The SPC is forecasting ...Outbreak of tornadoes and severe thunderstorms expected over parts of the southern Plains this afternoon and tonight....   Please read the latest public statement about this event.  View What is a Watch? clip.

   SPC AC 201255

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0755 AM CDT Mon May 20 2019

   Valid 201300Z - 211200Z

   ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   NORTHWEST TEXAS AND THE EASTERN TEXAS PANHANDLE TO WESTERN AND
   CENTRAL OKLAHOMA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   EASTERN NEW YORK AND NEW ENGLAND...

   ...SUMMARY...
   An outbreak of tornadoes, some potentially long-track and violent,
   is expected today into this evening over portions of northwest Texas
   into western and central Oklahoma.  More-isolated but still
   potentially dangerous severe weather, including tornadoes and
   destructive winds and hail, is possible in surrounding parts of
   Texas, Oklahoma, Kansas, and Arkansas.

   --- Technical Discussion ---

   ...Synopsis...
   Two primary features will serve as the main mid/upper-level
   influences on convective potential through the period:
   1.  A synoptic cyclone -- centered initially over NV and embedded
   within a persistent western CONUS longwave trough. The associated
   500-mb low is forecast to pivot southeastward across the Grand
   Canyon region today and the southern Rockies overnight, while the
   associated trough at that level evolves from positively to
   negatively tilted.  This will occur as an upstream, northeastern
   Pacific low moves southeastward to western OR.  The Southwestern
   synoptic wave will yield kinematic fields rarely seen in mid/late
   May at their latitudes, for example (by 12Z):  120-130-kt 250-mb jet
   max over the TX South Plains/Panhandle region, 90-100-kt 500-mb flow
   over northwest TX and western OK, 70-80-kt 700-mb winds over or near
   eastern OK, and an evening LLJ potentially exceeding 75 kt across
   parts of north TX and OK.
   2.  A shortwave trough -- now apparent in moisture-channel imagery
   over portions of Lower MI and Lake Huron.  This feature -- embedded
   within the southern belt of cyclonic flow around a more-substantial
   perturbation over the James Bay region -- will eject east-
   northeastward toward the lower St. Lawrence Valley and Maine through
   00Z.

   At the surface, 11Z analysis showed a diffuse low over southeast NM
   near ROW, with warm front eastward across the DFW Metroplex to
   northern LA.  A dryline was drawn from southeastern NM into the Big
   Bend region of southwest TX.  The dryline will mix/advect eastward
   across the southern High Plains through the day, reaching the
   eastern TX Panhandle and Permian Basin regions by late afternoon. 
   Thereafter, where not muddled by outflow, the dryline should retreat
   northwestward overnight.  The warm front is forecast to move/develop
   northward across AR, OK and portions of the TX Panhandle through the
   day, reaching the Ozarks this evening and overnight.  Warm-frontal
   progress will be tempered somewhat by the presence of abundant
   precip to its north; however, extreme mass response forecast in
   advance of the mid/upper-level trough should contribute to enough
   warm-sector thetae advection and maintenance to keep the warm-
   frontal baroclinic gradient sharp, and the dryline well-defined.

   ...Southern Plains and vicinity...
   For details regarding ongoing hail/wind threats from mainly elevated
   storms over the Panhandles/northwest OK area, refer to SPC watches
   195/196 and related mesoscale discussions.

   A serious outbreak of destructive, tornadic supercells is likely
   over parts of this region this afternoon into evening, especially in
   the high- and moderate-risk areas.  Given the expected fast storm
   motions, especially mid-afternoon into evening, a few of the
   best-organized supercells may reach an equilibrium with their
   already very favorable mesoscale environments long enough to sustain
   wide, long-track tornadoes.  Very large and damaging hail also is
   possible from some of the supercells, along with sporadic severe
   downdraft gusts.  The hail threat will extend farther north of the
   warm front than the wind and tornado potential.  The wind-damage
   threat may increase this evening as convection aggregates into
   clusters or lines, though tornadoes and hail still will be likely
   from any relatively discrete storms.  QLCS/line-embedded tornadoes
   also are possible.  Collectively, the multi-episode threat justifies
   maintaining a high risk for this outlook cycle.

   Scattered to numerous thunderstorms are expected to develop, in
   aggregate, across a substantial portion of the southern Plains from
   the dryline eastward across OK, perhaps into western/central AR,
   southward to the Red River region and northwest TX.  Isolated to
   widely scattered convection also may develop farther south in north
   TX and west-central/southwest TX along/ahead of the dryline this
   afternoon and evening.  The first episode should involve moist-
   sector convection building gradually into potentially tornadic
   supercells by early-mid afternoon, between north TX and
   central/western OK, perhaps even over eastern OK/western AR, in
   subtle confluence/convergence zones.  Without a strong EML, MLCINH
   will be modest, with only subtle diabatic heating and ambient
   theta-e advection necessary to support them.  

   Thereafter, several supercells should form near the dryline in a
   regime of strengthening deep-layer flow and shear.  This activity
   will move rapidly northeastward into the warm sector, which will be
   characterized by large low-level hodographs and buoyancy.  With 
   steep midlevel lapse rates, upper 60s to 70s F surface dew points
   and deep moist layers being common, MLCAPE ranging from 2000-5000
   J/kg should develop in the moist sector, amidst 55-70-kt effective-
   shear magnitudes.  Effective SRH 300-500 J/kg should develop --
   locally higher, especially near the warm front.  Some areas will
   experience a rare combination of 3000+ MLCAPE and 400+ J/kg
   effective SRH.  The parameter space over much of the region today
   will support a threat of violent tornadoes, and the likelihood of
   multiple significant (EF2+ events).

   ...Northeast...
   Widely scattered to scattered thunderstorms are forecast to develop
   this afternoon along/ahead of a surface cold front forecast to cross
   the region from west to east.  Occasional severe hail and damaging
   gusts will be the main concerns, though some supercell potential
   exists over the northern parts of the risk area, and a tornado
   cannot be ruled out.

   Destabilization of the boundary layer is expected throughout the day
   and into afternoon, following the passage of a belt of precip now
   crossing parts of southern New York and New England in the warm-
   advection zone.  Both diabatic surface heating and boundary-layer
   theta-e advection will contribute to airmass recovery from south to
   north ahead of the front.  Surface dewpoints 60s F and steep low-
   level lapse rates will support 1000-2000 J/kg MLCAPE.  Deep shear
   will increase northward, supported mainly by speed strengthening
   with height, with 35-45-kt effective-shear vectors possible over
   parts of New England.  The threat should diminish with time this
   evening as the remaining inland warm sector shrinks and stabilizes
   diabatically near the surface.

   ..Edwards/Jewell.. 05/20/2019

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