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    Day 2 Outlook >
Feb 18, 2017 1300 UTC Day 1 Convective Outlook
Updated: Sat Feb 18 12:57:53 UTC 2017 (Print Version | 20170218 1300Z Day 1 shapefile | 20170218 1300Z Day 1 KML)
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 Forecast Discussion
   SPC AC 181257

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0657 AM CST Sat Feb 18 2017

   Valid 181300Z - 191200Z


   Isolated thunderstorms will be possible over portions of the
   Southeast States and southeastern California and southern Arizona. 
   Isolated thunder may occur overnight along the Oregon coast.  No
   severe weather is expected.

   In mid/upper levels, a high-amplitude southern-stream pattern will
   prevail, featuring two well-defined troughs:
   1.  Formerly closed low with a pronounced vorticity max/circulation
   center over western AR.  This trough, which currently extends
   southeastward over the LA coast and over the northern Gulf, will
   move eastward to the Carolinas, eastern GA and northern/central FL
   by the end of the period.
   2.  Pacific trough extending along the northern/central CA coast,
   then just offshore southern CA and extending as far south in
   latitude as the tropics around 20 deg N, well to the west-southwest
   of Los Cabos (Baja).  This perturbation is forecast to move eastward
   to western NV, southern AZ, the southern/central Gulf of CA, and the
   Pacific south of the tip of Baja.

   At the surface, a weak frontal-wave/lee low was analyzed at 11Z over
   west-central KS, along a cold front extending northeastward across
   southeastern IA to Lake Superior, and southward over western OK and
   the Texas South Plains.  These features should remain well-separated
   from sufficient moisture/instability for thunderstorms this period.

   ...Southeastern States...
   Cold-core instability near the midlevel vorticity center --
   currently over western AR -- has supported a cluster of convection
   with occasional/embedded thunderstorms for the past few hours.  This
   regime is expected to shift eastward to east-southeastward across AR
   through at least midday, with sporadic thunder possible until the
   associated lift/cooling gradually diminishes enough to shrivel the
   thunder potential below 10%.

   Substantial reductions to the thunder outlook area farther southeast
   account for pronounced dry-slot processes apparent in satellite
   imagery and 12Z LCH/LIX soundings, behind the ongoing plume of
   precip located over the Gulf and adjacent coastal plain.  Thunder
   potential should continue to shift eastward across the remaining
   outlook area through this afternoon and evening.  With the leading
   plume of ascent progressing faster than previously progged across
   the Southeast, additional, elevated convection -- with isolated
   embedded thunder -- may develop in its wake across portions of GA
   and the Carolinas.  This would be associated with a zone of subtle
   cooling/destabilization aloft, prior to the arrival of the
   southern-stream shortwave trough.

   ...Southern CA/AZ...
   Isolated thunderstorms should continue to develop sporadically
   within a plume of warm-conveyor precip moving eastward across the
   outlook area.  Isolated convection occurring behind this plume also
   may briefly reach depths suitable for lightning-supporting charge
   separation, as low/middle-level lapse rates steepen with the
   approach of the strong Pacific trough.  Aggregate coverage of
   thunder over this region, through the remainder of the period, may
   be marginal for a general-thunder outlook, but will maintain roughly
   the eastern 2/3 of the previous outlook area at this time.

   ...Coastal OR...
   Isolated shallow thunderstorms may reach the coast tonight as a
   late-period shortwave trough approaches.  DCVA-related cooling aloft
   will steepen lapse rates atop a weakly capped marine layer with
   marginally supportive theta-e.

   ..Edwards/Peters.. 02/18/2017



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