SPC AC 281252

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0752 AM CDT Fri Aug 28 2020

   Valid 281300Z - 291200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   SOUTHERN WISCONSIN...NORTHERN ILLINOIS AND CENTRAL/EASTERN IOWA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
   IOWA TO WESTERN PENNSYLVANIA...AND ACROSS PARTS OF THE MID-SOUTH AND
   TENNESSEE VALLEY REGIONS INTO KENTUCKY...

   ...SUMMARY...
   Severe storms with large hail, damaging winds and a couple tornadoes
   are possible today over parts of the upper Mississippi Valley and
   southern Great Lakes.  Tornadoes and damaging thunderstorm winds are
   possible across parts of the Mid-South and Tennessee Valley region
   ahead of the remnants of Laura.

   ...Synopsis...
   In mid/upper levels, a progressive northern-stream pattern will
   continue near the Canadian border, with some amplification through
   the period related to two strong shortwave troughs.  The first --
   currently over central/southern MB -- is forecast to dig
   southeastward across northwestern ON and Lake Superior.  A compact
   cyclone is progged to develop tonight, north of Lake Superior, with
   the trough south-southeastward over Lower MI by 12Z tomorrow.  The
   second major shortwave trough is evident in moisture-channel imagery
   over the Gulf of AK, southwest of a cyclone moving inland over
   northwest BC, and will amplify and move southeastward to coastal WA
   and southwestern BC by the end of the period.  Meanwhile, a weaker
   perturbation now located over northern AZ will move around the
   persistent anticyclone, reaching  eastern CO by 12Z (perhaps with
   vorticity augmentation from convection).  Elsewhere, the
   deep-tropospheric remnants of former Hurricane Laura -- now over
   northeastern AR -- will move generally east-northeastward then
   eastward over KY.

   At the surface, 11Z analysis showed a somewhat wavy, quasistationary
   frontal zone from CT across western NY, southern lower MI, and WI,
   intercepted by an arc of outflow from an ongoing MCS.  That outflow
   boundary was drawn from western WI across extreme southeastern MN
   and northwestern IA, and should progress across portions of central
   WI and northern/western IA before decelerating through the day.  The
   frontal zone to its east should remain nearly stationary, with minor
   modulations by convective processes.  A cold front was drawn across
   the eastern Dakotas and central/southwestern NE, through a low
   between GLD-AKO, to south-central CO.  This front should advance by
   00Z to a position from a low over northern Lake Michigan across
   southern WI, southern IA, and central/southwestern KS.  By 12Z the
   front should reach northwestern OH, southern IL, southern MO, and
   northwestern OK, perhaps interacting with a weak low over
   central/western OK, then extending across the southern TX Panhandle
   and east-central NM.  Upslope flow behind the front, across the
   south-central High Plains, will contribute to isolated, marginal
   severe potential in that region.

   ...Upper Mississippi Valley, southern Great Lakes...
   An ongoing complex of thunderstorms that produced overnight severe
   gusts over SD has weakened considerably since, but remains organized
   enough to pose a threat for isolated strong/locally damaging gusts
   as it moves across northern IA and western WI the next few hours. 
   See SPC mesoscale discussion 1609 for more details.

   Widely scattered to scattered thunderstorms are expected to develop
   near the associated outflow boundary and cold front this afternoon,
   offering a threat for large hail and severe gusts.  Some potential
   for aggregation upscale into a line or cluster of severe convection
   is possible.  Favorable destabilization is expected over the warm
   sector, with surface dew points commonly in the upper 60s to mid 70s
   F over most of this corridor.  This will support peak preconvective
   MLCAPE in the 2000-3000 J/kg range, decreasing somewhat with
   eastward extent due to areas of cloud cover and weaker midlevel
   lapse rates.  Effective-shear magnitudes of 35-45 kt should be
   common, even with an increasing westerly surface-wind component.

   Some concern exists regarding convective coverage in this regime,
   especially given some veering of winds in the warm sector
   (potentially limiting low-level convergence) and strong
   inconsistencies among various convection-allowing guidance. 
   However, the greatest threat for development of an organized
   convective complex capable of severe gust still appears to be in and
   near the "enhanced" area, which resides south of outflow boundary
   from the morning activity, and southeast of the cold front
   overtaking it today.  Those boundaries should serve as primary foci
   for convection leading to increased severe potential this afternoon
   into evening.

   ...Remnants of Laura...
   Isolated to scattered thunderstorms are expected to occur across
   this region through tonight, some of which will be supercells
   capable of producing tornadoes.  Sometimes the most prolific tornado
   day of a tropical cyclone is in the inland-decay phase, as clearing
   over remaining favorable low-level shear allows sufficient
   destabilization to support sustained supercells.  That may occur
   here, and an upgrade could be needed if a mesoscale area of
   heightened potential becomes better apparent during the day.

   Despite the overall weakening of the core winds with the remains of
   Laura, a strong low/middle-level height gradient will be maintained
   to the south and southeast of the circulation center today into
   tonight.  The associated 45-55-kt LLJ should be maintained through
   the day while shifting east, and may expand in size somewhat, atop a
   diurnally destabilizing boundary layer with very rich
   moisture/theta-e.  Effective SRH should range from 150-400 J/kg
   across much of TN, northern MS, and northern AL, decreasing
   southward.  This will overlap a gradient of buoyancy from marginal
   to very favorable, with greatest instability under and around the
   edge of the mid/upper dry slot, where sustained surface heating can
   occur.  MLCAPE will vary widely, ranging from around 2000-2500 J/kg
   over southern parts of the outlook area this afternoon to 500 J/kg
   or less over northern areas and into the southern Appalachians
   tonight.

   ..Edwards/Leitman.. 08/28/2020

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