SPC AC 231254

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0754 AM CDT Sat May 23 2020

   Valid 231300Z - 241200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   NORTHERN ILLINOIS...EASTERN IOWA AND EXTREME SOUTHERN WISCONISN...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   THE SOUTHERN AND NORTHERN HIGH PLAINS...

   ...SUMMARY...
   Scattered severe thunderstorms, some with tornadoes, will cross
   parts of eastern Iowa, northern Illinois and southern Wisconsin from
   midday through this afternoon.  Thunderstorms with very large hail
   and severe wind will affect parts of the northern and southern Great
   Plains.

   ...Synopsis...
   In mid/upper levels, a longwave trough will persist over the western
   CONUS, with a series of shortwaves (and two subsynoptic-scale
   cyclones) penetrating the mean ridging to the east.  The first of
   those cyclones -- now apparent in moisture-channel imagery over PA
   and parts of adjoining states -- will move eastward off the Mid-
   Atlantic Coast over the next 12 hours.  The other -- currently
   centered over IA -- has been reinforced by convectively generated
   vorticity.  This feature is forecast to move east-northeastward
   across the Mississippi River today while slowly weakening, then as
   an open shortwave trough across WI and Lake Michigan to northern
   Lower MI by 12Z tomorrow.  To its south, a smaller shortwave trough
   -- initially over eastern OK and north-central/northeast TX --
   should move slowly eastward to eastward to east-northeastward across
   the Mid-South region.  This includes an embedded MCV evident in
   radar reflectivity composites over southeastern OK.

   In the West, the main cyclone/vorticity max of the broader trough
   was apparent over the ID/UT border, and should move slowly
   northeastward to eastern MT through the period.  Several low-
   amplitude shortwaves -- some convectively induced/enhanced, will
   eject from the mean trough position, in a broad fetch of
   southwesterly flow covering most of the High Plains and central/
   southern Rockies. 

   At the surface, a low was evident over southwestern IA, stacked
   under the midlevel cyclone center, and should move northeastward
   across the southeast corner of MN and into adjoining WI through
   early evening.  A warm front will become better defined from the low
   east-southeastward across eastern IA and northern parts of IL/IN,
   moving across extreme southern WI by 00Z.  A weak cold front,
   arching southward/southwestward from the low, will stall then move
   northward as a warm front today over central/northern KS and
   western/central MO.  An extensive composite outflow boundary should
   stall and weaken across parts of western TN to northern LA and the
   Arlkatex region, into central/northwest TX.  An area of low pressure
   between CYS-LHX should consolidate today and move to east-central/
   northeastern CO by 00Z.  A dryline will mix eastward over the High
   Plains again today, reaching from its intersection with the front,
   just east of the low, across west-central KS, the eastern TX/OK
   Panhandles, and the Permian Basin by late afternoon before being
   modulated by outflow from nearby convection.  Another weak cold
   front and surface trough will move slowly eastward across eastern WY
   and the western Dakotas, reaching north-central ND, the Black Hills,
   and near the WY/NE line by 00Z. 

   ...Eastern IA to southern Lake Michigan vicinity...
   Scattered thunderstorms should develop across portions of eastern IA
   and northwestern IL from mid/late morning through early/mid
   afternoon.  Amidst weak MLCINH, activity should form in episodic,
   broken arcs, then move northward to northeastward across the
   remainder of the IA/IL/WI part of the outlook through the afternoon.
   Additional convection, more widely scattered to isolated in nature,
   should form farther south into central IL and move eastward to
   northeastward.  Several of these thunderstorms could become
   supercells, capable of all severe hazards, including tornadoes.  The
   potential for fairly dense coverage of such convection drives the
   increase in tornado probabilities for this outlook. 

   As the compact cyclone approaches, a channel of difluent and
   increasing mid/upper-level flow, and related strengthening deep
   shear, will spread across the outlook area.  This will be
   conterminous with increasing large-scale DCVA/ascent and diurnal
   heating/destabilization of the favorably moist boundary layer. 
   Meanwhile, the mass response to the perturbation aloft will
   strengthen both shear and convergence in the low levels, with
   favorable hodographs in the eastern sector of the surface cyclone,
   especially near the warm front.  Warm advection and diurnal heating
   within a favorably moist boundary layer will boost MLCAPE to the
   1500-2000 J/kg range over parts of northern IL, and 500-1500 J/kg
   bending back into a narrowing corridor closer to the deep-layer low.
   Although surface winds are not forecast to be strong (generally
   10-15 kt), and the hodographs are not progged by most models to be
   extremely large, they should be well-curved with sufficient SRH
   (effective SRH of 150-250 J/kg) to support supercells and tornado
   potential.  Threats for severe hail and wind will extend farther to
   the north, east and especially south, with deep shear diminishing
   markedly toward the Ohio Valley and storm modes trending
   multicellular there. 

   ...Northern Plains...
   Scattered thunderstorms are expected to develop this afternoon near
   the front, where deep-layer lift will be relatively maximized, and
   over the Black Hills, amidst preferential removal of MLCINH by
   heating of higher terrain.  Relatively early in the convective cycle
   (first 2-3 hours), the hail potential will be maximized with
   discrete to semi-discrete storm modes, with a mix of supercells and
   multicells possible.  Additional hail threat may develop from
   nocturnal convection farther east across parts of northern NE or SD
   amidst low-level war/moist advection.

   Cold-pool aggregation is possible from one or more clusters of
   thunderstorms into the evening -- especially across central/southern
   parts of the outlook area in portions of SD and NE where low-level
   moisture and supportive low-level warm advection will be stronger. 
   Max instability will lessen with northward extent, with peak
   preconvective MLCAPE ranging from around 1500-2000 J/kg over south
   western NE to patchy 500-1200 J/kg values over western/central ND. 
   Deep shear should strengthen with westward extent, closer to the
   eastern rim of the stronger southwest flow from the mid/upper trough
   -- but with effective-shear magnitudes generally 25-40 kt near the
   front. 

   ...Southern Plains...
   Widely scattered to scattered thunderstorms should develop this
   afternoon near the dryline and move eastward into a very unstable
   air mass, offering sporadic severe gusts and hail.  Some of the hail
   may exceed 2 inches in diameter, particularly during the first few
   hours after convective initiation when the storm modes (mostly
   large/deep multicells, but with isolated/transient supercell
   character possible) still are relatively discrete.  Strong surface
   heating, rich low-level moisture (dew points commonly 60s F east of
   the dryline), and a deep troposphere will lead to MLCAPE commonly in
   the 3000-4000 J/kg range, locally higher.  Though winds will veer
   with height, modest flow in mid/upper levels will limit organization
   somewhat, and keep effective-shear magnitudes generally under 35 kt
   region-wide.

   This evening, the potential for cold-pool aggregation and
   development of a line or arc of storms exists, evolving the main
   threat to wind as convection shifts eastward into portions of the
   Low Rolling Plains and Concho Valley regions.  Eastward extent of
   the wnd threat is uncertain, and depends strongly on storm-scale
   contributions to cold-pool organization, but the outlook has been
   expanded eastward to given these processes room to work before
   activity weakens tonight, in strengthening low-level static
   stability.

   ..Edwards/Smith.. 05/23/2020

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