SPC AC 151255

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0755 AM CDT Fri May 15 2020

   Valid 151300Z - 161200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS PARTS OF NY TO
   SOUTHERN NEW ENGLAND AND PARTS OF WEST/CENTRAL TX INTO SOUTHERN
   OK...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS CO/NE BORDER
   AREA...

   ...SUMMARY...
   Scattered damaging winds, severe hail, and a couple tornadoes are
   probable across parts of New York into western New England this
   afternoon through early evening. Scattered to numerous severe wind
   gusts, along with scattered large hail, are possible across southern
   Oklahoma into western and central portions of Texas.

   ..Northeast States...
   Overall change from the 06Z outlook is minimal with concerns over
   the amplitude of boundary-layer moistening depicted in latest
   guidance.

   A shortwave trough over Lake Superior will translate east into Maine
   by early Saturday. Attendant mid-level speed max is progged to
   amplify with 500-mb flow in excess of 50 kt overspreading much of NY
   and northern New England by peak heating. Surface cyclone should
   diurnally deepen as it reaches the NY/VT border area around 21Z.
   Surface temperatures should warm through the 70s and low 80s south
   of the cyclone, ahead of an impinging cold front. While low-level
   moistening is underway, the bulk of guidance may be too aggressive
   with depiction of warm-sector dew points in the low 60s.
   Nevertheless, enlarged low-level hodographs should favor a few
   supercells developing from central NY and spreading towards southern
   VT/NH and western MA. Modest mid-level lapse rates will likely
   curtail significant severe potential.

   ...OK/TX...
   Primary changes are to expand severe risk east across the TX Gulf
   Coastal Plain for late in the period and to upgrade to a category 3
   risk for wind.

   A surface cyclone should drift south in the southeast TX Panhandle
   as a convectively reinforced cold front is pushed south from the
   OK/KS border area. Convective outflow ahead of the cold front has
   made steady southward progress early this morning across northeast
   OK as moderate to strong convection sags south across the Ozarks.
   Lower-than-average confidence exists in where these boundaries will
   lie by peak heating. South of them, a strongly to extremely unstable
   air mass will develop a plume of MLCAPE from 3000-5000 J/kg amid
   very steep 700-500 mb lapse rates of 8-9 C/km. 

   MLCIN will become negligible along the dryline in west TX and
   scattered thunderstorms will develop during the late afternoon along
   it to the intersection with the composite front, aided by a difluent
   upper-level pattern with approach of a minor trough ejecting east
   from the NM/Chihuahua border. Compared to a typical mid-May severe
   setup, deep-layer shear will be marginal outside of the
   dryline/front intersection and along the Rio Grande Valley. As such,
   outflow-dominated convection is expected with several consolidating
   cold pools this evening. Consensus of CAMs suggest convection on the
   front should sag south, merging with cold pools emanating east from
   the dryline and higher terrain of Coahuila. This should yield
   widespread convective overturning this evening through tonight with
   multiple large MCSs possible as a moderate southerly low-level jet
   develops across south TX. While deep-layer shear will be modest,
   potential exists for several swaths of severe wind gusts into
   central TX given the supportive thermodynamic setup for robust cold
   pools. 

   ...Central High Plains...
   Minimal change to the 06Z outlook with a corridor of severe hail and
   wind possible, centered on the Colorado-Nebraska border in the late
   afternoon and early evening.

   A zone of low-level convergence is expected to remain oriented
   north-south across eastern CO into the NE Panhandle. While low-level
   moisture is not particularly high across the central High Plains,
   adequate PW should be in place for robust convection along this
   boundary ahead of a shortwave trough over the northern Rockies.
   Around 40-kt effective shear suggests a few weak supercells should
   form before convection evolves into an eastward-moving cluster that
   outruns the instability axis this evening.

   ..Grams/Goss.. 05/15/2020

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