Severe Storm Outlook Narrative (AC)
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ACUS03 KWNS 180748
SWODY3
SPC AC 180747

Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0147 AM CST Wed Jan 18 2017

Valid 201200Z - 211200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FRIDAY NIGHT
INTO SATURDAY MORNING FOR PARTS OF THE LOWER MISSISSIPPI VALLEY
REGION...

...SUMMARY...
Isolated severe storms may occur late Friday night and Saturday
morning across parts of the lower Mississippi Valley.

...Synopsis...
A broad cyclonic flow regime is expected from the Pacific coast to
the MS Valley, with the primary jet core likely to extend from near
the southern CA coast to northern Mexico by the end of the period.
A series of embedded shortwave troughs will eject northeastward over
the central Plains, and from the northwest Gulf coast to the lower
MS Valley.  This latter wave will interact with a moist low-level
air mass that is already present across the northwest Gulf coast,
and support a risk of isolated severe storms late in the period.

...Lower MS Valley overnight...
A relatively quiet period is expected Friday across the lower MS
Valley, in the wake of the wave forecast to pass on day 2.  A moist
air mass, with boundary layer dewpoints well into the 60s, should
remain across the northwest and north central Gulf coast from now
through Friday, which will contribute to at least weak buoyancy
within the warm sector.  However, little or no forcing for ascent,
and related minimal low-level mass response, suggest that
thunderstorm coverage during the day should be sparse.

An increase in low-level warm advection and ascent is expected
Friday night beginning across the Upper TX coast, and continuing
across LA/MS/AL through Saturday morning.  Strengthening low-level
southerly flow will result in stronger moisture flux near and just
above the surface across the lower MS Valley, which will support
clusters of thunderstorms.  Steeper midlevel lapse rates will also
spread east-northeastward from south TX above the moist layer,
resulting in a gradual increase in buoyancy overnight.  On the edge
of the lapse rate plume, low-level shear will increase overnight,
supporting the risk for embedded rotating storms.  Given the late
arrival of the subtle midlevel trough, and the usual mesoscale
concerns this far in advance, will just introduce low severe
probabilities, including some hail/wind and perhaps isolated tornado
risk.

..Thompson.. 01/18/2017

$$



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