SPC AC 220040

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0640 PM CST Sun Jan 21 2018

   Valid 220100Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS EVENING
   ACROSS PARTS OF SOUTHEASTERN OKLAHOMA AND NORTHEASTERN TEXAS INTO
   WESTERN/CENTRAL ARKANSAS AND NORTHWESTERN LOUISIANA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
   SURROUNDING AREAS OF THE SOUTHEASTERN PLAINS AND MISSISSIPPI
   VALLEY...

   ...SUMMARY...
   A risk for severe storms capable of producing damaging wind gusts,
   and perhaps a couple of tornadoes, continues this evening, mainly
   across parts of southeastern Oklahoma and northeastern Texas into
   western and central Arkansas and northwestern Louisiana.

   ...01Z Outlook Update...
   Latest model output suggests that a period of more rapid deepening
   of the surface cyclone may not be until later this evening into the
   overnight hours, across parts of northeastern Kansas into
   northwestern Missouri, before it begins to occlude.  To this point,
   thunderstorm development has been largely confined to the narrow
   plume of modest low-level moisture return emanating from the western
   Gulf of Mexico, where  a combination of relatively warm mid-levels
   (allowing for only weak CAPE) and weak low-level lapse rates have
   tended to minimize the overall severe weather potential.
   There may be little overall change to convective trends for another
   couple of hours, before the leading edge of stronger lower/mid
   tropospheric cooling begins to overspread the better low-level
   moisture return toward the 03-04Z time frame.  This may contribute
   to at least some further intensification of storms, and perhaps
   organization of storms into an evolving squall line.  

   Although this will coincide with a  gradual cooling and
   stabilization of the axis of weak boundary layer instability
   (supported by daytime heating) across central/eastern Texas into
   eastern Oklahoma/western Arkansas, a period of increasing severe
   weather potential still appears possible this evening.  This may be
   primarily in the form of damaging wind gusts, aided by downward
   momentum transfer near a southerly 850 mb jet which is forecast to
   strengthen to 50-70 kts across the Ark-La-Tex and Ozark Plateau,
   through the lower/middle Mississippi Valley.  However, sizable
   clockwise curved low-level hodographs will also contribute to at
   least some risk for tornadoes, particularly with any continuing
   discrete storm development.  As storms tend to develop eastward into
   the lower/middle Mississippi Valley, weaker instability,
   particularly in the near surface layer, is expected to result in
   diminishing severe weather potential by late tonight.

   ..Kerr.. 01/22/2018

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