SPC AC 241728

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1228 PM CDT Mon Aug 24 2020

   Valid 251200Z - 261200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
   THE MID-ATLANTIC REGION...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE UPPER
   OHIO VALLEY INTO MUCH OF THE NORTHEAST...AND PORTIONS OF CENTRAL
   IDAHO INTO SOUTHWEST MONTANA...

   ...SUMMARY...
   Thunderstorms capable of isolated severe gusts (55-65 mph) resulting
   in scattered wind damage are probable for portions of the
   Mid-Atlantic states.

   ...The Northeast...
   A strong cold front is forecast to shift across the U.S./Canada
   border and move quickly eastward across New England, while
   progressing more slowly southeastward across the Mid Atlantic region
   and southward across the central Appalachians.  Isolated general
   thunderstorms may be ongoing Tuesday morning and lending some
   uncertainty regarding destabilization across lower elevations near
   the coast ahead of this activity.  However, west-northwesterly flow
   in the mid levels is forecast to strengthen during the day across
   the region and support relatively fast multicell storms.  

   It appears the greatest risk for severe gusts and associated
   tree/wind damage will occur farther south across parts of eastern
   PA/NJ into the Chesapeake Bay vicinity.  Strong heating and mean
   mixing ratios in the 15-16 g/kg range, will result in a moderately
   to very unstable boundary layer by mid afternoon (2000-3000 J/kg
   MLCAPE).  Effective shear 20-30 kt will favor a multicell convective
   mode supportive of clusters and line segments potentially capable of
   isolated 55-65 mph gusts and accompanying scattered wind damage.  

   Farther west, a strong storm cannot be ruled out during the morning
   over Lower MI vicinity.  However, additional storms either
   rejuvenating on earlier-day outflow or associated with a weak
   disturbance, will foster a risk for strong to severe thunderstorms
   across the upper OH Valley during the afternoon/early evening.  

   ...Northern Intermountain region into the northern Plains...
   A series of cyclonic mid-level disturbances will continue to move in
   an easterly anticyclonic arc across the northern Intermountain
   region and northern Plains, embedded within a belt of 40-50 kt 500mb
   flow around the northern periphery of the upper ridge.  

   Afternoon heating will support the development of steep low-level
   lapse rates and isolated to widely scattered storm initiation over
   the higher terrain.  Given strong mid-level flow promoting
   fast-moving storms, and inverted-v profile, evaporative cooling
   coupled with stronger momentum in the mid levels will favor
   severe-gust potential with the stronger cores.  

   Farther east into the northern Plains, a dry/capped boundary layer 
   is expected, that should act to substantially limit convective
   development though the afternoon.  Still, with fast flow aloft
   supporting potential for updraft organization, gusty/damaging winds
   would be possible with any sustained storm which could develop.

   Overnight, evolution of a southwesterly low-level jet may foster an
   increase in convection coverage from the Dakotas into the Great
   Lakes region, though storms would likely be primarily elevated above
   the capping layer.  Some risk for hail would likely exist given
   steep mid-level lapse rates supporting ample CAPE above the cap, and
   a locally strong wind gust cannot be ruled out.

   ...Upper Midwest...
   The potential for some increase in coverage is expected after dark
   as warm advection increases in conjunction with a southwesterly
   low-level jet.  Given the anticyclonic belt of fast flow overhead, a
   couple of stronger storms will be possible, with attendant potential
   for locally gusty winds or marginal hail.

   ...Arizona...
   Model forecasts continue to depict small mid-level cyclonic
   disturbances moving southwestward across Arizona Tuesday and Tuesday
   night, within a belt of enhanced northeasterly mid-level flow.

   As the airmass heats/destabilizes through the afternoon, storms
   should develop over higher terrain/the Mogollon Rim, and then shift
   southeastward into the lower deserts.  With a very deep mixed layer
   promoting evaporative downdraft acceleration, potential for locally
   damaging wind gusts is evident with a couple of the stronger
   storms/storm clusters.

   ...Central/Western Gulf Coast region...
   The latest National Hurricane Center forecasts continue to show
   Marco weakening with time as it moves westward along the Louisiana
   coast, followed by Laura expected to be moving northwestward across
   the eastern and central Gulf of Mexico as a
   strengthening/substantial hurricane.  With sufficient low-level
   shear expected across parts of Louisiana due to the presence of
   Marco, risk for a brief/weak tornado or two will persist through the
   day.  Meanwhile, with Laura expected to reach the north-central Gulf
   south of southeastern Louisiana late Tuesday night, additional
   strengthening of the low-level wind field is expected.  While
   questions persist regarding the forecast track of Laura, a continued
   risk for a couple of weak/brief tornadoes is expected across parts
   of the central/western Gulf Coast region through the evening and
   overnight hours.

   ..Smith.. 08/24/2020

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