SPC AC 121628

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1128 AM CDT Wed Aug 12 2020

   Valid 121630Z - 131200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON AND EVENING ACROSS PARTS OF WESTERN AND NORTHERN NORTH
   DAKOTA...

   ...SUMMARY...
   Widely scattered strong storms may impact portions of the the
   northern and central Great Plains late this afternoon into tonight,
   accompanied by at least some risk for severe hail and wind.  Other
   thunderstorms with locally strong wind gusts are possible near and
   east of the Blue Ridge Mountains into the northern Mid Atlantic
   coast vicinity.

   ...Synopsis...
   A belt of westerlies emanating from northern mid-latitude Pacific
   has undergone some amplification, and this appears likely to
   transition eastward near/north of the Canadian/U.S. border today
   through tonight.  This includes large-scale troughing across western
   Canada and the Pacific Northwest, gradually shifting east of the
   Canadian and northern U.S. Rockies, through the Canadian Prairies
   and higher portions of the northern U.S. Great Plains.  Downstream
   ridging is forecast to continue to build across lower portions of
   the northern Great Plains, Upper Midwest and adjacent Great Lakes
   region through the Hudson Bay vicinity.

   In lower latitudes, mid-level subtropical ridging will remain
   influential across much of the central and southern tier of the
   U.S., including a prominent high centered over parts of the
   Southwest into the southern Great Plains.

   Steep lower/mid tropospheric lapse rates associated with a plume of
   warm and capping elevated mixed-layer air will be maintained across
   much of the Great Plains.  Seasonably high moisture content lingers
   south of a weak surface front extending across parts of the
   Northeast into the Ohio Valley, and generally to the east of weak
   mid/upper troughing within the larger-scale ridging across the lower
   Mississippi Valley.

   ...Great Plains...
   Coverage of thunderstorm development and associated severe weather
   potential remain unclear for today through tonight.  Stronger
   forcing for ascent associated with a short wave impulse migrating
   northeastward out of the base of the larger-scale troughing appears
   likely to remain mostly north of the international border, but could
   perhaps extend southward into the vicinity of lee surface troughing
   across parts of northwestern North Dakota late this afternoon and
   evening.  Otherwise, the lee surface trough extending southward
   through central portions of the higher plains could provide a focus
   for widely scattered late afternoon and early evening thunderstorm
   initiation.  Additional thunderstorms may initiate, aided by warm
   advection, later tonight, east of the warmer and more strongly
   capping elevated mixed-layer air across parts of northern and
   central Minnesota, as well as portions of the central Great Plains.

   Where storms form, thermodynamic profiles characterized by steep
   mid-level lapse rates and sizable CAPE, coupled with perhaps modest
   shear, may provide support for severe hail and locally strong
   surface gusts.

   ...Northern Mid Atlantic into central Appalachians...
   Orographic forcing along the Appalachians may provide the primary
   support for thunderstorm initiation this afternoon, along with
   perhaps weak pre-frontal low-level convergence extending east of the
   higher terrain, across southern Pennsylvania into the northern Mid
   Atlantic coast region.  Although deep-layer ambient mean flow is
   generally on the order of 10 kt or less, seasonably high moisture
   content and sizable CAPE may contribute to heavy precipitation
   loading and the risk for potentially damaging wind gusts in the
   stronger storms.

   ..Kerr/Cook.. 08/12/2020

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