SPC AC 220735

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0235 AM CDT Sat Aug 22 2020

   Valid 241200Z - 251200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE GREAT LAKES AND INTO WESTERN NEW ENGLAND...

   ...SUMMARY...
   A few thunderstorms capable of producing locally gusty/damaging
   winds, and possibly hail, may affect portions of the Great Lakes and
   vicinity Monday.

   ...Synopsis...
   Some amplification within the main belt of westerly flow -- residing
   over southern Canada and the northern U.S. -- is expected this
   period.  This will be particularly true over the Northeast, as a
   pronounced, fast-moving short-wave trough crosses Ontario and the
   adjacent Upper Great Lakes region during the period.  The result
   will be increasing cyclonic flow aloft expanding across the
   northeastern quarter of the country through the end of the period.

   At the surface, a low associated with the aforementioned trough will
   shift out of Ontario and into Quebec, deepening substantially with
   time.  As this occurs, a trailing cold front will move eastward
   across Quebec toward New England, while farther west the front
   becomes increasingly elongated in a west-to-east manner, sagging
   slowly southward across the northern Plains/upper Midwest/Great
   Lakes region.

   Elsewhere, short-wave troughing will move quickly across the
   Canadian Rockies and adjacent northern Intermountain region, while a
   compact low moves into central California late in the period. 
   Meanwhile, latest model forecasts -- and National Hurricane Center
   forecasts -- show that Tropical Storm Marco will continue crossing
   the western Gulf of Mexico as it moves northwestward toward the
   Texas coast, while Tropical Storm Laura shifts west-northwestward
   across Cuba and eventually enters the Gulf of Mexico west of the
   Florida Keys through the end of the period.

   ...Great Lakes region into western New England...
   Convection may be ongoing over the northern Minnesota vicinity at
   the start of the period, possibly lingering into the afternoon. 
   Thereafter, as a west-to-east surface cold front sags southward, an
   increase in convective coverage and intensity is expected in tandem
   with afternoon heating/destabilization.

   Stronger flow aloft is forecast to remain to the cool side of the
   front, but shear may support evolution a few stronger storms/storm
   clusters.  Locally damaging winds, and hail, would be possible as a
   result.  

   Farther east, the eastward-moving portion of the Canadian front will
   approach the Lower Great Lakes/western New England with time.  This
   will likely permit late afternoon/evening storms to spread across
   the international border.  While flow aloft will gradually increase
   across the Lower Lakes and New England with time, the bulk of the
   stronger/more organized convection ahead of the front may not shift
   into the U.S. until after dark.  Given the unfavorable time of day,
   the degree of severe risk -- which would mainly be in the form of
   locally damaging wind gusts -- remains uncertain.  As such, only
   5%/MRGL risk is being included across this region at this time.

   ...Western Gulf Coast region...
   Tropical Storm Marco -- now evident in the vicinity of western Cuba
   and the tip of the Yucatan Peninsula, is expected to move slowly
   northwestward into the Gulf of Mexico.  The latest forecasts from
   the National Hurricane Center show Marco moving northwestward across
   the northwestern Gulf, toward the Texas coast, through the end of
   the Day 3 period.  This would suggest an increase in both convection
   and the low-level wind field across southern Louisiana and the Texas
   coastal areas with time.  However, substantial uncertainty is noted
   by NHC with respect to forecasts of both the track, and the
   intensity, of Marco.  As such, it appears premature at this time to
   areally highlight any possible/heightened risk for tornadoes with
   this system.

   ...South Florida and the Keys...
   Per the most recent National Hurricane Center forecast, Tropical
   Storm Laura will be crossing Cuba through the day, before emerging
   into the Gulf west of the Keys overnight.  This would suggest an
   increase in associated convection across southern Florida and the
   Keys.  With an increase in low-level flow/shear that would occur
   presuming the current forecast track of Laura, some tornado
   potential would be possible.  However, similar to forecasts of
   Tropical Storm Marco at this time, substantial uncertainty regarding
   both strength and track of Laura -- as noted in NHC forecasts --
   exists.  This precludes the inclusion of any severe-weather risk
   area at this time.

   ..Goss.. 08/22/2020

   CLICK TO GET WUUS03 PTSDY3 PRODUCT

   NOTE: THE NEXT DAY 3 OUTLOOK IS SCHEDULED BY 0730Z