Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS02 KWNS 181729
SPC AC 181728
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1128 AM CST Wed Jan 18 2017
Valid 191200Z - 201200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF THE LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COASTAL REGION...
A marginal severe threat will be possible across parts of the Lower
Mississippi Valley and central Gulf Coast states on Thursday.
Substantial progression/evolution of the large-scale upper flow
field is expected across the U.S. Day 2/Thursday. An upper trough
over the West is forecast to move eastward across the Rockies, while
digging energy on the back side of the trough shifts across the
eastern Pacific toward the West Coast later in the period. This
will result in overall expansion of longwave troughing over the
western half of the country.
Farther east, an upper low initially progged to lie over the Central
Plains is forecast to weaken with time, as it moves east-northeast
toward/into the Midwest while evolving into an open wave.
At the surface, a weakening cool front moving eastward across east
TX/AR/LA early in the period is expected to wash out with time,
while a weak warm front shifts north across the TN Valley area. In
the West, a generally weak surface pattern is forecast, though a
deep low and associated front over the eastern Pacific -- associated
with the aforementioned digging upper system -- should begin to
approach the coast late in the period.
...Lower MS Valley and central Gulf Coast region...
Widespread showers and scattered thunderstorms are expected to be
ongoing early in the period from parts of east/southeast TX and the
adjacent western Gulf into the mid and lower MS and TN Valleys, near
and ahead of a weakening cool front and associated short-wave
troughing moving east across the region. Though a high theta-e
low-level airmass will support modest buoyancy -- allowing
convection to spread east across the central Gulf Coast region with
time, widespread clouds and precipitation (driven by
quasi-geostrophic ascent) should hinder potential for appreciable
steepening of lapse rates, therefore limiting updraft intensity.
Thus, while low-level shear should remain otherwise sufficient, lack
of stretching potential in low levels suggests only brief/weak
Some risk for locally gusty/damaging winds will exist, particularly
with any banded structures (as hinted at by some high-res CAMs).
However, with the weakening surface front and thus lack of strong
linear forcing, a well-organized line is not expected. Convection
is expected to spread northeast with time, though generally
weakening/decreasing in coverage during the second half of the
...Coastal southern CA...
A short-wave upper trough expected to be moving onshore at the start
of the period will support showers and a few embedded thunderstorms
over portions of CA, and later across the CO Valley into portions of
AZ. While weak instability across most of the area should hinder
convective intensity, a few stronger low-topped cells may move
onshore early in the period across coastal southern CA. With
low-level flow not expected to be particularly strong, any risk for
damaging convective gusts appears too low to warrant introduction of
a risk area at this time.