Severe Storm Outlook Narrative (AC)
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ACUS48 KWNS 131007
SWOD48
SPC AC 131006

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0406 AM CST Wed Dec 13 2017

Valid 161200Z - 211200Z

...DISCUSSION...
A Pacific trough forecast to move inland across British Columbia and
the western U.S. on Day 3 (Friday) should amplify on Day 4
(Saturday) across the Rockies, Four Corners region, and the adjacent
Plains.  As a result of this evolution, the Day 3 northern Mexico
trough will eject northeast across TX toward AR/LA.  Further
amplification of the Rockies trough is expected on Day 5 (Sunday)
across the Plains and upper MS Valley, with the TX shortwave trough
weakening as it progresses downstream through the OH/TN Valleys to
the off the middle and northern Atlantic coast.

...Day 4 (Saturday)...
Strengthening southerly low-level winds into East TX to the lower MS
Valley Saturday, and especially Saturday night will result in
moisture return ahead of the ejecting northern Mexico shortwave
trough.  Although strong vertically veering winds would be
supportive of organized storms, forecast soundings suggest stronger
instability should remain offshore the northwestern Gulf coast.
There is some potential for surface-based storms near the TX coast
to the LA coast Saturday night, though overall weak instability
precludes the issuance of a 15% severe probability at this time.

...Day 5 (Sunday)...
The Day 4 TX shortwave trough will continue ejecting northeast and
away from the Gulf coast region.  There will be the potential for
showers and thunderstorms to spread across the central Gulf coast
region and southeast states, though weak instability should limit
the severe-weather threat.

...Day 6 (Monday)...
Models diverge, especially after Day 5, with the eastward
progression of the central U.S. large-scale trough.  The ECMWF is
faster and more amplified with this trough, shifting it from the
southern Plains to off the eastern seaboard by 12Z Tuesday.  The
ECMWF suggests the potential for organized severe storms across part
of northern FL on Monday, though weak instability and lack of model
agreement preclude the introduction of a 15% severe probability area
at this time.

..Peters.. 12/13/2017



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