SPC AC 200556

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1256 AM CDT Mon May 20 2019

   Valid 201200Z - 211200Z

   ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF WEST
   AND NORTHWEST TEXAS...THE EASTERN TEXAS PANHANDLE AND WESTERN
   OKLAHOMA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   SOUTHEAST NEW YORK AND NEW ENGLAND...

   ...SUMMARY...
   An outbreak of strong tornadoes and severe thunderstorms is expected
   today across parts of the southern and central Plains. In addition,
   many of the storms will have very large hail and wind damage. The
   severe threat will be concentrated from west Texas and the Texas
   Panhandle eastward across Oklahoma, Kansas into western Missouri and
   western Arkansas. Additional severe storms with wind damage and hail
   will be possible this afternoon from southeast New York into
   southern and central New England.

   ...Tornado Outbreak Expected Across the Southern Plains Today Into
   Tonight...

   ...Southern and Central Plains...
   An impressive and potent upper-level trough will move quickly
   eastward across the Desert Southwest today as a powerful 75 to 90
   knot mid-level jet rounds the base of the trough. Ahead of the
   system, a corridor of strong instability is forecast across the
   Southern Plains from west Texas into the eastern Texas Panhandle and
   eastward into western and central Oklahoma. This combined with steep
   mid-level lapse rates and strong low-level shear will be very
   favorable for severe storms. As the mid-level jet ejects
   northeastward across the southern High Plains this afternoon and
   evening, a tornado outbreak is likely across the southern Plains.
   The tornado outbreak is expected to continue into the overnight
   period. This event should result in a significant threat to life and
   property.

   RAP forecast soundings late this afternoon along the instability
   corridor from Childress, Texas northeastward to Clinton, Oklahoma
   show MLCAPE values of 3000 to 4500 J/kg with 0-6 km shear in the 50
   to 65 kt range. In addition, hodographs are large and looped with
   0-1 km shear values in the 30 to 40 kt range. This environment
   should be very favorable for supercells and tornadoes. 0-3 km
   storm-relative helicities are forecast to steadily increase from
   about 300 m2/s2 in the late afternoon to about 450 m2/s2 by early
   evening as an anomalously strong low-level jet becomes better
   focused. This will be ideal for a tornado outbreak with strong
   tornadoes upstream and to the west of the low-level jet.

   The current thinking concerning the details is that the first round
   of severe storms will begin early as thunderstorms initiate in west
   Texas and move northeastward into the eastern Texas Panhandle by mid
   to late morning. Initially, large hail will be the main threat but
   as these storms mature, tornadoes will be possible with the stronger
   and more dominant cells. Additional supercells with tornado
   potential are expected to initiate near the warm front in western
   and central Oklahoma. A tornado threat will transition to large hail
   and wind damage as the storms move into Kansas to the north side of
   the boundary. This first round of severe storms is forecast to move
   northeastward, away from the warm sector, allowing for the
   development of extreme instability during the mid to late afternoon
   from northwest Texas into southwest Oklahoma.

   The second round of severe storms is expected to start during the
   late afternoon as repeat initiation takes place in west Texas. Rapid
   supercell formation is forecast along the I-27 corridor from the
   vicinity of Lubbock northward to just south of Amarillo. A cluster
   of tornadic supercells is then forecast to move northeastward into
   northwest Texas and the southeastern Texas Panhandle during the
   early evening. Additional tornadic supercells are forecast to
   rapidly develop in southwest Oklahoma and move northeastward into
   west-central Oklahoma. At that time, the strengthening low-level jet
   will couple with a highly progressive and seasonably strong
   mid-level jet, making conditions favorable for long-track strong
   tornadoes and possibly violent tornadoes.

   The greatest threat for long-track tornadoes will exist along the
   corridor from near Lubbock northeastward to Childress, Altus,
   Lawton, Clinton to just west of the Oklahoma City and Enid
   vicinities. After considerable deliberation, a High risk will be
   issued for parts of west Texas, the southeastern Texas Panhandle
   into western Oklahoma.

   In addition to the tornado threat, conditions will be very favorable
   for large hail and wind damage. Strong instability, enhanced
   deep-layer shear and steep mid-level lapse rates will make
   hailstones of greater than 2 inches in diameter possible with the
   more intense supercells, mainly in west Texas and the western half
   of Oklahoma. A wind-damage threat will also exist across much of the
   southern Plains due to the expected intense nature of the storms.
   The wind-damage threat, including some gusts above 70 kt, should
   become widespread during the late evening into the overnight period
   as squall line organizes and moves quickly eastward across the
   southern Plains.

   ...Southeast New York/New England...
   An upper-level trough will move across southern Quebec and the
   Northeast today. At the surface, a cold front will advance eastward
   across New York and into western New England. Ahead of the front,
   surface dewpoints in the mid 60s F should result in destabilization
   of the airmass by afternoon from southeast New York into much of
   southern and central New England. Scattered thunderstorm development
   is expected around midday along the cold front with this convection
   moving eastward across the moist sector during the afternoon.
   Forecast soundings along the instability axis from Springfield,
   Massachusetts northeastward into Vermont at 21Z show MLCAPE values
   peaking in the 1000 to 1500 J/kg range and have steep low-level
   lapse rates. This combined with 0-6 km shear values around 35 kt
   should be sufficient for multicells with wind damage potential. A
   few rotating storms with a large-hail threat may also develop.

   ..Broyles/Wendt.. 05/20/2019

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