SPC AC 261248

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0748 AM CDT Wed Aug 26 2020

   Valid 261300Z - 271200Z

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

   ...SUMMARY...
   Hurricane Laura will pose an increasing threat for tornadoes this
   evening and overnight across much of Louisiana.  Elsewhere, damaging
   thunderstorm winds and sporadic hail will be possible from the Upper
   Midwest across the Great Lakes.

   ...Synopsis...
   In mid/upper levels, a persistent anticyclone will remain centered
   over the Four Corners region.  Meanwhile, to its east, a slow-moving
   midlatitude trough over the southern Plains will contribute to the
   northward turn of Laura toward its NHC-projected landfall tonight as
   a major hurricane, in the BPT-LCH area, then northward up the Sabine
   River region.  Meanwhile, deamplification is forecast for the upper
   ridge now located from the high across MN to Hudson Bay.  A series
   of mostly low-amplitude perturbations will traverse the associated
   northern-stream belt near the Canadian border from the Northwest to
   the Great Lakes, then southeastward across the northeastern CONUS. 
   A strong cold-core cyclone will pivot erratically/slowly eastward
   across eastern QC, Labrador and the Gulf of St. Lawrence. 

   At the surface, 11Z analysis showed a triple-point low near FAR with
   cold front southwestward to a slightly stronger low near PIR, then
   southwestward across eastern/central WY.  A warm front was drawn
   from the FAR low across northwestern WI to near MKE, FWA, CMH, and
   HTS, becoming a cold front across the VA/NC border.  The warm front
   should move northeastward over the remainder of eastern/northeastern
   MN and WI, Lake Michigan and lower MI through the period.  By then,
   the cold front should reach Upper MI, northern WI, southern MN, and
   southwestern NE.

   ...Hurricane Laura...
   Given the currently expanding and fairly large forecast size of this
   hurricane, favorable low-level wind fields may spread somewhat
   inland later today, before substantial convection arrives.  As such,
   any outer-fringe convection may become briefly supercellular as
   early as this afternoon near the coast.  Such cells may pose at
   least marginal tornado potential with inflow from what remains of
   diurnal/inland heating inland, amidst gradually strengthening
   low-level shear.  However, the greatest supercell/tornado threat for
   day-1 should be from evening onward, in middle/outer spiral
   convergence bands north-northeast through southeast of center.  The
   threat will spread northward across LA overnight into tomorrow
   morning, as the hurricane penetrates well inland.  

   Though the hurricane's surface wind speeds will lessen with time as
   it moves deeper inland, low-level shear will remain strong. 
   Forecast hodographs indicate effective SRH may approach 500 J/kg
   over parts of central/southern LA tonight.  Extremely moist Gulf air
   will advect inland in the eastern sector -- which will be largely
   downshear with respect to ambient, deep-tropospheric/850-200-mb
   shear vectors.  Large theta-e will offset both modest lapse rates
   aloft and the lack of diurnal heating, to support 500-1000 J/kg
   MLCAPE in some areas (decreasing northward).  Depending on the
   number of sustained supercells that can develop in that setting,
   tornado threat may become enhanced in a smaller, embedded corridor. 
   However, for now, a broad 5% area is maintained in deference to both
   mesoscale uncertainties with the character of the precip/convective
   field, and the climatological nocturnal dip in TC-tornado frequency.
    See NHC advisories for tropical watches/warnings and the latest
   forecast track/intensity guidance for Laura.

   ...Upper Midwest, Great Lakes...
   Episodic areas of thunderstorms are possible across this belt
   through tonight,contributing collectively to an overlapping threat
   for strong-severe thunderstorms with hail and damaging gusts.

   A belt of thunderstorms moving southeastward across parts of MI and
   Lake Michigan this morning may produce sporadic hail and isolated
   damaging gusts before shifting eastward out of the region today.  A
   narrow corridor of favorable heating and surface-based
   destabilization along its western fringes will yield a potential
   inflow layer characterized by 1000-2000 J/kg MLCAPE, a well-mixed
   boundary layer, and 35-45-kt effective-shear magnitudes in support
   of storm organization.  Convective coverage is uncertain by
   afternoon near the warm front, however, because of modest lift near
   that boundary.

   Farther west, surface dew points generally in the mid 60s to mid 70s
   F and diurnal heating, beneath steep/EML-originated midlevel lapse
   rates, will yield a corridor of MLCAPE in the 2000-3000 J/kg range
   near the cold front over northeastern SD, southeastern ND and MN,
   decreasing to around 1000-2000 J/kg south of Lake Superior. 
   30-40-kt effective-shear magnitudes will combine with that buoyancy
   to support a blend of multicell and supercell characteristics, and
   some clustering/bowing of convection also will be possible. 
   Afternoon/evening convection may evolve upscale into one or more
   larger clusters with locally greater damaging-wind potential.

   ...Northern Rockies/High Plains...
   Isolated to scattered thunderstorms are expected to develop
   initially this afternoon over the higher terrain in western parts of
   the outlook area, then farther east with time into the
   evening/overnight hours.  A surface-based thunderstorm or two also
   cannot be ruled out over lower elevations in the western Dakotas or
   eastern MT.  The post-frontal upslope regime of eastern WY, the
   Black Hills, or the NE Panhandle also may contribute to potential
   for storm initiation.  Isolated severe gusts and marginal hail will
   be possible.  Activity will be episodic, supported by a corridor of
   modest but sufficient low-level moisture and diurnal heating behind
   the western cold front.  A well-mixed boundary layer will enable
   strong/locally damaging downdrafts.

   ..Edwards/Leitman.. 08/26/2020

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