Z, and
   western MN 12 hours later.

   At the surface, 11Z analysis depicted a low over northeastern MT,
   with cold front southwestward across east-central MT to northwestern
   WY.  A warm front was drawn from the low east-southeastward across
   southwestern ND, northeastern SD, southwestern MN, and western/
   southern IA.  The cold front is forecast to move eastward to the
   central Dakotas by 00Z, then reach northwestern MN, eastern SD, and
   central WY by 12Z.  The warm front will move northward across much
   of ND, MN and WI through the period, possibly reaching western Lake
   Superior before the front at that longitude is overtaken by the
   convective complex discussed below.

   ...Northern Plains, Upper Midwest...
   Widely scattered thunderstorms should develop from mid to late
   afternoon along/ahead of the cold front, from the triple point near
   the international border southward across west-central or central
   ND.  Supercells with severe to very large hail, a few tornadoes and
   sporadic severe winds will be possible in the first few hours of the
   convective cycle across portions of ND.  Activity should evolve
   upscale with eastward extent into this evening, building a
   well-organized cold pool, with severe wind becoming the dominant
   hazard, a QLCS tornado threat on a localized/brief basis, and
   isolated severe hail.  The wind threat should sweep eastward across
   northern and perhaps central MN this evening and tonight,
   potentially reaching parts of Lake Superior, northern WI and western
   upper MI late tonight.

   The most important change for this outlook cycle was twofold:
   1.  A northward compression of the probabilities in deference to
   forecast forward-propagational MCS-motion vectors closer to the warm
   front, where both lift and storm-relative winds in low levels will
   be maximized as well.  
   2.  An eastward extension of probabilities for wind potential
   extending into the late-overnight time frame.  The warm front will
   be aligned west-northwest/east-southeast ahead of the MCS, but also
   should move northward across northern MN/WI and the western U.P.
   prior to its arrival in those areas, further supporting a more
   eastward probable track.

   Activity will be encouraged by combination of steep low/middle-level
   lapse rates related to the EML, and modest MLCINH on the EML's
   northern rim that can be overcome with frontal lift,  diurnal
   heating and very rich low-level moisture.  Forecast soundings
   reasonably indicate MLCAPE reaching 3500-4500 J/kg, amidst roughly
   westerly, 35-45-kt effective-shear vectors.  Hodographs should be
   relatively enlarged along and near the warm front as well, in
   support of supercell potential early and a tornado threat where the
   later MCS encounters the boundary. 

   Depending on mesobeta- to storm-scale processes like cold-pool
   width/depth and associated isallobaric/thermal perturbations,
   resultant leading-edge acceleration may thrust damaging-wind
   potential into western/central upper MI late in the period, before
   the forced-ascent arc outruns favorable buoyancy at or just above
   the surface.  The probabilities are tapered gently eastward due to
   uncertainties over MCS speed.  Extent of southward building of the
   MCS is still uncertain, but will be limited by the EML, related
   warmer temperatures above the boundary layer, and weaker
   convective-scale forcing.

   ...Central/southern MT...
   Widely scattered thunderstorms should form this afternoon over the
   higher terrain of southwestern MT, as diurnal heating of the
   elevated ground preferentially overcomes MLCINH, amidst marginal but
   sufficient low-level moisture.  Activity will move northeastward in
   strengthening mid/upper-level flow, across a deep, increasingly
   well- mixed boundary layer supporting strong/locally severe gusts
   from the most intense cells.  Large-scale ascent will strengthen
   gradually ahead of the progressive Pacific Northwest trough, helping
   to steepen midlevel lapse rates while low-level instability
   strengthens diabatically.  Lack of more-robust moisture, with
   surface dew points mostly in the 40s F in lower elevations, should
   keep peak/preconvective MLCAPE below 500 J/kg in the area, except on
   a brief/spotty basis.  The threat should end this evening as
   low-level lapse rates stabilize greatly.

   ..Edwards/Thompson.. 07/17/2020

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