SPC AC 180555

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1255 AM CDT Sat May 18 2019

   Valid 181200Z - 191200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   SOUTHEAST OKLAHOMA...EAST TEXAS...ARKANSAS...FAR SOUTHERN MISSOURI
   AND NORTHWEST LOUISIANA...

   ...SUMMARY...
   Severe storms are possible over a large area today, from Texas to
   southern Minnesota, with the greatest threat from northeast Texas
   across southeast Oklahoma into Arkansas and northwest Louisiana. A
   few tornadoes, widespread damaging winds and large hail are
   expected.

   ...Significant Event Including Widespread Wind Damage and a Few
   Strong Tornadoes Expected Today across parts of the Southern Plains,
   Arklatex and southern Ozarks...

   ...Southern Plains/Arklatex/Southern Ozarks...
   A negatively-tilted upper-level trough will move eastward into the
   southern High Plains today as a moist and unstable airmass advects
   northward into the southern Plains. A cluster of strong to severe
   thunderstorms is expected to be ongoing across north-central Texas
   and southwest Oklahoma during the morning, associated with a 40 to
   50 kt low-level jet. Model forecasts develop this cluster upscale
   into an MCS, moving a squall-line eastward across Oklahoma and north
   Texas during the day. As surface heating takes place ahead of the
   line, cell intensity within the cluster is expected to gradually
   increase. Surface dewpoints ahead of the MCS should be in the upper
   60s and lower 70s F with MLCAPE values reaching the 2500 to 3500
   J/Kg range by early afternoon. In addition, a 60 to 70 kt mid-level
   jet is forecast to move across the southern Plains. This feature
   will enhance lift and create strong deep-layer shear profiles
   favorable for bow echoes within the squall-line. An isolated tornado
   threat along with potential for wind damage and isolated large hail
   is forecast during the early afternoon across parts of eastern
   Oklahoma and northeast Texas.

   As the linear MCS moves into far eastern Oklahoma, western Arkansas
   and far northeast Texas, the current thinking is that new convection
   will develop on the southern end of the MCS. At the same time, the
   strengthening low-level jet will become coupled with the mid-level
   jet. In addition, a cluster of discrete cells could also develop
   ahead of the squall-line across western Arkansas and far northeast
   Texas with a shear environment favorable for supercells and
   tornadoes. There is conditionality associated with this scenario.
   RAP forecast soundings at 21Z in southwest Arkansas show MLCAPE
   values of 2000 to 3000 J/kg with 0-6 km shear in the 35 to 45 kt
   range. This combined with looped hodographs and 0-3 km storm
   relative helicities of 300 to 400 m2/s2 would be favorable for
   strong tornadoes if a cluster of discrete cells can develop. The
   more certain scenario would be for widespread wind damage to occur
   along the leading edge of the linear MCS. The threat is expected to
   be maximized across western and central Arkansas extending
   southwestward into far northeast Texas in the 20Z to 00Z time-frame.
   The severe threat should be less with south south-westward extent
   into east Texas where wind damage and large hail will be possible. A
   wind damage threat and potential for tornadoes associated with the
   squall-line is expected to continue into the evening as far east as
   the Mississippi River.

   ...South-central and Southwest Kansas/Northwest and North-central
   Oklahoma...
   A negatively-tilted upper-level trough will move into the southern
   High Plains today. At the surface, a low is forecast to develop in
   the northeastern Texas Panhandle as a cold front advances
   southeastward across western Kansas. Surface heating ahead of the
   front should enable a corridor of moderate instability to develop
   from western Kansas southeastward into northwest Oklahoma by
   afternoon. Isolated to scattered thunderstorm initiation is expected
   to occur along this corridor during the mid to late afternoon just
   ahead of a shortwave trough. RAP forecast soundings from Dodge City
   to northwest of Oklahoma City late this afternoon show MLCAPE values
   in the 1500 to 2000 J/kg range with 0-6 km shear of 45 to 55 kt.
   This combined with steep mid-level lapse rates approaching 8.0 C/km
   should be favorable for supercells with large hail. Supercells that
   develop where instability becomes strongest, may have potential to
   produce hailstones of 2 inches or greater in diameter. A wind damage
   threat is also expected but cell coverage may remain somewhat
   isolated across the area.

   ...Lower to Mid Missouri Valley/Mid to Upper Mississippi Valley...
   South-southwesterly mid-level flow will be in place today across the
   central Plains and lower to mid Missouri Valley. At the surface, a
   cold front will advance eastward across the mid Missouri Valley as a
   low develops across northwest Iowa. The moist sector should be
   located to the southeast of the low across much of Iowa extending
   southward into north-central Missouri. Surface heating will likely
   enable a pocket of moderate instability to develop across the moist
   sector by afternoon. This combined with increasing low-level
   convergence to the southeast of the surface low should result in
   scattered thunderstorm development during the mid to late afternoon.
   RAP forecast soundings at Des Moines late this afternoon show MLCAPE
   values of 2000 to 3000 J/kg with 0-6 km shear around 30 kt. This
   combined with steep mid-level lapse rates would be enough for
   rotating cells or organized multicells capable of producing isolated
   large hail. Wind damage would also be possible, especially with fast
   moving short line segments. The severe threat could persist into the
   early evening mainly in areas where instability is the strongest.

   ..Broyles/Wendt.. 05/18/2019

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