Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 190555
SWODY1
SPC AC 190554

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1154 PM 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...

...SUMMARY...
A marginal severe threat, with isolated strong to locally damaging
winds and perhaps a tornado, will be possible across parts of the
lower Mississippi Valley and central Gulf Coast states on Thursday.

...Synopsis...
An upper trough/low centered over the central Plains Thursday
morning will move slowly northeastward to the upper Midwest as an
embedded shortwave trough moves from TX to the lower MS Valley
through the day. A low-level jet is forecast to strengthen across
the lower MS Valley by 12Z Thursday, and showers and thunderstorms
should be ongoing across LA/MS in association with low-level warm
air advection and ascent at the beginning of the period. A weak
surface low over the Mid-South should develop northward to the mid
MS Valley/lower OH Valley by Thursday evening as a trailing cold
front becomes increasingly ill-defined across the lower MS Valley. A
warm front will lift slowly northward across the TN Valley/Mid-South
in conjunction with the surface low, and a moist low-level airmass
characterized by low 60s to low 70s surface dewpoints will be in
place across much of the lower MS Valley into the central Gulf Coast
region. A shortwave trough will move across southern CA Thursday
morning, as a large-scale trough develops eastward over the western
CONUS.

...Lower MS Valley and Central Gulf Coast Region...
The effect of early-day precipitation and clouds across LA/MS will
likely serve to limit substantial diurnal heating downstream across
the central Gulf Coast region through the afternoon. 00Z soundings
from LCH, SHV, and JAN all show the presence of poor low- to
mid-level lapse rates (generally 5.5-6.0 degrees C/km), and the
prospect for steepening of these lapse rates appears very low given
the sub-tropical origin of the upstream airmass. NAM/RAP forecast
soundings suggest that even with modest diurnal heating, MLCAPE
should struggle to exceed 500 J/kg on a widespread basis across the
warm sector Thursday afternoon.

A gradual intensification of early morning convection as it moves
eastward across LA/MS/AL and the FL Panhandle/far western GA appears
to be the most probable convective scenario for Thursday
afternoon/evening. Effective bulk shear values on the order of 35-45
kt and modestly enlarged low-level hodographs will generally support
rotating updrafts with this activity even though instability will
likely remain weak. Primary severe threats should be isolated strong
to locally damaging winds as convection attempts to form into short
line segments, and perhaps a brief embedded tornado or two.

A more organized line of thunderstorms may develop from the
early-day convection, but this does not appear overly likely at this
time given the limited thermodynamic environment. Additionally, the
cold front is forecast to weaken through the day, and flow aloft
should remain largely parallel to the front and forecast morning
convection. The marginal severe threat should slowly diminish
through the evening across AL, far western GA, and the FL Panhandle
with the loss of daytime heating and related instability.

...Coastal Southern CA...
A line of low-topped thunderstorms associated with a shortwave
trough may be approaching the southern CA Coast at 12Z Thursday.
While isolated lightning strikes may occur with this activity,
forecast instability and low-level flow both appear too weak to
support the introduction of low wind probabilities.

..Gleason/Picca.. 01/19/2017

$$



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