Severe Storm Outlook Narrative (AC)
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046
ACUS01 KWNS 220058
SWODY1
SPC AC 220057

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0657 PM CST Tue Feb 21 2017

Valid 220100Z - 221200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
The risk for severe storms appears negligible across the U.S.
through tonight.

...Synopsis...
Despite the presence of seasonably mild conditions across much of
the U.S., under the prominent influence of split belts of westerlies
emanating from the mid-latitude Pacific, thermodynamic profiles
across most areas remain largely unsupportive of appreciable
convective potential.  However, within broader scale upper troughing
gradually shifting across and inland of the Pacific coast, through
the Intermountain West and Rockies, forcing for ascent and
destabilization associated with a number of smaller scale
perturbations have been contributing to a few areas of generally
weak thunderstorm activity.  The most significant of these
perturbations have been progressing east northeastward along a
strong, but weakening cyclonic mid-level jet, across northern
California through the northern Great Basin and northern Rockies.

East of the Rockies, a lower/mid tropospheric closed low within the
southern-most branch of mid-latitude westerlies has been supporting
some thunderstorm activity across the lower Mississippi Valley late
this afternoon.  With the unstable warm sector of the associated
occluded surface low confined to the eastern Gulf of Mexico, this
has largely been aided by daytime heating beneath the mid-level cool
pool, and does not seem likely to persist beyond the 01-03z time
frame.

...Northern Plains...
One of the impulses emerging from the mid-latitude Pacific now
appears to be in the process of progressing east of the northern
Rockies, into the crest of low amplitude ridging across the northern
Plains.  Mid-level moisture return to an eastward developing zone of
stronger lower/mid tropospheric warm advection appears to be
contributing to the maintenance of ongoing thunderstorm activity
across the western Dakotas.  This activity seems likely to persist
and spread eastward across the Dakotas through the 03-06Z time, and
perhaps continue into portions of the Upper Midwest overnight.

...North California/southern Oregon/northwestern Nevada...
Objective analysis suggests that weak to modest boundary layer
destabilization has taken place across the region, beneath cold
mid-level temperatures along/north of the stronger mid-level jet
axis.  Before this environment stabilizes appreciably with the loss
of daytime heating this evening, forcing associated with another
inland migrating short wave impulse may still contribute to an
increase in scattered thunderstorm activity.

...Florida Peninsula...
Latest guidance suggests that vigorous warm sector thunderstorm
activity may at least approach Gulf coastal areas of the Florida
Peninsula by 12Z Wednesday.  However, destabilization appears
unlikely to become supportive of appreciable severe weather
potential, given the weak to modest nature of the wind fields and
vertical shear to the east of the approaching mid-level closed
low/trough.

..Kerr.. 02/22/2017

$$



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