SPC AC 191957

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0257 PM CDT Sat May 19 2018

   Valid 192000Z - 201200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE CENTRAL AND SOUTHERN PLAINS TO THE LOWER MISSOURI VALLEY...

   ...SUMMARY...
   Severe thunderstorms will develop across the central and southern
   Plains into the lower Missouri this afternoon and evening.  Very
   large hail and damaging winds are the primary threats. Damaging
   winds also are possible into portions of the middle Mississippi
   Valley through this evening. A few strong storms are also possible
   from eastern Ohio and Kentucky into western Pennsylvania and West
   Virginia.

   ...Central and Southern Plains into the Lower Missouri Valley and
   Mid-Mississippi Valley...

   Minor adjustments have been made to the Enhanced Risk area based on
   latest observations and hi-res guidance. A small eastward expansion
   has been made on the eastern edge to account for potential upscale
   growth of current cluster over north-central OK. Should this occur,
   storm motion may be more to the right of more isolated cells and
   could impact more of northeastern OK into southeast KS.

   An additional expansion of the Marginal and Slight Risk areas has
   been made across parts of the Mid-MS Valley from the MO Bootheel,
   into far northeast AR, western TN and far northern MS. Strong
   instability coupled with steep low level lapse rates along an axis
   of PW values around 1.75 inch may continue to support a damaging
   wind threat into the evening hours, especially across Severe
   Thunderstorm watch 107.

   Finally, the Marginal Risk across parts of northwest KS has been
   trimmed southeastward to better reflect current position of the
   surface cold front and a continued stabilization of the boundary
   layer on the cool side of the boundary.

   ...Ohio Valley to the north-central Appalachians...

   No changes have been made to this portion of the outlook. Scattered
   storms could produce isolated strong wind gusts.

   ..Leitman.. 05/19/2018

   .PREV DISCUSSION... /ISSUED 1129 AM CDT Sat May 19 2018/

   ...Central and Southern Plains into the Lower Missouri Valley...
   The large scale pattern is characterized by southwesterly winds in
   the mid-upper levels to the south of a weak upper low over the
   central Plains.  The low is forecast to move toward the east and
   slowly weaken through tonight as several weak perturbations embedded
   in the flow progress east-northeastward from the central/southern
   High Plains toward the lower Missouri Valley.  

   A complicated convective scenario is expected this period as active
   nocturnal storms moved across Kansas in particular, and with ongoing
   convection continuing from southeast Kansas into southern Missouri. 
   The storms have generated a composite outflow boundary from
   southwest Missouri across northern Oklahoma, and the evolution of
   this boundary will likely have an impact on the intensity and mode
   of subsequent convective development along/north of the boundary
   this afternoon.  To the northwest of this boundary, a cold front is
   expected to move southeast across the region, preceded by a gravity
   wave/bore-like feature seen in satellite imagery moving across
   central Kansas.  Over the southern High Plains a dry line is
   expected to mix eastward over west Texas.

   Visible imagery shows limited clouds over Oklahoma and west Texas,
   with diminishing clouds spreading from central into eastern Kansas
   in the wake of earlier convection.  12Z soundings show steep
   mid-level lapse rates present from west Texas and Oklahoma into
   western and central parts of Kansas, and this plume is expected to
   spread into eastern Kansas in association with the deeper southwest
   flow aloft.  Moisture will be sufficient to develop moderate
   instability in areas where strong diabatic heating occurs with
   MLCAPE of 1500-2500 J/kg expected.  Southwest mid-level winds of
   35-45 kt will contribute to bulk shear of 40-45 kt which will
   enhance storm organization and intensity including potential for a
   few supercells to form.  

   CAM guidance suggests initial storms may develop over parts of
   western/north central Oklahoma and south central Kansas by 21-22Z,
   with additional storms then forming by 00Z farther north in north
   central/northeast Kansas, and southward from southwest Oklahoma into
   Texas.  The steep lapse rates suggest potential for large hail,
   including some producing significant hail during the initial phases
   of storm maturation.  Guidance indicates storms will grow upscale
   into several bowing line segments by evening with the severe threat
   transitioning to primarily a damaging wind threat.  In addition,
   backed surface winds with an easterly component north of the
   aforementioned outflow boundary may result in stronger vertical
   shear with a threat of a few tornadoes from north central Oklahoma
   into parts of eastern Kansas and northwest Missouri.

   .....Ohio Valley to the north-central Appalachians...
   Modestly enhanced low/mid-level winds are expected across the region
   to the east of an gradually weakening upstream shortwave trough. 
   Generally moist profiles are anticipated with the resulting modest
   instability and weak mid-level lapse rates precluding robust 
   updrafts. Even so, a few bowing segments/stronger downdrafts capable
   of locally damaging wind gusts are possible.

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