SPC AC 191249

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0749 AM CDT Sun Jul 19 2020

   Valid 191300Z - 201200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   THE GREAT LAKES/OHIO VALLEY REGIONS AND CENTRAL GREAT PLAINS...

   ...SUMMARY...
   Thunderstorms with severe gusts and hail are possible today over
   portions of the Great Lakes/Ohio Valley regions and central Great
   Plains.

   ...Synopsis...
   In mid-upper levels, the main belt of prevailing westerlies will
   remain across the northern CONUS and southern Canada, perturbed
   primarily by a cyclone now over northwestern ON and its shortwave
   trough, initially extending southward over the upper Mississippi
   Valley region.  The cyclone will open up, and the trough as a whole
   will move eastward across much of northern ON, Lake Superior, Upper
   MI, and WI by 00Z.  Thereafter, the trough should deamplify and
   eject northeastward over southwestern/western QC.

   On the southern periphery of the westerlies, a shortwave trough was
   evident in moisture-channel imagery from southeastern NE across
   north-central KS to southeastern CO, associated with an MCV apparent
   in radar composites near CNK.  The northern part of this
   perturbation, including the MCV, is progged to break away in
   piecemeal fashion and move eastward across the lower Missouri and
   Ohio Valleys through the period, while a vorticity lobe is left
   behind across the south-central High Plains, potentially to be
   reinforced by more convection this afternoon and tonight.  A poorly
   defined corridor of relatively large heights interspersed with weak,
   slow-moving vorticity maxima, or a "dirty ridge" in synopticians'
   maproom parlance, will persist across the southern tier of states
   from SC across the southern Plains to the Desert Southwest.

   At the surface, 11Z analysis showed a low near the Superior Lakehead
   region of northwestern ON, with cold front southward across central
   Upper MI, southern WI, southeastern IA, and extreme northwestern MO,
   becoming nearly stationary close to the KS/NE border and over
   northeastern CO.  By 00Z, the cold front should extend from
   southeastern ON across northwestern OH and central portions of IL/
   IN, becoming quasistationary westward across central MO, and a
   slow-moving warm front over south-central to northwestern NE and
   perhaps southwestern SD.  A area of low pressure, related both to
   lee troughing and diurnal surface heating, will cover parts of
   eastern CO, southeastern WY and the NE Panhandle today into this
   evening, along and south of the warm front.

   ...Great Lakes to Ohio Valley and lower Missouri Valley...
   A complex of thunderstorms is moving east-southeastward across parts
   of Lower MI, initially positioned from Saginaw Bay to the
   southeastern Lake Michigan shoreline.  Isolated severe gusts may
   occur as parts of this complex encounter a diabatically
   destabilizing air mass through midday across the remainder of Lower
   MI.  Refer to SPC mesoscale discussion 1273 for near-term details.

   Another episode of thunderstorms is forecast along and ahead of the
   surface cold front, moving eastward to southeastward across the
   remainder of the outlook area.  Damaging to severe gusts will be the
   main concern, and isolated large hail also is possible.  Some of
   this activity may evolve as an outgrowth of initially subsevere
   convection located across portions of northern IL into southwestern
   Lower MI, shifted eastward and impinging upon a diurnally
   destabilizing boundary layer.  Although mid/upper-level large-scale
   support associated with the shortwave trough will pass mostly or
   entirely north of the area, frontal lift and buoyancy should remain
   strong enough to support a convective corridor through the day and
   perhaps into the evening.

   Little directional shear is expected, with the low/middle-level
   winds oriented only slightly rightward of the frontal orientation, 
   and mid/upper winds weakening with southwestward extent into larger
   buoyancy.  This will support a dominant quasi-linear storm mode.
   With areas of strong heating and rich moisture offsetting modest
   midlevel lapse rates, peak preconvective MLCAPE should range from
   1500-3500 J/kg, with the corridor of greatest instability generally
   weakening/narrowing northeastward from IL across OH/PA/western NY.

   ...Central Great Plains...
   Widely scattered to scattered thunderstorms are expected to form by
   mid/late afternoon, in a corridor close to the lee trough/low-
   pressure area and south of the warm front, from northeastern WY
   across eastern CO.  Strong surface heating of relatively high
   terrain will weaken MLCINH preferentially and foster this convection
   initiation.  The activity is expected to move eastward across the
   adjoining High Plains, into a well-mixed boundary layer supporting
   strong-severe gusts and maintenance of hail to the surface.

   Forecast soundings suggest 500-1200 MLCAPE in an narrow corridor
   across northeastern WY, southwestern SD and the northern NE
   Panhandle, increasing to 2000-3000 J/kg around the northeastern and
   eastern edge of CO into southwestern NE and western KS.  Multicells,
   with some clustering and forward-propagational aggregation possible
   into tonight, should prevail roughly near and south of I-80.  A few
   of the thunderstorms in the northern parts of the outlook area,
   across eastern WY and adjoining parts of southwestern SD and the NE
   Panhandle, may become high-based supercells, in an environment of
   favorable deep shear.  Low-level winds will be backed near the warm
   front, extending the deep-shear vectors somewhat, and veering of
   flow with height should be strong area-wide.  Effective-shear
   magnitudes are progged to range from around 50 kt along the front to
   25-30 kt over northwestern KS, generally weakening with southward
   extent as moisture and CAPE increase.  However, weakness of
   boundary-layer wind speeds overall should limit 0-1-km SRH/shear
   over most of the region.

   ..Edwards/Dean.. 07/19/2020

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