SPC AC 271250

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0750 AM CDT Wed May 27 2020

   Valid 271300Z - 281200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER CENTRAL
   TEXAS...

   ...SUMMARY...
   The main severe-weather threat today will be over central Texas and
   vicinity, where very large hail, severe wind, and a few tornadoes
   are possible.

   ...Synopsis...
   Split flow will characterize the upper-air pattern across the CONUS
   today, with the main feature being a cyclone centered initially over
   southeastern OK.  The associate 500-mb low should drift
   northeastward over the Ozarks through the period, while cyclonic
   flow persistently covers much of the area from the southern/central
   Plains to the southern Appalachians, lower Ohio Valley and much of
   the Gulf Coast.  To its southeast, a weak but persistent shortwave
   trough -- evident in moisture-channel imagery from western/southern
   GA southeastward toward the northern Bahamas -- will move northward
   and weaken further.  An associated low/middle-level vortex --
   centered just offshore from CHS -- will move generally northwestward
   to upstate SC, western NC and southwestern VA through the period,
   while gradually weakening inland.

   At the surface, a cold front was analyzed from extreme northern ON
   across western Lake Superior, becoming wavy and quasistationary
   through an elongated area of low pressure over southeastern MN, IA
   and western MO, then to western AR and north-central TX, to a low
   near INK. Only minor/mesoscale movements of the baroclinic zone are
   expected away from convective outflow, given the rather unchanging
   pattern above.

   ...Central TX and vicinity...
   Scattered thunderstorms are forecast this afternoon across parts of
   central TX, over and near the Hill Country, some of which should be
   supercells offering the full spectrum of severe hazards. This will
   include the threat for very large/damaging hail.  Convection may
   evolve into one or more forward-propagational clusters this evening
   into tonight, with the severe threat becoming more weighted toward
   convective wind, though hail and some tornado threat may remain.

   This activity will occur in a field of strong northwesterly mid/
   upper-level flow around the southwestern rim of the cyclone aloft,
   with 500-mb wind speeds of 55-65 kt over the region.  Preconvective
   surface winds mostly will be weak, but backing through the afternoon
   into a southerly to southeasterly direction.  This will yield near-
   maximum directional shear, and contribute to 45-60 kt magnitudes of
   effective shear in support of supercell potential.  Though forecast
   soundings reasonably depict small 0-1-km hodographs, given the
   modest ambient boundary-layer speeds, storm-scale and boundary
   processes also may support a tornado risk, especially when
   convection is still relatively discrete.  The lengthy, relatively
   straight character of forecast hodographs also suggest some
   potential for left-moving storms that would travel eastward in this
   flow regime, while also offering large hail.

   A field of -13 to -15 deg C 500-mb temperatures was analyzed last
   night at 00Z, and is expected to persist over the area through
   today, as part of a plume of steep midlevel lapse rates.  Beneath
   that, expect surface dew points in the 50s over western parts of the
   area, transitioning to mid-upper 60s southeastward out of the Hill
   Country and onto the coastal plain.  This will support max afternoon
   MLCAPE of 1500-2000 J/kg in the Hill Country and 2000-3000 J/kg over
   lower elevations.  A lack of substantial MLCINH may yield a large
   number of thunderstorms in this environment, with severe threat most
   concentrated in the enhanced-risk area this afternoon into evening.

   ...Carolinas...
   The cyclone offshore from SC has organized into Tropical Storm
   Bertha.  Please refer to the latest NHC advisory for forecast
   guidance and watch/warning information on this system.

   A plume of at least marginally favorable shear and buoyancy -- the
   former sampled well by the Wilmington/LTX radar VAD winds --  should
   spread inland across the outlook area today.  The most favorable
   CAPE/shear parameter space should be in the outer north through east
   sector of the cyclone.  Though cloud cover will temper diabatic
   surface heating, there should be enough, in concert with inland
   advection of a richer-theta-e marine air mass, to foster MLCAPE of
   500-1500 J/kg amidst little or no MLCINH.  Subtle confluence/
   convergence lines, outflow boundaries, and corridors of differential
   heating all may provide enough lift to support development that may
   include a few small supercells and bowing segments.  Well-curved
   low-level hodographs are expected, with 150-300 J/kg of effective
   SRH possible.  Isolated severe gusts and/or a tornado cannot be
   ruled out.  If favorable mesoscale convective trends (i.e., a well-
   organized band containing supercells) become evident, a subset of
   the existing area of marginal tornado probabilities may need to be
   upgraded a level in a succeeding outlook.

   ...Central Gulf Coast to northern AL and western GA...
   Scattered to locally numerous thunderstorms are expected to develop
   through the afternoon and move northward to northeastward over the
   outlook area, offering strong to locally severe gusts and isolated
   hail.

   The peripheral influence of the Carolinas low, combined with mass
   response to the larger OK/Ozarks cyclone, will set up a deep,
   low-level confluence/convergence zone today over the outlook area,
   with associated lift quickly overcoming weak ambient MLCINH.  This
   will occur with a very moist and diurnally well-heated boundary
   layer, containing surface dew points commonly in the upper 60s F. 
   1500-2000 J/kg preconvective MLCAPE may be attained, with locally
   higher values. Flow and shear in low levels will be weak, though a
   deep layer of 50-65-kt southerlies to southeasterlies in upper
   levels will aid with venting aloft.  Multicell clusters with
   water-loaded downbursts are expected, and some of this activity may
   aggregate into loosely organized, forward-propagating complexes
   capable of sporadic damage and isolated severe gusts, along with
   hail in the deepest/strongest cores.

   ...Northern ME...
   Widely scattered to scattered thunderstorms are possible this
   afternoon, with a localized threat for marginally severe hail/gusts.
   Buoyancy will increase with northward extent due mainly to a
   combination of stronger boundary-layer moisture (dew points upper
   50s to mid 60s F), slightly greater low-middle-tropospheric lapse
   rates, and sufficient heating near a diffuse warm front to weaken
   MLCINH substantially.  Forecast soundings reasonably suggest 1000-
   1500 J/kg peak preconvective MLCAPE, with a well-mixed subcloud
   layer supporting maintenance to the surface of hail and strong/
   severe downdrafts.  Weak vertical shear should limit overall
   organization and render the dominant mode as multicellular.

   ..Edwards/Smith.. 05/27/2020

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