SPC AC 201257

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0757 AM CDT Sun Jun 20 2021

   Valid 201300Z - 211200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   EXTREME EASTERN IOWA ACROSS CHICAGOLAND TO NORTHWEST OHIO AND
   SOUTHERN LOWER MICHIGAN...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENHANCED RISK...AND ALSO OVER PARTS OF GEORGIA AND THE CAROLINAS...

   ...SUMMARY...
   Scattered severe storms are expected across the Midwest into the
   lower Great Lakes/upper Ohio Valley. Large hail, severe/damaging
   winds, and a few tornadoes are possible. Damaging winds and a couple
   of tornadoes are also possible across the Southeast.

   ...Synopsis...
   The mid/upper-level synoptic pattern will become much more
   cyclonically curved, with height falls common across the northern/
   central Plains and much of the Great Lakes by 12Z tomorrow.  This
   will occur as a series of shortwave troughs moves southeastward to
   eastward over the northern/central Rockies, Plains, and Upper
   Midwest.  This will include an MCV and associated perturbation now
   evident over the OMA area into northeastern KS, which should move
   eastward to the northwestern IN/western Lower MI region by 00Z.  A
   strong shortwave trough -- apparent in moisture-channel imagery over
   the northern Rockies just north of the Canadian border -- will move
   southeastward and lose some amplitude, reaching CO by 12Z.

   The 11Z surface analysis showed a wavy quasistationary to warm front
   from south of Long Island across southern PA, northern parts of
   OH/IN, northern IL, and southern MN, to a low near ABR.  A cold
   front was drawn from there southwestward across northeastern CO, and
   is expected to move southeastward across the central Plains and
   upper/mid Mississippi Valley through the period.  Meanwhile the warm
   front should shift northward across northern IL and into
   southern/central Lower MI ahead of the low, which should cross
   northern lower MI and Lake Huron overnight. 

   ...Midwest/Great Lakes...
   Multiple rounds of scattered to locally numerous thunderstorms are
   possible through tonight within a broad corridor from the mid/upper
   Mississippi Valley to the area between southern Lake Michigan and
   Lake Erie.  All severe hazards are possible -- wind, hail (some
   significant where supercells can occur) and tornadoes -- with the
   most dense and widespread concern being severe thunderstorm wind.

   Predictability does not always improve closer to an event, and this
   is an example.  A messy, conditional and very mesoscale-dependent
   scenario is evident, with convective trends from the prior overnight
   to this morning only augmenting the uncertainties.  The main
   influences for now appear to be:
   1.  MCV-related convection, which has weakened over the last few
   hours across IA and northern MO as it moves into a pronounced
   low-level theta-e deficit and CINH maximum related to the next
   factor below.  Still, the associated mass and thermal perturbations
   may reinvigorate convection and organize a severe MCS over IL into
   northern IN/southern MI, as it encounters a diabatically
   destabilizing and suitably moist boundary layer...
   2.  A separate, earlier area of convection and precip -- now mostly
   dissipated -- that left behind its cold pool across northern MO and
   parts of western IL.  This theta-e deficit will be entrained into
   prevailing low-level flow downstream into eastern IA and northern
   IL, with mixing and diurnal heating potentially diluting its
   negative impacts on convective potential somewhat.
   3.  The cold front later this afternoon into evening, to the extent
   it encounters favorable pockets of relatively undisturbed
   boundary-layer air across IA/WI/MI/IL/MO.  The amount and extent of
   favorable instability will depend strongly on recovery behind both
   ongoing convective factors 1 and 2 above.  The front is likely to
   support development south of the MCV and cold pools, over part of
   northern MO and eastern KS, where instability will be strong but
   shear weaker than farther north and northeast.

   Given these factors and related uncertainties, only peripheral
   changes are being made to the existing outlook, pending both more
   clarity on mesoscale trends, and numerical guidance from 12Z onward
   that will better incorporate the effects of these cold pools and MCV
   influences than earlier progs did.

   ...Southeast, T.D. Claudette...
   Following a nocturnal relative min in both convective and severe
   (tornado, gust) potential, a ramp up in those is expected during the
   remainder of this morning and into afternoon across parts of GA and
   the Carolinas.  See SPC mesoscale discussion 1030 for near-term
   details.  The greatest tornado potential will be bounded by
   relatively low-theta-e air and cloud cover/precip with weaker shear
   to the north and northwest, weaker deep-layer winds and areas of
   stabilizing precip to the south, and veering of boundary-layer flow
   (reducing hodograph size) to the southwest.

   A well-defined mid/upper-level dry slot was apparent in
   moisture-channel imagery wrapping around the southern semicircle of
   the system, and was sampled well upstream by the 12Z JAN and LIX
   soundings.  The eastern fringe of this slot may overlap areas where
   surface flow remains more southerly to easterly across the eastern
   semicircle, low-level moisture remains very rich, and
   deep-tropospheric vertical-shear vectors also will be aimed.  In an
   environment of modest midlevel lapse rates, even subtle diabatic
   heating-related enhancement to low-level lapse rate will yield
   favorable buoyancy, with MLCAPE commonly around 1000-1500 J/kg and
   effective SRH 150-300 J/kg supporting supercell development with any
   sustained convection. [See NHC advisories for the latest
   track/intensity guidance, as well as any tropical watches/warnings,
   regarding Claudette.]

   ...Central High Plains...
   Isolated to scattered thunderstorms are possible this afternoon and
   overnight in two main episodes, cumulatively contributing to this
   expanded area of severe probabilities:
   1.  Diurnal development near and north of the frontal zone, where a
   combination of heating of elevated terrain and weak moist advection
   will contribute just enough buoyancy for some high-based
   thunderstorm potential, with MLCAPE in the 300-800 J/kg range.  A
   well-mixed subcloud layer will support localized strong/isolated
   severe downburst potential.  Strong mid/upper winds will contribute
   to around 35-45 kt effective-shear magnitudes (which are limited by
   lack of greater CAPE depth).
   2.  Overnight convection developing ahead of the northwest-flow
   perturbation, in a regime of both DCVA and low-level theta-e
   advection.  Lack of more-robust moisture still will be a limiting
   factor, but an area of convection may develop and move southeastward
   across the outlook area, offering potentially damaging gusts and
   marginal hail potential.  Though much of the boundary layer will
   stabilize diabatically, a relict diurnal mixed layer above that may
   support enough downdraft acceleration to push strong/locally severe
   gusts to the surface.

   ..Edwards/Broyles.. 06/20/2021

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