SPC AC 211628

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1028 AM CST Sun Jan 21 2018

   Valid 211630Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS EAST TX/OK TO AR
   AND NORTHWEST LA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS MO SOUTH TO
   PARTS OF TX/LA...

   ...SUMMARY...
   Scattered severe storms are likely after 4 PM CST, mainly from parts
   of east Texas/Oklahoma into Arkansas and northwest Louisiana. A few
   tornadoes, damaging winds, and isolated large hail will be possible.

   ...East TX/OK to AR/LA...
   Overall minor changes to the SLGT and MRGL risks in this region.
   Considered an upgrade to ENH centered on Texarkana but continued
   concerns over the degree of buoyancy and potential weakness in
   forecast hodographs around 700 mb suggest the tornado threat may not
   yet warrant greater probabilities. 

   12Z soundings sampled mean mixing ratios of 10-12 g/kg within the
   modifying western Gulf air mass that is characterized by surface dew
   points of 60-65 F. These dew points should spread as far north as
   eastern OK and western/southern AR during the afternoon/evening.
   Abundant low cloud cover will limit diabatic surface heating within
   the moist sector. However, low-level convergence will intensify
   along the dryline during the late afternoon as mid-level height
   falls associated with the southern Rockies shortwave trough
   overspread the boundary. This should yield storms gradually
   increasing along/ahead of the dryline across eastern OK into
   east-central TX. 

   Forecast soundings show modest low/middle-level lapse rates across
   much of the moist sector. Nearly saturated low-level thermodynamic
   profiles should result in predominant MLCAPE values of 300-800 J/kg.
   Low-level and deep-layer shear will strengthen tonight and become
   increasingly favorable for updraft rotation, however some forecast
   soundings indicate weaknesses in the hodograph around 700 mb. Given
   the positive tilt of the mid/upper wave, the related geometry of
   winds aloft should lead to a substantial component of mean flow
   parallel to the developing convective corridor. These factors should
   contribute to a mixed mode of semi-discrete cells transitioning to a
   largely quasi-linear mode. Damaging winds should be the primary
   hazard, but a few tornadoes are possible, particularly centered from
   northeast TX into southeast OK and southwest AR between 00-06Z where
   the combination of buoyancy/shear should be relatively maximized. 

   A broken band of storms will likely expand/develop slowly eastward
   towards southern MO through the Sabine Valley tonight. The severe
   threat should diminish towards the Lower MS Valley during the early
   morning as the warm sector narrows from north to south ahead of the
   convective band, though isolated damaging wind potential may persist
   through 09-12Z.

   ...MO...
   Relative to areas farther south, colder midlevel temperatures will
   offset weaker boundary-layer theta-e to maintain weak buoyancy for
   storms, amid an intense warm-advection conveyor. Forecast soundings
   suggest MLCAPE will be meager, below 500 J/kg along and south of the
   warm front in the 00-06Z time frame. A shallow, near-surface stable
   layer reinforced by sporadic precip should temper downdraft strength
   somewhat. Still, convective downward momentum transfer and related
   isolated damaging winds may penetrate that layer. Low wind
   probabilities have been maintained in this regime of favorably
   strong deep-layer lift and weak buoyancy immediately preceding the
   ejecting low-level cyclone.

   ..Grams/Gleason.. 01/21/2018

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