SPC AC 262000

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0300 PM CDT Wed Aug 26 2020

   Valid 262000Z - 271200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   LOUISIANA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE UPPER MIDWEST AND GREAT LAKES...

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

   ...Louisiana and East Texas...
   Major Hurricane Laura is forecast by the National Hurricane Center
   to make landfall along the TX/LA Coast later tonight. The enhanced
   low-level winds associated with Laura have increased in both
   magnitude and spatial extent with the intensification that has
   already occurred. In addition to the rain band ongoing along/near
   the LA Coast, confidence is increasing that one or more additional
   outer rain bands will form in the eastern semicircle of the cyclone
   later this evening and overnight. Forecast soundings from short-term
   guidance across portions of southern LA that will remain to the east
   of the eyewall show substantial low-level shear, with effective SRH
   potentially exceeding 500 m2/s2 late this evening and continuing
   into early Thursday morning. Low-level moisture is also expected to
   further increase across this area as the low-level winds shift from
   more an easterly to southerly component, which will support at least
   weak destabilization. The potential for several tornadoes occurring
   with low-topped supercells during the nighttime hours in one or more
   of these rain bands has increased enough to include higher tornado
   probabilities and an Enhanced Risk.

   ...Eastern Ohio/Northern West Virginia...
   Have expanded the Marginal Risk for hail/wind into parts of eastern
   OH and northern WV to account for ongoing strong to locally severe
   storms near a warm front. Overall longevity of this convection
   remains unclear given nebulous large-scale ascent aloft. Regardless,
   an isolated severe threat will probably continue for at least the
   next couple of hours before eventually diminishing with the loss of
   daytime heating.

   ...Upper Midwest/Great Lakes...
   The potential for robust surface-based storms remains unclear across
   this region, especially across parts of MN and vicinity where 12Z
   soundings observed a substantial cap. Still, attempts at convective
   initiation will likely occur across MN along a weak front as a
   low-amplitude shortwave trough moves across the Upper Midwest. If
   storms can form and remain sustained, they would be capable of
   producing isolated large hail and damaging wind gusts given the
   favorable environment. The potential for surface-based storms across
   the Great Lakes region also appears highly uncertain and conditional
   with the lack of a more obvious large-scale forcing mechanism. Have
   not made any changes to the Slight Risk across this area, but severe
   probabilities may need to be lowered in the next outlook update if
   storms do not form this afternoon or early evening.

   ..Gleason.. 08/26/2020

   .PREV DISCUSSION... /ISSUED 1129 AM CDT Wed Aug 26 2020/

   ...Hurricane Laura...
   Latest VWP data from KLCH and KLIX have sampled notable 0-1 km SRH
   (150-225 M2/S2) as the expansive wind field north of Hurricane Laura
   impinges on the coast. Thus far, little rotation has been noted with
   outer bands offshore of the LA coast, but an increasing risk for at
   least a few tornadoes will develop late this afternoon and continue
   tonight as higher theta-e air and intense low/mid-level wind fields
   move inland. Higher tornado probabilities may still be warranted for
   tonight in later outlooks as the mesoscale details become clearer.

   ...Upper Midwest/Great Lakes...
   Clusters of thunderstorms continue near and north of a warm front
   that extends from northern MN east across the Upper Peninsula of
   Michigan and then southeast into northern OH, and along a cold front
   over eastern ND. Low-amplitude perturbations within stronger
   mid-level flow, and/or weakening low-level warm advection are
   contributing to the maintenance of these storms. Scattered
   thunderstorms may intensify/redevelop late this afternoon/evening
   along the cold front in the presence of moderate/strong instability
   aided by steep mid-level lapse rates and 35-40 kts of westerly
   mid-level flow. Although some question remains concerning overall
   coverage of the severe threat, large hail and damaging gusts will be
   possible with storms that do develop.  

   Farther east/southeast, thunderstorms may re-intensify near the warm
   front later today across the eastern Great Lakes and into portions
   of western NY/PA aided by impulses within the mid-level flow and
   strengthening warm advection tonight. This area will be on the
   eastern fringes of the EML, despite substantial morning cloud cover
   expect sufficient instability/shear to support multicell clusters
   capable of strong/locally damaging winds.  

   ...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 tonight. 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 the potential for storm
   initiation. Isolated severe gusts and marginal hail will be
   possible. This thunderstorm potential will be 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 help promote strong/locally damaging downdrafts.

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