Severe Storm Outlook Narrative (AC)
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ACUS48 KWNS 170959
SPC AC 170957

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0357 AM CST Tue Jan 17 2017

Valid 201200Z - 251200Z

The medium range models including the ECMWF and GFS solutions begin
the day 4 to 8 period with a negatively tilted upper-level trough
over the eastern Gulf Coast States and move this feature
northeastward into the Mid-Atlantic on Friday/Day 4. The models
suggest that moisture advection will take place across the western
and central Gulf Coast States on Friday into Friday night. A severe
threat appears possible Friday night in the lower Mississippi Valley
and central Gulf Coast States although uncertainty is increased due
to instability and timing concerns. On Saturday/Day 5, both the
ECMWF and GFS show an impressive 90 to 110 kt mid-level jet moving
through northern Mexico. The exit region of the mid-level jet is
forecast to spread eastward across the lower Mississippi Valley on
Saturday over the top of strong moisture advection. The exit region
of the mid-level jet will create strong deep-layer shear profiles
and enhance broadscale lift being supportive of supercells and
organized line segments. This potential for severe storms is
forecast to continue into Sunday/Day 6 as the core of the mid-level
jet moves into the western Gulf of Mexico and influence of the jet
spreads eastward across the Gulf Coast region. GFS forecast
soundings early Sunday at Mobile, AL show moderate instability, 0-6
km shear of 50 to 55 kt and unseasonably mid-level steep lapse rates
suggesting a significant severe threat along with a threat for
tornadoes, wind damage and large hail will be possible with this
system. The main uncertainty is the timing of the system. The
current model solutions have slowed the mid-level jets eastward
progression by about 9 hours suggesting a starting point for the
event further to the west on Saturday. It appears that the greatest
severe weather impact could occur on Sunday when a higher end severe
weather episode will be possible. A 30 percent contour could be
added once certainty increases concerning the timing of the system.
On Monday/Day 7, both the ECMWF and GFS solutions move the
upper-level system into the eastern states and push the warm sector
into the Atlantic. If the system ends up being slower than is
currently forecast, then a chance for severe storms could exist
along the Atlantic seaboard early on Monday. The current solutions
suggest severe thunderstorms will be unlikely across the continental
United States on Tuesday/Day 8.

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