SPC AC 060556

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1256 AM CDT Mon Jul 06 2020

   Valid 061200Z - 071200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
   NORTHERN PLAINS INTO THE UPPER GREAT LAKES...

   ...SUMMARY...
   Severe thunderstorms are expected across portions of the northern
   Plains into the upper Great Lakes region later today. Large hail and
   damaging winds are the primary threats, though a few tornadoes may
   be noted from southeast Montana into northwest South Dakota. Strong
   winds may also accompany storms across the Middle Atlantic region.

   ...Northern Plains to Upper Great Lakes...

   An expansive area of convection has developed across the northern
   Plains ahead of a short-wave trough that should progress into the
   eastern Dakotas by sunrise. This feature will shift into the upper
   MS Valley during the afternoon before shifting into the upper Great
   Lakes overnight. Modest deep-layer flow is expected along a corridor
   from the northern Plains into WI ahead of a surface front that
   should settle south into this region during the day. Current
   thinking is ongoing thunderstorm complex over SD will spread into MN
   ahead of the short wave. This activity should propagate into the
   upper MS Valley region and possibly reintensify as boundary-layer
   warming aids buoyancy. While forecast soundings do not exhibit
   strong shear, adequate flow will exist for organized thunderstorm
   clusters capable of producing wind/hail. Behind the short wave, it's
   not entirely clear how productive the surface front will be draped
   across southwest MN into southeast SD. Forecast soundings suggest
   convective temperatures will be breached and there is some reason to
   believe isolated discrete storms will evolve after 22z.

   Of more concern will be regions north of the frontal zone across the
   northern High Plains. Southeasterly low-level flow will maintain
   moisture across southern MT where strong boundary-layer heating is
   forecast. Latest model guidance suggests a weak mid-level
   disturbance will approach south-central MT by 07/00z. If this
   feature is real there is increasing confidence that scattered
   thunderstorms will evolve within a wind profile favorable for
   supercells. NAM forecast sounding for 4BQ at 07/01z exhibits
   surface-6km shear of 60kt with effective SRH in excess of 300 m2/s2.
   With surface dew points forecast near 60F, cloud bases should be
   adequately low for a threat of tornadoes. Additionally, latest CAMs
   are fairly aggressive across southeast MT into northwest SD. An
   upscale-evolving complex is possible by late evening as LLJ
   strengthens into southwest SD after sunset. 

   ...Middle Atlantic...

   Strong short-wave trough is forecast to shift off the New England
   Coast early in the period. As a result, northwesterly flow will be
   maintained across the Middle Atlantic, albeit relatively weak with
   roughly 20kt expected at 500mb through late afternoon. As the short
   wave passes, surface front should orient itself southeast-northwest
   across NY as surface high settles over southern ME. This frontal
   orientation should prove favorable for strong heating across much of
   the Middle Atlantic and surface temperatures should warm rapidly by
   early afternoon. NAM forecast soundings by early afternoon suggest
   surface parcels will reach their convective temperatures in the
   upper 80s/90F. With SBCAPE between 2000-3000 J/kg, robust updrafts
   are expected due to relatively steep lapse rates. Locally damaging
   winds are expected with storms that form within this northwesterly
   flow regime.

   ..Darrow/Moore.. 07/06/2020

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