SPC AC 181959

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0259 PM CDT Fri May 18 2018

   Valid 182000Z - 191200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF
   CENTRAL KANSAS INTO PARTS OF NORTHERN OKLAHOMA...

   ...SUMMARY...
   Severe thunderstorms are possible near the Front Range of the
   Rockies eastward into parts of the central and southern Plains late
   this afternoon and tonight. Damaging wind is most likely across
   Kansas this evening.

   ...Central Plains...
   Changes to this outlook update include expanding the Enhanced Risk
   for damaging wind farther east across KS as an MCS is expected to
   materialize and persist through the evening. Satellite imagery and
   surface analysis continues to show heating and destabilization south
   of the primary front across central KS, along with expanding CU
   fields and moisture advection out of OK. A strong, relatively backed
   low-level jet this evening will provide favorable storm relative
   inflow into any convective system, while steep midlevel lapse rates
   maintain updraft vigor. Large hail is also possible especially with
   early cellular activity. For more information see mesoscale
   discussion 450.

   ...Southeast LA and coastal MS and AL...
   Scattered storms currently extend from central LA into the FL
   Panhandle, beneath a weak upper trough which is providing cool
   temperatures aloft. Heating and the existence of low 70s F dewpoints
   has led to a zone of over 2000 J/kg MLCAPE which is currently
   supporting a hail threat in the strongest cells. These storms are
   tied to the diurnal cycle and should dissipate by around 00Z. Until
   then, a small Marginal Risk area has been added for hail and wind.

   ..Jewell.. 05/18/2018

   .PREV DISCUSSION... /ISSUED 1128 AM CDT Fri May 18 2018/

   ...Central and Southern Plains...
   Nocturnal and morning convection now located over eastern Nebraska
   and north central Kansas has generated an east-west low-level
   boundary that extends from central Kansas into southeast Colorado. 
   Areas of widespread stable low clouds, especially over eastern
   Colorado but also evident over northwest and north central Kansas,
   will inhibit diabatic heating into at least early afternoon, and may
   promote maintenance of a differential heating discontinuity that
   will help focus later thunderstorm development.

   The large scale pattern will continue to be characterized by modest
   southwest flow aloft advancing eastward over the central and
   southern Plains as weak perturbations embedded into the flow move
   toward the High Plains.  A surface low over southeast Colorado may
   drift southward as strong heating occurs this afternoon, with the
   aforementioned boundary serving as a focus for thunderstorm
   development later this afternoon and evening.  North of the
   boundary, an easterly component to the low-level flow will provide
   moist upslope winds, and dependent on the degree of cloud
   dissipation and development of pockets of stronger heating, may
   support additional storm development by mid-to-late afternoon.

   Sufficient low-level moisture is in place over the central and
   southern High Plains with surface dew points in the 50s and lower
   60s.  12Z regional soundings continue to exhibit steep mid-level
   lapse rates, and strong diabatic heating south of the central plains
   boundary will contribute to moderate instability with MLCAPE of
   1500-2500 J/kg.  Although mid-level winds will be in the 25-35 kt
   range, veering wind profiles will result in 35-45 kt bulk shear
   which will enhance storm organization and intensity.  In addition,
   low-level shear will be stronger along and north of the boundary
   where backed near-surface winds will be present.

   Storms are expected to develop initially over parts of southeast and
   north-central/northeast Colorado as the cap weakens this afternoon. 
   There is potential for a few discrete supercells to form in the
   earlier stages of the convective cycle.  Steep lapse rates will
   enhance hail growth with these storms with potential for significant
   hail to occur with stronger cells as they spread eastward with time
   into western Kansas.  The presence of a well-mixed deeper PBL will
   also be supportive for strong wind gusts to occur.

   Other severe storms are expected to develop near the dry line
   extending southward into west Texas, with primary threats for large
   hail and damaging winds.  The overall thermodynamic environment will
   be supportive of cold pool development and potential for storms to
   grow upscale into one or more bowing line segments this evening. 
   This scenario is indicated by most morning CAM guidance and suggests
   a transition to primarily a wind damage threat.  A strengthening
   southerly low-level jet is forecast to develop by this evening from
   Texas across Oklahoma into southern Kansas, and the associated
   convergence and warm advection near the terminus of the LLJ is
   likely to maintain strong-to-severe convection spreading eastward
   across Kansas into the overnight hours.  

   ...Tennessee/Lower Ohio Valley and the Southeast...
   Widespread convection is anticipated this afternoon as the moist air
   mass over the region destabilizes amidst diurnal heating.  Weak
   vertical shear across the area suggests that the only severe
   potential would come from water-loaded downbursts which result from
   updraft strengthening due to cell mergers, but this is expected to
   occur only on a very isolated/episodic basis.

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