SPC AC 181257
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0657 AM CST Sat Feb 18 2017
Valid 181300Z - 191200Z
...NO SEVERE THUNDERSTORM AREAS FORECAST...
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.
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.
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.
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.
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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z