SPC AC 251949

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0249 PM CDT Mon Jun 25 2018

   Valid 252000Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON AND EVENING ACROSS MUCH OF THE LOWER MISSOURI VALLEY
   REGION...

   ...SUMMARY...
   Isolated severe thunderstorms are possible late this afternoon and
   evening across much of the lower Missouri Valley, while other severe
   thunderstorms are expected across the southern Appalachians vicinity
   and Carolinas during the afternoon and evening.

   ...20Z Outlook Update..
   Some changes to categorical and probabilistic lines have been made,
   mainly across the lower Missouri Valley region to account for
   ongoing trends concerning destabilization and apparent convective
   potential.  

   Strongest destabilization, characterized by moderate CAPE in excess
   of 2000 J/kg has been largely focused across parts of central and
   southern Missouri into adjacent portions of the lower Missouri
   Valley.  And there appears a signal in latest model output,
   including recent runs of the high-resolution Rapid Refresh, that the
   warm frontal zone near through east southeast of the St. Louis MO
   area may become a focus for strong/severe thunderstorm development
   late this afternoon and evening.  Deep layer shear appears more than
   sufficient for organized convective development, including isolated
   supercells and an upscale growing convective system.  Supporting
   this idea, deepening convective development is now evident to the
   west and southwest of the St. Louis area, where 2 hourly 2 mb
   surface pressure falls have been observed in 19Z surface data.

   Otherwise, along the warm frontal zone to the northwest,
   destabilization has remained more modest across northern Missouri
   into southern Iowa, with persistent rain continuing to inhibit
   boundary layer destabilization across western Iowa into the middle
   Missouri Valley.  However, forcing for ascent aided by the
   approaching closed low could still provide support for strong/severe
   thunderstorm development where the boundary layer has destabilized. 
   This may include the risk for a tornado or two, though stronger
   low-level shear appears to be in the process of becoming focused to
   the cool/stable boundary layer side of the frontal zone.  If strong
   storm development can commence within the next hour or two, this
   potential may be maximized near the Missouri/Iowa border vicinity.

   ..Kerr.. 06/25/2018

   .PREV DISCUSSION... /ISSUED 1104 AM CDT Mon Jun 25 2018/

   ...IA/MO area this afternoon/evening...
   A closed midlevel low over the southwest NE/northwest KS border will
   drift generally eastward toward IA/MO by the end of the period, as
   an upstream shortwave trough crosses the northern Rockies.  At the
   surface, a weak cyclone will move northward from northeast KS to
   eastern NE as an embedded speed max rotates around the NE closed
   low.  A diffuse warm front will move northward with the surface
   cyclone, and a belt of southeasterly low-level flow (30-40 kt at 850
   mb) will be maintained along the warm front.  Assuming enough cloud
   breaks occur to allow destabilization along the fringe of the
   ongoing band of rain and thicker clouds, there will be some
   potential for supercells and a couple of tornadoes this
   afternoon/evening across southern/central IA.

   Farther south, a corridor of low-mid 70s boundary-layer dewpoints
   will be maintained along the MS River, to the south of a synoptic
   front and southwest of early convection and a weak MCV moving
   eastward over the lower OH Valley.  Some cloud breaks by this
   afternoon will allow surface temperatures to warm well into the 80s,
   resulting in afternoon MLCAPE approaching 2500-3000 J/kg.  Some
   enhancement to midlevel flow (30-40 kt) is expected with the
   approach of a vorticity lobe rotating around the southeastern
   periphery of the NE closed low, which will favor organized
   multicells and some supercell structures as convection develops
   southward in advance of the diffuse wind shift across MO.  Some
   clustering of the storms will be possible across east central and
   southeast MO, where the damaging wind probabilities have been
   increased.

   ...Southern Appalachians into the Carolinas this
   afternoon/evening...
   A series of remnant MCVs will move from KY toward VA/NC around the
   southwestern periphery of a midlevel trough over New England. 
   Surface heating is underway across the Carolinas in the wake of
   overnight convection and along/south of the thicker cloud band and
   slow-moving synoptic front from eastern KY to north central NC.  As
   low-level destabilization continues, thunderstorm development is
   expected this afternoon from northeastern TN and southeastern KY
   across western and central NC in the zone of differential heating
   and near the southern fringe of the remnant MCVs now crossing
   southwestern VA and KY/TN.  Multicell clusters will be the primary
   convective mode, with the potential for occasional wind damage with
   downbursts through this evening as storms coalesce on outflows and
   move southeastward toward the coast.

   ...Central OK to southwest MO this afternoon into tonight...
   The remnants of overnight convection linger this morning across
   central OK, though this convection should weaken by midday.  Gradual
   thinning of cloud debris and modification of the outflow air mass
   from overnight storms will result in destabilization up to the I-44
   corridor, just southeast of the quasi-stationary front in OK. 
   Renewed thunderstorm development will be possible late this
   afternoon continuing into tonight along the boundary, where moderate
   buoyancy and the southern periphery of the stronger mid-upper flow
   will support a mix of multicell clusters and marginal supercells
   capable of producing isolated damaging gusts and large hail. 

   ...Southwest into north central MT this evening...
   A pronounced midlevel trough and embedded jet streak crossed eastern
   WA early this morning, while the larger-scale trough will move
   eastward over the northern Rockies later today through tonight. 
   Low-level moisture is somewhat limited this morning, though strong
   surface heating and residual boundary-layer dewpoints around 50 F
   will support weak surface-based CAPE this afternoon.  Some
   high-based convection is expected to form over the higher terrain of
   southwestern MT late this afternoon and subsequently spread
   northeastward across west central into north central MT late this
   evening through early tonight.  An increase in midlevel flow and
   inverted-v profiles in the low levels will support a threat for
   isolated strong/damaging outflow winds and marginally severe hail
   for a few hours this evening.

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