SPC AC 281259

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0759 AM CDT Mon May 28 2018

   Valid 281300Z - 291200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS PRIMARILY OVER
   THE CENTRAL HIGH PLAINS...

   ...SUMMARY...
   Severe thunderstorms with isolated very large hail, wind damage and
   a tornado threat are expected across parts of the Great Plains.  A
   couple of brief tornadoes will also be possible in the Southeast in
   conjunction with Alberto.

   ...Synopsis...
   The main upper-air feature for this forecast will be a broad cyclone
   -- its center evident in moisture-channel imagery over western UT. 
   As the associated synoptic-scale trough moves slowly eastward
   through the period, a series of shortwave troughs and vorticity
   maxima will pivot through the southern and eastern quadrants of the
   cyclone -- most notably a perturbation now located over AZ.  That
   feature is expected to eject northeastward over CO toward the CO/WY
   state line by 00Z.  This should become the primary vorticity center
   and 500-mb low overnight across eastern WY.

   Meanwhile, Subtropical Storm Alberto is forecast by NHC to move
   inland over the western FL Panhandle today and AL thereafter, moving
   generally north-northwestward into a longstanding mid/upper height
   weakness over the east-central U.S.  Please refer to NHC advisories
   for specific track/intensity guidance on Alberto, as well as related
   tropical watches/warnings.

   At the surface, 11Z analysis showed a wavy frontal zone was analyzed
   from the Delmarva Peninsula across central PA to a low over southern
   ON, then westward over southern/central Lower MI, being overtaken by
   a convective outflow pool across WI.  The frontal zone was evident
   again from a low over southern MN southwestward to eastern CO. 
   except where effectively shifted by mesoscale and smaller convective
   processes, the central U.S. frontal segment should move little
   through this evening.  A dryline -- initially drawn from
   southeastern CO roughly southward across the TX/NM border region to
   between FST-MRF, is expected to move just slightly eastward today
   across southeastern CO, the western OK/TX Panhandles and the Llano
   Estacado.

   ...High Plains region...
   Convective coverage on the High Plains is expected to be even
   greater than yesterday as the cyclone aloft and associated shortwave
   perturbation approach, and low-level mass response occurs amidst
   generally greater boundary layer moisture content from eastern
   CO/southwestern NE southward.  Scattered to locally numerous
   thunderstorms are forecast to develop this afternoon near the
   dryline, as well as the Palmer Divide area and Front Range to its
   north.  Initial convection may assume supercellular characteristics,
   offering the potential for a few tornadoes, along with damaging/
   significant hail greater than 2 inches in diameter.  Activity should
   aggregate with time, forming outflow-dominant/cold-pool-driven
   clusters across portions of CO/southeastern WY and NE, and a
   generally meridional belt of severe convection from southwestern NE
   across western KS into the TX Panhandle.  Coverage generally should
   diminish with southward extent across the Panhandle toward the South
   Plains and adjacent Caprock.

   Upslope/post-frontal flow across the CO/WY portion of the outlook
   will provide a combination of enhanced storm-relative flow, enlarged
   low-level hodographs and lift in support of the severe potential 
   there, with MLCAPE generally 1000-2000 J/kg, beneath strengthening
   large-scale lift and southwesterly deep-shear vectors.  Meanwhile,
   somewhat weaker shear but substantially greater buoyancy is expected
   ahead of the dryline.  In that regime, warm-sector dew points 60s F
   and strong diabatic heating, beneath steep deep-tropospheric lapse
   rates, will drive preconvective MLCAPE to around 3000 J/kg, locally
   higher.  For what relatively discrete storms remain, a window of
   relatively maximized tornado potential may develop within a couple
   hours after sunset as the LLJ enlarges low-level hodographs. 
   Thereafter, overall severe potential should diminish through the
   evening, as the airmass stabilizes from a combination of diabatic
   cooling and expansive outflow coverage.  However, a few clusters of
   strong-severe storms may linger into the overnight hours across
   portions of the central/northern Plains.

   ...Southeast...
   Isolated, transient supercells are possible east through north of
   the center of Alberto as it penetrates inland today, and a tornado
   or two may occur.  Any more-concentrated tornado potential remains
   too conditional for a categorical upgrade at this time, but
   mesoscale boundary and convective trends will be monitored for the
   emergence of a more tightly focused zone of potential within this
   rather broad marginal risk.

   IR and early VIS imagery show a broad area of relative breaks in
   cloud cover over much of the existing marginal-risk area, indicating
   favorable duration/intensity of diabatic destabilization.  Despite
   modest lapse rates aloft -- characteristic of landfalling tropical
   and subtropical cyclones -- boundary-layer heating with surface dew
   points commonly 70s F may boost MLCAPE into the 1000-2000 J/kg
   range.  Kinematically, areas forecast to experience the strongest,
   most sustained heating away from the central/core convective region
   of the cyclone also will have weaker near-surface winds, with
   generally smaller hodographs and low-level shear.  The possible
   exception would be differential-heating/precip-reinforced
   boundaries, and any inner bands that have access to inflow air from
   the more strongly heated sector.  Otherwise, an inner-band supercell
   may form every few hours or so in relatively weak buoyancy but
   stronger low-level shear, offering at least conditional/brief
   tornado potential.

   ..Edwards/Peters.. 05/28/2018

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