SPC AC 251730

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1230 PM CDT Tue Aug 25 2020

   Valid 261200Z - 271200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   FAR EAST TX AND MUCH OF LA...AND PORTIONS OF THE UPPER MIDWEST INTO
   THE GREAT LAKES AND OH VALLEY/MID-ATLANTIC...

   ...SUMMARY...
   A threat for a few tornadoes will exist Wednesday and Wednesday
   night primarily across parts of Louisiana and far east Texas in
   association with Hurricane Laura. Otherwise, a few strong to severe
   storms with damaging winds and marginally severe hail will be
   possible over portions of the Upper Midwest, Great Lakes, Ohio
   Valley, and Mid-Atlantic regions. A few severe storms may also occur
   from the northern Rockies through the northern Plains, and in
   Arizona.

   ...Texas/Louisiana Gulf Coast into the Lower Mississippi Valley...
   Hurricane Laura is forecast to strengthen by the National Hurricane
   Center as it approaches the coast of TX/LA Wednesday evening into
   early Thursday morning. A corresponding increase in the low-level
   flow field is expected across this area as Laura moves northward,
   with most guidance suggesting 850 mb winds strengthening to 50-75+
   kt in the eastern half of the circulation. The low-level airmass
   should be sufficiently moist, with generally low to mid 70s surface
   dewpoints present, to support weak instability across far east TX
   into LA and parts of the lower MS Valley. Given the very favorable
   low-level shear that will be present across this region, a few
   tornadoes should be the main convective threat with any low-topped
   supercells that can form in outer rainbands. Tornado probabilities
   have been expanded slightly westward given the latest forecast track
   from NHC showing the center of circulation moving northward
   along/near the TX/LA border through the end of the period.

   ...Upper Midwest into the Great Lakes, Ohio Valley, and
   Mid-Atlantic...
   A front will likely extend southeastward from the Upper Great Lakes
   into parts of the OH Valley and Mid-Atlantic Wednesday morning. Weak
   low-level warm advection across these areas will likely be
   supporting a band of mainly elevated storms across the Great Lakes
   and vicinity at the start of the period. Most guidance suggests this
   convection will weaken through the morning, with additional storm
   development in the afternoon rather unclear owing to weak
   large-scale forcing for ascent. Still, the forecast combination of
   moderate instability coupled with strong deep-layer shear across the
   warm sector will conditionally support severe storms should they
   form and subsequently move southeastward. Have opted to maintain the
   ongoing Slight Risk for mainly damaging winds with some modification
   on its eastward extent into the Mid-Atlantic based on the placement
   of the surface front in the latest forecast guidance. Robust
   convective development appears somewhat more probable Wednesday
   afternoon along a southeastward-moving cold front across central MN
   and the U.P. of MI as a weak upper trough also moves eastward across
   the Upper Midwest. These storms may initially pose a large hail
   threat, before damaging winds potentially become the primary severe
   hazard as storms merge into small clusters/bows.

   ...Northern Rockies into the Northern Plains...
   A cold front will advance southward through the northern Plains
   during the day within a moderately unstable environment. However,
   the stronger large-scale ascent should remain in the post-frontal
   zone with modest convergence along the front beneath a capping
   inversion. Overall convective coverage appears limited over much of
   this region given the nebulous forcing aloft. Still, any storms that
   do form will be capable of producing isolated large hail and severe
   wind gusts given the forecast combination of instability and shear.
   A better chance of scattered storms will evolve over the higher
   terrain of the northern Rockies and adjacent High Plains where
   moisture and instability should remain more limited. However, storms
   will be high-based with steep lapse rates and moderate deep-layer
   shear supportive of both multicells and possibly a few supercells
   capable of a few severe/damaging wind gusts and perhaps large hail.

   ...Arizona...
   At least isolated storms should form Wednesday afternoon across the
   higher terrain of central/southern AZ as an upper high remains
   centered over the Four Corners region. Mid-level flow is forecast to
   remain quite weak over this area, and confidence in storms moving
   southwestward off the higher terrain remains low. Regardless,
   isolated strong to severe wind gusts appear possible through
   Wednesday evening given the deeply mixed boundary layer and steep
   low-level lapse rates.

   ..Gleason.. 08/25/2020

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