SPC AC 181252

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0752 AM CDT Sat Jul 18 2020

   Valid 181300Z - 191200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
   THE UPPER MIDWEST AND UPPER GREAT LAKES STATES...

   ...SUMMARY...
   Damaging thunderstorm gusts, large hail and a few tornadoes are
   possible this afternoon and tonight, across portions of the Upper
   Midwest and Upper Great Lakes States.

   ...Synopsis...
   In mid/upper levels, a belt of mostly zonal northern-stream flow
   will prevail over the northern tier of states, predominantly
   perturbed by a shortwave trough now extending from northeastern to
   south-central MT.  This feature will move eastward to southeastern
   MB, eastern ND and north-central to southwestern SD by 00Z.  By 12Z,
   the trough should extend from the MN Arrowhead northward to
   adjoining parts of ON, and southwestward over eastern/southern MN.
   To the south, a messy ridge with multiple/subtle 500-mb highs will
   persist from SC/GA across the southern Plains to NM/AZ.  A weak
   perturbation -- evident in moisture-channel imagery over south-
   central CO and northwestern NM -- should move slowly eastward
   through the period, reaching western KS by 12Z tomorrow.

   At the surface, 11Z analysis showed a low over central SD near PIR,
   with cold front west-southwestward to east-central WY, and trough
   south-southwestward across southwestern NE and southeastern CO.  A
   well-defined outflow boundary -- produced by an ongoing/residual MCS
   over the U.P., northern Lake Michigan and northeastern WI -- was
   drawn from that convection across southwestern WI and northern IA. 
   Additional thunderstorms over northeastern IA were reinforcing the
   boundary and continuing its southward motion over northern IA.  The
   low should shift eastward along the outflow/cold-frontal triple
   point through the period, with the front reaching northwestern IA,
   eastern/southern NE and northeastern CO by 00Z.  The surface trough
   and convergence axis should intersect the front over south-central/
   southwestern NE by that time, extending southwestward across
   northwestern KS to east-central/southeastern CO.  By 12Z, the cold
   front should reach central upper MI, eastern WI, southeastern IA,
   and northeastern through west-central KS, overtaking the trough from
   north to south across KS.

   ...Upper Midwest/Great Lakes...
   Scattered thunderstorms are expected to develop near the cold-front/
   outflow-boundary intersection by late afternoon, with additional
   development possible equatorward along the cold front. In the first
   few hours of the convective cycle, supercells with all severe
   hazards are possible, including the potential for significant/
   damaging hail and a few tornadoes -- especially near the residual
   outflow boundary, where low-level shear/vorticity/SRH will be
   relatively maximized.  Upscale growth and formation of another
   largely eastward-propagating MCS is expected this evening and
   tonight, mainly along and north of the old outflow boundary, with a
   corridor of severe-wind potential centered close to or just north of
   the boundary.

   The warm sector will become very unstable this afternoon, as strong
   diabatic heating occurs in a field of rich, evapotranspiratively
   aided, low-level moisture.  Mid/upper 70s F surface dew points are
   possible along and south of the boundary, yielding MLCAPE in the
   3500-4500 J/kg range.  Favorable vertical shear will develop near
   the boundary as winds aloft strengthen ahead of the trough, with
   forecast soundings depicting 35-45-kt effective-shear magnitudes.

   Major concerns linger on location of the greatest severe potential
   this evening/tonight.  The outflow boundary remains progressive at
   this hour along much of its length, and has surged south of the
   positions progged by all of last night's operationally used 00Z and
   06Z model runs -- both synoptic and convection-allowing -- including
   HREF members.  The closest of all those progs has been the 3-km NAM,
   which itself appears somewhat too poleward.  Yet the boundary's
   convective-reinforcing processes are weakening from west-east from
   SD, and northward retreat amidst strong diurnal heating will occur
   to some extent today.  The main uncertainties involve:
   1.  How far north the boundary and associated undisturbed warm
   sector will spread with optimal theta-e, and
   2.  The extent to which favorable surface-based destabilization will
   occur to its north.

   While a corridor of relatively maximized/30%+ wind potential appears
   probable near the residual boundary, and within the broader tier of
   15% probabilities, too many mesoscale uncertainties remain to assign
   a well-focused, unconditional, 30%/enhanced area at this time. 
   Mesoscale observational trends and 12Z-onward guidance may inform an
   upgraded subset of the current "slight" area in a succeeding outlook
   cycle.

   ...ND...northeastern MT...
   Widely scattered thunderstorms are possible this afternoon,
   initially across portions of northeastern MT into northwestern ND,
   spreading/developing eastward across the remainder of the outlook by
   early evening.  The main concerns will be isolated severe gusts and
   hail.  The threat will be tied strongly to diurnal processes and
   should diminish markedly after sunset.  Cooling aloft accompanying
   the shortwave trough will overlie adequate/residual boundary-layer
   moisture with surface dew points 50s to low 60s F.  A field of
   mid/late-afternoon MLCAPE in the 500-1200 J/kg range should develop
   across the outlook area, with minimal MLCINH.  Favorable mid/upper
   flow will contribute to 35-40 kt effective-shear magnitudes, despite
   the westerly surface-wind components.  This will aid in storm
   organization enough to maintain concern for isolated severe
   potential.

   ...Central Plains...
   Isolated to scattered thunderstorms are forecast to develop this
   afternoon near the front and trough, and for the central High Plains
   portion, ahead of the CO perturbation aloft.  Intense surface
   heating and boundary-layer mixing will remove CINH in concert with
   lift near those boundaries.  For a few hours, the most vigorous
   cells may produce damaging to severe gusts, with encouragement from
   strong DCAPE related to the deep/well-mixed subcloud layers. 
   Deep-layer shear is expected to be less than farther north under the
   belt of stronger westerlies, and generally will weaken with
   southward extent.

   ..Edwards/Thompson.. 07/18/2020

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