SPC AC 231738

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1238 PM CDT Sun Aug 23 2020

   Valid 241200Z - 251200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PARTS OF THE
   CENTRAL GULF COAST AND FOR PORTIONS OF THE UPPER MIDWEST...

   ...SUMMARY...
   A few tornadoes are possible across the central Gulf Coast on Monday
   and Monday night.  Severe thunderstorms capable of large hail and
   damaging gusts are possible across parts of the Upper Midwest.

   ...Central Gulf Coast...
   Tropical cyclone Marco is forecast to make landfall over the LA
   coast and move inland into parts of southern LA during the evening
   Monday into early Tuesday per the latest National Hurricane Center
   forecast.  An increasingly moist/unstable airmass will move ashore
   along the immediate coast during the day and gradually spread
   further inland Monday night.  Forecast hodographs show clockwise
   curvature initially near the coast from the FL Panhandle westward to
   the mouth of MS River.  The risk for a supercell tornado will
   correspondingly be focused near the coastal counties during the day
   with the risk gradually penetrating northwestward into the areas
   north/northwest of Lake Pontchartrain into southern MS late.
     
   ...Upper Midwest into the Great Lakes...
   Not much change to the previous forecast, as the trailing portion of
   a cold front crossing Ontario and Quebec, sags southward across the
   Upper Great Lakes region.  Thunderstorms may be ongoing early Monday
   morning with a possible risk for a strong storm or two, mainly from
   northern Wisconsin and Upper Michigan east across northern Lower
   Michigan.  As this convection spreads east across Lake Huron and
   into Ontario with time, new convective development -- driven by warm
   advection near and atop the southward-moving front -- may occur,
   across portions of southeast Minnesota and southern Wisconsin, and
   shifting southeastward with time.  Given ample CAPE and
   veering/increasing flow with height, risk for large hail is
   apparent.  The development of an MCS may spur a risk for damaging
   gusts and possibly a tornado as it moves east-southeast during the
   evening into the overnight.

   ...Lower Great Lakes into New England...
   While it appears that the cold front advancing across Ontario/Quebec
   should remain northwest of the Lower Great Lakes/New England through
   the end of the period, modest heating/destabilization of the
   pre-frontal airmass is expected during the day across New York/and
   into New England.  Gradual height falls and presence of a
   weak/subtle warm-frontal boundary extending northwest-to-southeast
   across northern New York and southern New England may focus a
   cluster of afternoon storms.  Though stronger flow will remain
   across Canada through the day, shear may support potential for gusty
   winds -- and perhaps local tree damage with the afternoon storms.

   Later in the period, storms nearer the advancing front may spill
   across the international border.  While a less unstable boundary
   layer after dark should limit severe risk, strengthening flow aloft
   may be sufficient to maintain low-end risk for wind through much of
   the period.

   ...FL Keys and Everglades...
   Tropical cyclone Laura will continue to move to the west-northwest
   across the northwest Caribbean/Cuba and into the southeast Gulf of
   Mexico according to the latest National Hurricane Center forecast. 
   The northeastern portion of the strong wind envelope associated with
   Laura will overspread the FL Straits and the Keys during the day and
   into the overnight.  Forecast soundings show a very moist/tropical
   airmass across this region and hodographs show strong
   east-southeasterlies at 1km AGL over the Keys.  The hodographs
   across the southern FL Peninsula appear less favorable compared to
   areas farther southwest across the Keys.  As a result, it appears
   the risk for transient supercell rotation may focus over the Keys
   where the wind profile may support better storm organization and an
   isolated threat for a tornado.

   ..Smith.. 08/23/2020

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