Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 200559
SWODY1
SPC AC 200558

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1158 PM CST Thu Jan 19 2017

Valid 201200Z - 211200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
LA/MS/AL AND THE WESTERN FL PANHANDLE...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE LOWER SABINE/MS RIVER VALLEYS INTO THE GULF COAST STATES...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
COASTAL SOUTHERN CA...

...SUMMARY...
Severe storms may develop Friday night into early Saturday across
the lower Mississippi Valley and adjacent Gulf coastal areas, with a
few tornadoes, large hail, and damaging winds all possible. Strong
thunderstorms with at least some severe-weather potential are also
possible near the southern California coast Friday afternoon.

The potential for higher severe hail/tornado probabilities (and a
corresponding Enhanced Risk) exists for late in the Day 1 period
across parts of LA/MS/AL, but a possible upgrade has been deferred
to a later outlook due to:
1. Considerable uncertainty in convective coverage/mode across the
warm sector late Friday into early Saturday per latest guidance,
2. Relatively high potential for spatial error in the placement of
higher probabilities, and
3. The opportunity for later near-term guidance and observations to
provide additional details on the mesoscale evolution of this event.

A detailed discussion follows below.

...Synopsis...
A large-scale upper trough over the western CONUS will acquire an
increasingly negative tilt as it shifts northeastward across the
Plains Friday. One upper jet initially across the southern Plains
will develop eastward across the lower to mid MS Valley, TN/OH
Valleys, and adjacent Gulf Coast through early Saturday. A separate
upper jet is forecast to move from the eastern Pacific across CA and
parts of the Southwest through the period.

A shortwave trough embedded within the synoptic-scale
mid/upper-level trough will likely move from northern Mexico across
TX and into the lower MS Valley from Friday evening through 12Z
Saturday (end of Day 1 period). An elevated mixed layer with steep
mid-level lapse rates originating from the higher terrain of
northern Mexico appears likely to shift northeastward into much of
the southern Plains and parts of the lower MS Valley in tandem with
the embedded shortwave trough.

At low levels, Gulf moisture will stream northward across the lower
MS Valley and adjacent areas ahead of the approaching shortwave
trough Friday night into early Saturday. A southerly/southwesterly
low-level jet is forecast to strengthen to 40-50 kt across parts of
LA/MS/AL, mainly after 21/03Z. Higher surface dewpoints/moisture
will likely be slower to advance northward across the same area due
to a convectively reinforced boundary currently located over the
northern Gulf of Mexico from prior thunderstorms. The primary
surface cyclone is expected to remain well displaced to the
northwest (across the central/northern Plains) from the warm sector
across the lower MS Valley and Gulf Coast states.

...East TX/Lower MS Valley and Adjacent Gulf Coast States...
While isolated thunderstorms may occur near to just offshore parts
of coastal TX/LA/MS during the day Friday along a remnant boundary,
the better opportunity for scattered to numerous thunderstorms
appears to be Friday evening after 21/00Z. This later development
would be tied to increasing large-scale ascent associated with an
embedded shortwave trough over the southern Plains approaching the
region, as well as low-level warm air advection/ascent with a
strengthening low-level jet.

Considerable uncertainty remains regarding convective coverage and
mode late in the period. Regardless, forecast soundings across
LA/MS/AL show slowly steepening mid-level lapse rates, strengthening
low- to mid-level winds, and increasing low-level moisture Friday
night into early Saturday morning that will support supercell
structures. The threat for large hail, some potentially greater than
2 inches in diameter, will exist with any initially discrete
supercell storms that can form this environment, although the better
mid-level lapse rates will be arriving toward 06-12Z Saturday. In
addition, damaging winds and a few tornadoes will also be possible
across this region, particularly southern LA/MS/AL where higher
surface dewpoints may support surface-based convection.

...Coastal Southern CA...
Numerous showers and isolated thunderstorms may develop Friday
afternoon across coastal southern CA and adjacent eastern Pacific
tied to ascent with the nose of an upper-level jet moving over this
region. Forecast soundings indicate weak instability may combine
with marginal bulk shear values to support some organization with
this convective activity. Locally strong to damaging winds appear to
be the primary threat.

..Gleason/Picca.. 01/20/2017

$$



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