SPC AC 061955

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0255 PM CDT Thu Aug 06 2020

   Valid 062000Z - 071200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THROUGH TONIGHT
   ACROSS PARTS OF THE NORTHERN ROCKIES AND ADJACENT PORTIONS OF THE
   NORTHERN/CENTRAL GREAT PLAINS...AND PARTS OF THE CENTRAL AND
   SOUTHERN APPALACHIANS AND PIEDMONT...

   ...SUMMARY...
   Strong thunderstorms may impact parts of the northern Rockies and
   adjacent portions of the northern and central Great Plains late this
   afternoon and evening, accompanied by some risk for severe wind and
   hail.  Locally strong wind gusts may also accompany thunderstorms
   across the central and southern Appalachians and Piedmont this
   afternoon through tonight.

   ...Discussion...
   The only minor change this outlook update was to remove 5-percent
   wind probabilities over eastern KS for forecast thunderstorm
   activity late tonight.  An MCV over northern OK will continue
   eastward towards the western part of the Ozark Plateau this
   afternoon into the early evening.  Additional storms are forecast to
   develop this afternoon and a few of these may briefly be capable of
   isolated wind damage.  Elsewhere, the forecast remains largely
   unchanged.

   ..Smith.. 08/06/2020

   .PREV DISCUSSION... /ISSUED 1130 AM CDT Thu Aug 06 2020/

   ...Synopsis...
   Near the leading edge of a belt of strong zonal flow extending
   across the northern mid-latitude Pacific, the remnants of a fairly
   significant mid-level low continue to undergo deformation, with
   negatively tilted mid-level troughing now spreading across the
   British Columbia/Pacific Northwest coast.  This has come in phase
   with troughing in a weaker southern branch of westerlies, across
   California.  But models indicate that this will be short-lived, with
   the trough in the northern branch pivoting inland across the
   Canadian and northern U.S. Rockies by late tonight, leaving broad,
   weak troughing trailing to the southwest, across and west of the
   California coast.

   Within the branching downstream westerlies, mid/upper flow will
   remain generally weak across much of Canada and the U.S., in the
   wake of a significant short wave trough accelerating across and to
   the east of the Canadian Maritimes.  This is to the north of
   prominent mid-level ridging over the southern
   mid-latitude/subtropical western Atlantic, which may maintain a
   considerable influence as far west as southern Atlantic coastal
   areas.

   Fairly prominent subtropical ridging may also continue gradually
   shifting east of the central and southern Great Plains, with
   residual, weak, positively tilted mid-level troughing across the
   lower Great Lakes into portions of the central Gulf States, between
   the two ridges.  This includes one substantive short wave impulse
   gradually pivoting through the lower Ohio Valley.

   In association with this regime, warm elevated mixed-layer air will
   continue to gradually overspread the Great Plains, with a plume of
   seasonably high moisture content beginning to advect north-northeast
   of the southeastern Great Plains.  A plume of high moisture content
   also lingers ahead of the weak eastern troughing, around the western
   through northern periphery of the western Atlantic subtropical
   ridge. This generally extends along the southern into middle
   Atlantic Seaboard, into the vicinity of a frontal zone extending
   along and east of the northern Mid Atlantic and southern New England
   coast.

   ...Appalachians and Piedmont...
   Residual low-level moisture, coupled with daytime heating, appears
   likely to be sufficient to contribute to mixed-layer CAPE of
   1000-2000+ J/kg by this afternoon.  Aided by large-scale ascent
   ahead of the weak mid-level troughing, orographic forcing may
   contribute to the initiation of numerous thunderstorms, with
   guidance suggesting at least some potential to grow
   upscale/consolidate across the Virginia and Carolina Piedmont by
   this evening.  Perhaps aided by 10-20 kt southwesterly deep-layer
   mean flow and heavy precipitation loading, this convection will be
   accompanied by the potential to producing localized damaging wind
   gusts.

   ...Northern Rockies into adjacent portions of Great Plains...
   With sufficient low-level moisture to contribute to moderately large
   CAPE beneath steep mid-level lapse rates, surface troughing to the
   lee of the Rockies seems to provide the primary focus for widely
   scattered strong to severe thunderstorm development late this
   afternoon.  This activity may mostly initiate off the higher
   terrain, aided by orographic forcing, but lift ahead of the inland
   migrating short wave trough may contribute to convective development
   across Montana, where instability may remain weaker.  In general,
   due to a combination of factors, the environment appears conducive
   to a sparse coverage of any severe wind/hail.

   ...Lower Great Plains...
   Thunderstorm development and associated severe weather potential
   east of the eastward advecting elevated mixed-layer air remain more
   unclear.  However, models suggest that stronger low-level warm
   advection supportive of appreciable thunderstorm development will
   become generally focused across parts of eastern Kansas and adjacent
   portions of the plains later tonight, to the east of a nocturnally
   strengthening low-level jet.

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