Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 180550
SWODY2
SPC AC 180549

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1149 PM CST Tue Jan 17 2017

Valid 191200Z - 201200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THROUGH THURSDAY
EVENING FOR PARTS OF THE LOWER MS AND TN VALLEYS...

...SUMMARY...
A marginal severe threat will be possible across parts of the Lower
Mississippi and Tennessee Valleys on Thursday.

...Lower Mississippi Valley...
A southern-stream shortwave trough over NM and northern Mexico late
this evening will eject northeastward from TX to the lower MS Valley
Thursday, and reach the TN/OH Valleys Thursday night, in response to
an upstream trough crossing CA.  The TX trough will induce weak
cyclogenesis along a pre-existing front that will extend from
southeast TX to the Mid South by Thursday afternoon.  A moist air
mass is already present south of this stalled front, with boundary
layer dewpoints now near 70 F along the TX coast.  Little overall
change is expected in low-level trajectories or the character of the
moist layer across the Gulf basin, so it is likely that 65+ F
dewpoints will spread and remain inland from the TX coast to at
least southern LA/MS/AL.

The ejecting wave/weak cyclogenesis interacting with the moist warm
sector will support clusters of thunderstorms through the period,
some of which will likely be ongoing at the beginning of the period
from southeast TX into LA.  The early convection and clouds through
the day could limit surface heating as well as midlevel lapse rates,
thus the magnitude of buoyancy is somewhat in question across the
lower MS Valley.  Still, any recovery during the day, in combination
with increasing vertical shear, will support some risk for embedded
supercells or line segments with isolated damaging winds and perhaps
a tornado.  The severe risk should be focused during the day across
LA/MS, and into the evening across AL, and then diminish overnight.

...CA into western AZ...
Isolated thunderstorms will be possible along the CA coast, and
perhaps into parts of the San Joaquin Valley, during the day as the
midlevel trough and steeper lapse rates progress inland.  A few
lightning strikes may also occur closer to the lower CO River Valley
within the left exit region of the mid-upper jet.

..Thompson.. 01/18/2017

$$



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