SPC AC 051954

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0254 PM CDT Fri Jun 05 2020

   Valid 052000Z - 061200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON AND EVENING ACROSS PARTS OF THE LOWER MISSISSIPPI
   VALLEY...

   ...SUMMARY...
   Severe thunderstorms are possible late this afternoon into tonight
   across the lower Mississippi Valley. Other strong to severe
   thunderstorms may occur over the Northern Rockies and adjacent High
   Plains.

   ...20Z Outlook Update...
   There has been considerable adjustment to the categorical and
   probabilistic lines, in an attempt to better reflect convective
   potential based on latest trends in observational date and latest
   model output.

   ...Great Lakes vicinity...
   Mid-level forcing for ascent associated with short waves within the
   mid-latitude westerlies is generally focused to the north and
   northeast of the Great Lakes, and pre-frontal low-level convergence
   across southern portions of the Great Lakes region remains weak. 
   With boundary-layer destabilization remaining modest, and strongest
   south of the more favorable mid-level flow, severe weather
   probabilities through the remainder of the afternoon and evening
   appear less than 5 percent.

   ...Central Appalachians/Mid Atlantic...
   West-southwesterly deep-layer mean ambient flow across the region
   appears only on the order of 20 kts, with maximum CAPE a rather
   modest 1500 J/kg or so, largely focused from the immediate lee of
   the Blue Ridge, east-northeastward toward the northern Mid Atlantic
   coast.  This appears largely aligned with stronger surface
   heating/steeper low-level lapse rates within lee surface troughing. 
   Given some ongoing clustering of convection across this region,
   ahead of weak mid-level troughing, there could still be some
   increase in potential for strong surface gusts (perhaps aided by
   precipitation loading) with storms overspreading the northern
   Virginia, Washington D.C. and Baltimore vicinity late this
   afternoon.

   ...Great Basin...
   Strengthening southwesterly mid-level flow, coupled with the
   evolution of a deeply-mixed boundary layer, could provide support
   for strong wind gusts with thunderstorm development late this
   afternoon.  However, boundary-layer destabilization appears likely
   to remain quite weak, and anything more than very isolated
   thunderstorm initiation seems unlikely.  Due to the anticipated
   sparse coverage of storms, severe probabilities now appear less than
   5 percent.

   ..Kerr.. 06/05/2020

   .PREV DISCUSSION... /ISSUED 1129 AM CDT Fri Jun 05 2020/

   ...Mid-South area through early tonight...
   A remnant MCS (with an MCV) and surface outflow continue to move
   south-southeastward over the MO Bootheel as of mid-late morning. 
   The storms are expected to develop southeastward along a diffuse
   boundary toward western TN this afternoon, as well as southward into
   the unstable warm sector across AR.  Strong surface heating south of
   the ongoing convection and along the outflow boundary will
   contribute to large buoyancy this afternoon from AR into northern MS
   and western TN, where MLCAPE will likely reach 3000-4000 J/kg. 
   Though vertical shear will remain relatively weak over this area,
   the large buoyancy and strong downdraft potential (DCAPE in excess
   of 1200 J/kg) will favor multicell clusters capable of producing
   occasional damaging gusts.

   ...Northern Rockies this evening into early tonight...
   A northern stream midlevel trough will continue to progress eastward
   toward the Pacific Northwest, with strengthening deep-layer
   flow/shear expected downstream over the northern Rockies through
   tonight.  A few thunderstorms are expected along the primary
   baroclinic zone from ID into western MT through tonight, where wind
   profiles will favor organized severe storms.  However, low-level
   moisture/buoyancy will be somewhat limited in this same zone, which
   casts some doubt regarding the magnitude of the threat.  Will
   maintain low wind/hail probabilities in this update, but will
   continue to monitor through the afternoon for signs of any increase
   in the severe-storm threat.

   ...Northern High Plains overnight...
   Height rises are expected today across the northern High Plains,
   downstream from the trough approaching the Pacific Northwest. 
   Isolated high-based storms may form this afternoon near the higher
   terrain from WY into southern MT, but capping should limit the
   spatial extent and duration of these storms.  Warm advection will
   increase overnight across the northern High Plains in response to a
   strengthening low-level jet.  Isolated large hail may occur with
   elevated convection from eastern MT into western ND late in the
   period.

   ...NC to the Mid-Atlantic this afternoon/evening...
   A weak midlevel trough is moving eastward over the central/southern
   Appalachians as of midday.  Convergence along a weak associated
   surface trough near the Blue Ridge, in combination with daytime
   heating and boundary-layer dewpoints in the upper 60s, will support
   thunderstorm development through early-mid afternoon both along the
   trough/higher terrain, and in the moist axis farther southeast. 
   Regional 12z soundings revealed rather poor lapse rates which will
   tend to limit buoyancy somewhat, and also the potential for strong
   downdrafts.  Still, there should be enough steepening of low-level
   lapse rates to support a low-end downburst threat with multicell
   clusters spreading eastward this afternoon/evening.

   ...OH Valley/Great Lakes area this afternoon/evening...
   A few thunderstorms will be possible along a surface cold front and
   in advance of a midlevel trough crossing the upper Great Lakes. 
   Destabilization in advance of the front may be sufficient for a few
   strong storms with marginally severe wind/hail as far east as
   southeast Lower MI later this afternoon.  Buoyancy will be larger to
   the west, but forcing for ascent becomes more nebulous with westward
   extent, from northern IL westward, which will likely limit storm
   coverage.

   ...Four Corners area this afternoon into early tonight...
   A midlevel low off the northern Baja/southern CA coast is beginning
   to eject northeastward in response to height falls approaching the
   Pacific Northwest coast.  Moisture is initially limited in advance
   of this trough, but there should be sufficient midlevel moisture for
   a few high-based thunderstorms this afternoon into early tonight. 
   Deep inverted-v profiles and some increase in midlevel flow will
   support a few (mainly dry) thunderstorms with strong outflow winds.

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