SPC AC 210550

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1150 PM CST Sat Jan 20 2018

   Valid 211200Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON INTO TONIGHT ACROSS PARTS OF EASTERN OKLAHOMA AND
   NORTHEAST TEXAS INTO WESTERN ARKANSAS AND PARTS OF NORTHWEST
   LOUISIANA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
   SURROUNDING AREAS OF THE SOUTHEASTERN PLAINS...OZARK PLATEAU...LOWER
   MISSISSIPPI VALLEY AND NORTHWEST GULF COAST VICINITY....

   ...SUMMARY...
   Severe thunderstorms are possible late this afternoon into tonight,
   mainly across southeastern portions of the Plains into the Ozark
   Plateau region, where activity may be accompanied by the risk for
   damaging wind gusts and perhaps a couple of tornadoes.

   ...Synopsis...
   Mid/upper flow has become more zonal and progressive across much of
   the U.S. and will remain so through this period.  While temperatures
   have moderated considerably across most areas east of the Rockies,
   boundary layer recovery over the Gulf of Mexico is still ongoing. 
   In the wake of one short wave trough forecast to advance across
   parts of the Southeast into the western Atlantic today, seasonably
   high moisture content air is generally expected to remain confined
   to the Caribbean and southern Gulf of Mexico.  However, boundary
   layer modification has been sufficient for surface dew point
   increases into the lower/mid 60s across portions of the western Gulf
   of Mexico into the Texas coastal Plain.  

   At least some further deepening of this moist boundary layer appears
   possible, as a southerly return flow strengthens across the
   northwest Gulf coast into portions of the southeastern Plains and
   lower/middle Mississippi Valley today into tonight.  This is
   expected in response to the approach of a large/vigorous short wave
   trough, currently advancing into the Four Corners states.  As this
   feature continues east of the southern Rockies, through the central
   and southern Plains, models indicate that significant cyclogenesis
   (already underway) will proceed from the Texas/Oklahoma Panhandle
   region northeastward through northeastern Kansas/northwestern
   Missouri by 12Z Monday.  This appears likely to be accompanied by
   intensifying wind fields, including  90+ kt at 500 mb and 50-70+ kt
   at 850 mb, south through east of the developing lower/mid
   tropospheric circulation center.

   At the same time, upstream, another vigorous mid-level short wave
   trough is forecast to advance inland across the Pacific Northwest
   through portions of the northern Rockies, accompanied by areas of
   weak destabilization.

   ...Southeastern Plains into Ozark Plateau/lower Missouri Valley...
   Models remain suggestive that the developing cyclone may not be
   accompanied by the evolution of a particularly well-defined dryline,
   with stronger daytime heating generally occurring to the west of the
   plume of modest low-level moisture return.  Within this plume,
   considerable low cloud cover is expected to inhibit surface heating,
   resulting in generally weak near surface lapse rates, particularly
   with increasing northward extent to the north of the Red
   River/Ark-La-Tex region.  Increasing moisture near/above the surface
   will still probably result in increasing conditional instability,
   and perhaps increasing convection through the day, however
   convection likely will initially be capped by relatively warm
   elevated mixed-layer air.

   Toward late afternoon, erosion of mid-level capping with large-scale
   ascent ahead of the approaching short wave may allow for deepening
   convection and the development of thunderstorm activity,
   particularly across parts of northeast Texas and eastern Oklahoma. 
   However, guidance appears suggestive that a more substantive
   increase, intensification and organization of storms may not occur
   until the leading edge of the stronger lower/mid tropospheric
   cooling overspreads the more substantive low-level moisture return,
   near or perhaps after 22/00z.

   Once this occurs, mixed layer CAPE increasing to near or above 500
   J/kg across parts of northeast Texas and eastern Oklahoma, in the
   presence of strong deep layer flow and shear, is expected to support
   a period of increasing severe weather potential continuing
   into/through much of this evening.  Isolated supercells are possible
   early, but the evolution of a squall line may be fairly quick,
   spreading eastward toward the Mississippi Valley, and perhaps
   northeastward toward the lower Missouri Valley.  Forecast sizable
   clockwise curved low-level hodographs will contribute to at least
   some risk for tornadoes with stronger activity, although this may be
   hampered across even the Ark-La-Tex region by neutral to stable
   lapse rates within a shallow near surface layer.  Given the
   convective mode, potentially damaging wind gusts appear the primary
   severe threat, as stronger downdrafts aid the downward transfer of
   higher momentum to the surface.

   ..Kerr/Leitman.. 01/21/2018

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