Severe Storm Outlook Narrative (AC)
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000
ACUS03 KWNS 200656
SWODY3
SPC AC 200655

Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0155 AM CDT Fri Oct 20 2017

Valid 221200Z - 231200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
At this time, the risk for severe storms appears negligible across
the U.S., Sunday through Sunday night.

...Synopsis...
Subtropical ridging appears likely to become increasingly prominent
across southwestern North America and the lower latitude eastern
Pacific during this period, with ridging also building within the
mid-latitude westerlies, across the northeastern Pacific into
British Columbia and the Pacific Northwest.  As this occurs, within
larger-scale downstream troughing, one significant short wave trough
may dig into the central Canadian/U.S. border region and northern
U.S. Plains, while another pivots northeastward across the
Hudson/James Bay region, accompanied by a deep surface cyclone.
Another digging impulse, splitting away from northern cyclonic
regime, may gradually pivot eastward into the lower Mississippi
Valley.  However, as is readily evident by the large spread within
the ensemble output of the various models, the evolution and
progression of these latter perturbations (and associated
developments) remain highly uncertain.

...Lower Mississippi Valley/central Gulf Coast...
In general, the low-level environment across the Gulf States, ahead
of a cold front advancing southeast of the southern Plains, will
probably become seasonably moist by Sunday.  This could provide
potential for weak to moderate boundary layer destabilization across
parts of the lower Mississippi Valley and adjacent Gulf coastal
areas during the day Sunday.  However, at this time, there appears
little potential for substantive surface cyclogenesis along the
front, with the southwestern flank of low-level ridging centered off
the Atlantic Seaboard slow to weaken.  This seems likely to result
in the maintenance of relatively weak lower/mid tropospheric flow,
which may limit the potential for organized thunderstorm
development, even where CAPE may support vigorous convection.
Coupled with the aforementioned apparent low pattern predictability,
probabilities for severe thunderstorms still appear less than 5
percent at this time.

..Kerr.. 10/20/2017

$$



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