SPC AC 170603

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0103 AM CDT Wed Apr 17 2019

   Valid 171200Z - 181200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM THE FAR
   NORTHEAST TEXAS PANHANDLE INTO SOUTH-CENTRAL KANSAS...PORTIONS OF
   OKLAHOMA AND NORTHEAST INTO CENTRAL TEXAS...

   ...SUMMARY...
   Severe storms capable of damaging wind and hail are expected today
   and tonight across parts of western and central Texas, Oklahoma and
   Kansas. Additional storms capable of damaging winds, hail and a
   couple of tornadoes are possible from late evening into the
   overnight hours from eastern Texas into Arkansas and northern
   Louisiana. Additional isolated severe storms are also possible
   across Missouri into western Illinois and southern Wisconsin.

   ...Synopsis...

   A complex severe weather scenario is expected to evolve today
   encompassing portions of the southern/central Plains eastward to the
   ArkLaTex and northward through portions of the middle MS Valley
   vicinity.  Strong southerly flow will bring near-60s dewpoints as
   far north as far southern WI by late afternoon. Several features
   will promote areas of strong to severe thunderstorms from the
   afternoon hours through the overnight period as a large-scale trough
   shifts eastward from the Rockies and extends from the Upper MS
   Valley to the southern Rio Grande Valley by 12z Thursday. 

   The bifurcated nature of the upper trough will complicate the
   evolution of the severe threat. A northern shortwave trough will
   provide forcing for severe thunderstorms focused across parts of
   eastern IA, southern WI and northern IL in the afternoon. These
   storms will develop in the vicinity of a surface low and a warm
   front extending west-east across southern WI.  A cold front will
   stretch southwestward from this low to another surface low
   developing over the TX Panhandle. This second surface low is in
   response to a southern shortwave trough migrating eastward from NM
   into the southern Plains. A dryline extending southward from the
   Plains low will be sharply defined across western OK into central
   TX, with the cold front extending from northwest OK into southeast
   KS and eastern IA by 00z. These three features: 1) IA/WI/IL triple
   point, 2) Plains low and cold front and 3) Plains dryline, will be
   the main focus for severe thunderstorm development during the
   afternoon and continuing into the night time hours. 

   ...Eastern IA/Southern WI/Northwest IL Vicinity...

   The severe threat across the area will be somewhat conditional as
   weak capping is expected around 700 mb. However, vertical shear
   profiles will support supercells. Especially concerning would be any
   cell that develops in the vicinity of the triple point/warm front.
   Here, backed low-level winds will enhance low-level rotation
   potential and forecast 0-1 km SRH values greater than 200 m2/s2
   support this favorable shear parameter space for tornadoes.
   Boundary-layer moisture will be modest, with dewpoints increasing to
   around 58-62 F, but should be sufficient to support a severe threat
   given steep midlevel lapse rates and 0-6 km shear on the order of
   40-50 kt. Should storms develop in this area, a localized threat for
   tornadoes will occur.

   ...Mid-MS Valley into the Southern Plains...

   Thunderstorms are expected to develop in the vicinity of the surface
   low and along the cold front from the eastern OK/TX Panhandle into
   northwest OK and south-central KS during the late afternoon and
   evening. Strong height falls and cooling temperatures aloft will
   result in steep midlevel lapse rates in the 7.5-8.5 C/km range.
   Storms initially will pose a threat for all severe hazards,
   including very large hail, damaging winds and a few tornadoes.
   Storms are expected to develop into a line or several bowing line
   segments as stronger forcing overspread the region and the cold
   front sharpens during the evening hours. Further south, convective
   potential across central OK is a bit more uncertain. Storms in this
   area will likely be suppressed initially from capping, and then via
   a dry slot. Should a storm develop in this area however, very large
   hail and tornadoes will be possible. Eventually, damaging winds will
   be possible as convection associated with the cold front further
   northwest tracks east/southeast across the southern Plains
   overnight. 

   Convection across the Red River Valley into central TX also is
   somewhat conditional during the afternoon as capping slowly erodes
   as deep ascent increases with the approaching of the trough. Some
   CAMs guidance indicates a few isolated cells may develop in the warm
   sector ahead of the dryline.  If this occurs, much like in central
   OK, very large hail and a few tornadoes will be possible. More
   likely though, as the cold front overtakes the dryline late in the
   evening, vigorous thunderstorm development will occur across central
   TX  and bowing line segments will shift eastward across TX into the
   ArkLaTex vicinity, posing a threat for mainly hail and wind.
   However, some QLCS tornadoes also could occur given favorable shear
   profiles and a very moist boundary layer.

   ..Leitman/Bentley.. 04/17/2019

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