SPC AC 061954

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0254 PM CDT Mon Jul 06 2020

   Valid 062000Z - 071200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
   NORTHERN PLAINS/UPPER MIDWEST...AND THE MID-ATLANTIC...

   ...SUMMARY...
   Severe thunderstorms are expected over parts of the northern Plains
   to Upper Midwest this afternoon and evening, and also across the
   Mid-Atlantic States.

   ...20z Update...

   The Marginal risk area over TX has been shifted/expanded eastward
   with this update. This is based on the current location of a MCV,
   centered southwest of the Fort Worth area. Strong heating and richer
   boundary layer moisture (dewpoints low to mid 70s F) ahead of this
   feature are leading to an axis of greater instability across east
   TX, where MLCAPE values are currently as high as 2500 J/kg. Latest
   visible satellite imagery also shows deepening CU, and thunderstorms
   are expected through this evening. Strong downburst winds will be
   the main hazard with these storms given weak vertical shear but
   strong instability/high PW airmass. 

   Minor changes were made to the Slight risk area across the northern
   Plains. The Slight risk was expanded modestly across northeast NE
   and the Sioux Falls vicinity. Latest CAMs guidance and surface
   trends suggest strong to severe storms could develop southward this
   evening, and/or shift eastward across SD/NE during the evening
   overnight, posing a threat for strong winds and hail.

   ..Leitman.. 07/06/2020

   .PREV DISCUSSION... /ISSUED 1136 AM CDT Mon Jul 06 2020/

   ...Upper Midwest/Upper Great Lakes...
   Multiple MCVs and iterations of modifying outflow will influence
   much of the region ahead of slow-southeastward-moving cold front.
   However, somewhat more coherent/appreciable insolation is occurring
   at midday across parts of Wisconsin into Upper Michigan, where
   thunderstorm development should be further influenced by an
   eastward-moving mid-level impulse over northern Minnesota. The
   strongest deep-layer shear will be relegated to northern
   Wisconsin/western Upper Michigan, but a moist/unstable air mass will
   support the development/intensification of multiple eastward-moving
   thunderstorm clusters across Wisconsin and Upper Michigan this
   afternoon. Some severe hail is possible aside from more probable
   severe wind gusts, particularly as storms modestly organize/spread
   eastward through late afternoon/early evening.

   Farther west/southwest, other at least isolated severe thunderstorms
   are expected to develop across southeast South Dakota and southern
   Minnesota as outflow modifies across these areas into the afternoon
   and appreciable boundary layer destabilization occurs.
   Deep-layer/low-level winds will not be overly strong, but a few
   transient supercells and prevalent multicells are expected to yield
   at least isolated instances of severe hail/damaging winds late this
   afternoon and evening.

   ...Northern High Plains including MT/ND/SD/WY...
   Thunderstorm development, with some potential for supercells and
   eventual/upscale cold-pool growth, is expected over parts of
   southeast Montana and northeast Wyoming into southwest North
   Dakota/western South Dakota later this afternoon. All severe hazards
   are possible with this activity. The frontal zone and a weak surface
   trough should be the primary foci, with strong surface heating
   augmenting boundary lift to erode MLCINH. A moist axis,
   characterized by 60s F surface dew points, will contribute to
   1000-2000 J/kg MLCAPE over the area, with 35-45-kt effective shear
   and a strong veering of winds with height.

   Farther south, other at least isolated/higher-based deep convective
   development may occur across northern/eastern Wyoming and the Black
   Hills vicinity into western Nebraska, although overall coverage may
   be influenced by shortwave ridging and associated mid/upper height
   rises (especially with southern extent). However, a
   supercell/multicell-favorable CAPE/shear parameter will be in place,
   and at least isolated severe thunderstorms may occur late this
   afternoon and evening.

   ...Mid-Atlantic/Northeast States/Delmarva...
   Regional 12Z observed soundings sampled semi-cool mid-level
   temperatures (10-12C at 500 mb) this morning in conjunction with a
   moderately strong (by seasonal standards) belt of west-northwesterly
   flow aloft. Morning satellite trends show the dissipation of some
   morning cloud cover in vicinity of the Delmarva, with temperatures
   otherwise quickly warming through the 80s F with ample isolation.
   Give the prevalence of lower 70s F surface dewpoints, this suggests
   MLCAPE upwards of 3000 J/kg is plausible this afternoon with
   convective inhibition already minimal.

   An initial development/intensification of thunderstorms appears
   mostly like to occur across portions of central/eastern Pennsylvania
   into New Jersey, and also in vicinity of the mountains/Blue Ridge
   from parts of Maryland southwestward along the Virginia/West
   Virginia border region. Damaging winds and isolated severe hail will
   be possible. Over time, pending the number of storms, outflows may
   aggregate with some degree of upscale quasi-linear
   growth/organization as storms propagate south-southeastward toward
   and across the Delmarva/eastern Virginia vicinity. If so, this
   suggests a somewhat broader/less-isolated damaging wind potential
   may evolve across the region by late afternoon.

   ...Central/north-central Texas...
   Clouds/some precipitation lingers late this morning near/south of a
   convectively augmented frontal boundary, but thinning cloud cover
   this afternoon should allow for pockets of moderate destabilization
   within a moist air mass. A lingering weak MCV or two will influence
   thunderstorm redevelopment along with zone of differential heating.
   Sufficient heating/instability may exist for a couple of storms to
   produce near-severe-caliber wind gusts this afternoon/early evening.

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