SPC AC 131254

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0754 AM CDT Thu Jun 13 2019

   Valid 131300Z - 141200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
   NORTHEASTERN NEW MEXICO ACROSS PARTS OF THE TEXAS AND OKLAHOMA
   PANHANDLES...

   ...SUMMARY...
   The greatest concentration of severe thunderstorms today should be
   over parts of the south-central High Plains, with damaging wind and
   hail expected.

   ...Synopsis...
   In mid/upper levels, a high-amplitude synoptic trough was evident in
   moisture-channel imagery from the Hudson Bay area across the Ohio
   and Tennessee Valleys.  The northern component -- a closed cyclone
   centered near the northern tip of ON -- will shift slowly south-
   southeastward across northern ON through the period.  As this
   occurs, the open-wave southern component of the trough will move
   east-northeastward to NY, the southern Mid-Atlantic region, and the
   Carolinas.  Upstream, a weaker but still well-defined trough over OR
   and northern CA will shift eastward across the Great Basin and lower
   Colorado River region.

   The 11Z surface analysis showed an occluded cyclone over
   southeastern Lower MI, and another surface low over the Hampton
   Roads region of southeastern VA. A cold front extended from the
   latter low across coastal SC and the central/eastern FL Panhandle,
   becoming quasistationary over the northwestern Gulf and the Rio
   Grande area of southwest TX and adjoining northern MX.  The TX/MX
   segment of this front will become more diffuse through the period as
   return flow and low-level warm advection occur through and north of
   the frontal zone.  The FL portion of the front will move slowly
   southeastward and decelerate, preceded by a low-level convergence
   zone now extending southwest/northeast across the northern part of
   the peninsula.  A lee trough will deepen over eastern parts of
   CO/NM.

   ...Southern/central High Plains...
   Widely scattered to scattered, predominantly high-based
   thunderstorms are forecast to develop near the lee trough and where
   areas of diurnally heated higher terrain extend into the plume of
   return-flow moisture.  A few supercells are possible with large hail
   and severe gusts, the greatest confidence and potential
   concentration being in northeast New Mexico, downshear into parts of
   the TX/OK Panhandles.  The latter area will feature the most
   sustained zone of well-heated higher terrain and at least marginal
   moisture, along with a favorable CAPE/shear parameter space for
   early-stage supercells.  Upscale growth of convection in this regime
   is most probable relative to areas farther north and south, with the
   threat of a southeastward-moving, forward-propagational,
   severe-wind-producing MCS.  Isolated severe hail/gusts may occur
   elsewhere from far west TX to eastern CO and western KS.

   This should occur despite shortwave ridging over the area that will
   contribute to modest height rises through the day.  Activity will be
   supported by a narrow corridor of favorable return-flow moisture,
   generally characterized by surface dew points in the mid 40s to mid
   50s F.  Strong surface heating and steep low/middle-level lapse
   rates (only slightly less than dry adiabatic) will yield peak
   preconvective MLCAPE values of 1000-1500 J/kg, despite the large
   dewpoint depressions.  The related well-mixed subcloud layer will
   support organized severe-wind potential with any aggregating cluster
   that can develop during late afternoon into evening.  The convection
   should weaken tonight upon moving away from the low-level moist axis
   and encountering progressively more diabatically stabilized
   boundary-layer air.

   ...Central Intermountain West...
   Widely scattered to scattered thunderstorms are expected this
   afternoon as the Pacific Coast States mid/upper trough moves
   eastward.  That, along with diurnal surface heating, will reinforce
   already steep low/middle-level lapse rates across the region,
   preferentially removing MLCINH on higher elevations first.  Lack of
   greater moisture will temper buoyancy overall; however, pockets of
   500-1000 J/kg MLCAPE are possible atop a well-mixed boundary layer. 
   This will support the potential for locally strong-severe gusts from
   the most intense clusters and quasi-linear convective segments.  The
   threat should diminish after sunset with formation and rapid
   deepening of stable near-surface layers.

   ...Interior Mid-Atlantic...
   Widely scattered to scattered thunderstorms should develop through
   this afternoon, offering the potential for isolated severe gusts. 
   Small hail also cannot be ruled out.  Convection should form most
   commonly as the peak period of diurnal warming behind the departing
   morning cloud/precip plume removes already weak MLCINH.  This will
   underlie strong midlevel cooling/destabilization forecast with the
   approach of the Great Lakes portion of the mid/upper trough.  Time
   series of forecast soundings indicate substantial associated
   steepening of low/middle-level lapse rates (around 7 deg c/km from
   surface-500 mb).  Adequate residual moisture will be in place, with
   surface dew points generally in the 50s, to yield 500-1000 J/kg
   MLCAPE atop a well-mixed boundary layer.  Effective-shear magnitudes
   will be weak (less than 30 kt) amidst nearly unidirectional deep-
   layer flow.  Still, the most intense cells may produce damaging to
   isolated/brief severe gusts.

   ...Central/eastern FL Peninsula...
   Scattered to locally numerous thunderstorms should form from midday
   through the afternoon hours, southeast of an ongoing/nonsevere
   convective band over northern FL, in an area of sustained/strong
   diurnal heating.  With an ambient southwesterly to westerly flow
   regime aloft, activity should both develop along, and move eastward
   to interact with, the Atlantic coastal sea-breeze front.  That
   corridor will have the relatively maximized potential for storm/
   boundary interactions with locally damaging to severe gusts, given
   the dense concentration of convection expected and the potential for
   deep, water-loaded downdrafts.  Modified RAOBs and forecast
   soundings indicate that the moist boundary layer (dew points 70s F,
   mean mixing ratios around 17 g/kg) and depth of troposphere will
   offset modest mid/upper-level lapse rates enough for preconvective
   MLCAPE 2000-2500 J/kg -- locally higher.

   ..Edwards/Goss.. 06/13/2019

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