SPC AC 240551

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1251 AM CDT Mon Aug 24 2020

   Valid 241200Z - 251200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TODAY INTO TONIGHT
   ACROSS PARTS OF CENTRAL AND SOUTHEASTERN MINNESOTA THROUGH PORTIONS
   OF WESTERN AND SOUTHERN WISCONSIN..AS WELL AS A SMALL PORTION OF
   SOUTHEASTERN LOUISIANA AND COASTAL MISSISSIPPI...

   ...SUMMARY...
   The evolution of a small, but organized, southeastward moving
   cluster of storms is possible today across parts of the Upper
   Midwest.  Some of this activity may initially pose a risk for very
   large hail, and perhaps a tornado, before the risk for potentially
   damaging wind gusts becomes more prominent.  Elsewhere, Tropical
   Storm Marco may pose a risk for a couple of tornadoes across coastal
   southeastern Louisiana and southern Mississippi.

   ...Synopsis...
   Much of the nation appears likely to remain under the influence of
   mid-level subtropical ridging through this period.  One prominent
   center likely will be maintained along a major axis extending from
   the Southwest through the central Great Plains.  Another center,
   extending along axis from subtropical western Atlantic into the
   Southeast, may begin to weaken some.  However, Tropical Storm Laura
   likely will continue to migrate west-northwestward around its
   southwestern periphery, near the southern coast of Cuba, before
   progressing into the southeastern Gulf of Mexico where it may
   intensify into a hurricane by 12Z Tuesday.  Between the two high
   centers, after approaching southeastern Louisiana coastal areas
   early today, mid/upper remnants of Tropical Storm Marco may
   accelerate northeastward, inland of the Gulf coast, while the
   low-level circulation continues slowly west-northwestward along the
   southeastern Louisiana coast.

   Meanwhile, the mid-latitude westerlies may undergo some
   amplification east of the Canadian Prairies into the eastern
   Canadian provinces.  It appears that this will include a significant
   digging short wave perturbation across eastern Hudson Bay and
   Quebec.  A transition to broadly cyclonic mid-level flow is forecast
   downstream, across much of the Northeast by late tonight, but
   generally weak lower/mid tropospheric flow is expected to prevail
   through much of today into this evening.

   ...Upper Midwest/Great Lakes region...
   To the south of the main digging short wave perturbation, modest
   strengthening of northwesterly mid-level flow is ongoing from the
   northern Great Plains toward the Great Lakes region.  As this
   continues, a plume of warm elevated mixed-layer air to its south
   will be shunted southeastward through this period.  

   Vigorous convective development is ongoing associated with lower/mid
   tropospheric warm advection along the northeastern periphery of this
   plume, where steep mid-level lapse rates are contributing to sizable
   CAPE along an associated low-level frontal zone.  While activity is
   currently rooted above the stable air to the north of the front, at
   some point activity appears likely to shift or redevelop southward
   into closer proximity to the surface frontal zone.

   Most guidance seems to suggest that this will not occur until this
   afternoon, when a few supercells may initiate near the frontal zone,
   before possibly growing upscale into a small, but organized
   convective cluster.  It also might not be out of the question that 
   this occurs much earlier, aided by forcing for ascent on the nose of
   a remnant southwesterly 850 mb jet.  Regardless, given the potential
   instability and moderate to strong shear, there appears potential
   for the evolution of a sustained, severe mesoscale convective
   system, which could be accompanied by a swath of potentially
   damaging wind gusts across parts of central/southeastern Minnesota
   through western/southern Wisconsin by late this evening.

   It is possible severe weather probabilities could be increased
   across parts of this region, once lingering uncertainties become
   better resolved.

   ...Upstate New York into parts of northern New England...
   Modest boundary-layer destabilization may contribute to scattered
   vigorous thunderstorm development, aided considerably by orographic
   forcing.  Some of this activity may be enhanced by 30 kt westerly
   flow in the 850-500 mb layer, which may contribute to potentially
   damaging wind gusts this afternoon.

   ...Gulf Coast...
   Given the ongoing and anticipated evolution of Marco, potential for
   convection capable of producing tornadoes is becoming increasingly
   unclear.  This threat seems largely conditional at this point, and
   limited to immediate Gulf Coastal areas.

   ...Lower Florida Keys...
   Stronger wind fields associated with Laura increasingly appear
   likely to pass by to the southwest of the lower Keys.  However,
   low-level hodographs may still become at least marginally sufficient
   to support a risk for tornadoes in outer bands which could impact
   the  region, particularly late this afternoon and evening.

   ..Kerr/Moore.. 08/24/2020

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