Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 211956
SWODY1
SPC AC 211955

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0155 PM CST Sat Jan 21 2017

Valid 212000Z - 221200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON INTO TONIGHT ACROSS PARTS OF THE AR-LA-MISS REGION...

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS TONIGHT ACROSS
PARTS OF SOUTHERN AND CENTRAL ALABAMA AND ADJACENT PORTIONS OF THE
FLORIDA PANHANDLE INTO SOUTHWESTERN GEORGIA...

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS
SURROUNDING AREAS OF THE GULF INTO SOUTH ATLANTIC COAST STATES...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING
AREAS OF THE SOUTHEAST...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS IMMEDIATELY
SURROUNDING THE SLIGHT RISK AREA...

...SUMMARY...
Severe thunderstorms remain possible across parts of the Southeast
late this afternoon, and are expected to develop across parts of the
lower Mississippi Valley into the Southeast this evening through the
overnight hours.  This includes a risk for supercells capable of
producing tornadoes, particularly across parts of southern and
central Alabama, and adjacent portions of the Florida Panhandle,
into southwestern Georgia.

...20Z Outlook Update...
Severe weather potential has been complicated considerably by at
least a couple of short waves impulses, emerging from a strong
mid/upper jet which continues to migrate inland across the southern
tier of the U.S., in advance of one much more significant
perturbation still evolving near the southern Rockies.

The lead impulse is now in the process of shifting across/northeast
of the southern Appalachians.  This feature has provided support for
a severe mesoscale convective system which maintains considerable
strength as it shifts across southern Georgia and portions of the
lower Savannah River Valley, coincident with a 40-50 kt
southwesterly 850 mb jet.  Severe probabilistic lines have been
adjusted some to account for the progression of this feature.

Upstream, another impulse is already in the process of shifting
through the Ark-La-Tex region,  with the main amplifying upper
trough not likely to reach the lower Mississippi Valley until late
tonight, though another smaller scale impulse or two may still
emerge from it, and progress toward Gulf coastal areas ahead of it.
At the same time, the boundary layer from the vicinity of the north
central Gulf coast region into the Ark-La-Tex is still in the
process of recovering, as a convective outflow boundary and
developing warm frontal zone begin to advance northeastward.

Based on the variability that has been evident in short term model
forecast guidance, the details of the potential convective evolution
across the lower Mississippi Valley into the Southeast through the
remainder of the period is far from certain.  However, based on
current observations and model output, severe categorical and
probabilistic lines across the lower Mississippi Valley have
generally not been changed, and appear to reflect potential that
currently exists.  Farther east, confidence continues to increase
concerning severe weather potential associated with restrengthening
of west southwesterly 850 mb flow (50+ kt) across portions of
southern Alabama into southern Georgia, late this evening into the
overnight hours.  As the boundary layer moistens and destabilizes
across this region, and low-level hodographs enlarge, supercells
capable of producing tornadoes (some strong) appear increasingly
likely.

..Kerr.. 01/21/2017

.PREV DISCUSSION... /ISSUED 1027 AM CST Sat Jan 21 2017/

...AL/FL/GA/SC This Afternoon...
A fast moving and intense squall line is moving rapidly eastward
across parts of the FL Panhandle, extreme southeast AL, and
southwest GA.  This line will progress across GA and into SC later
today.  Transient rotating updrafts have been occurring along the
line this morning, producing locally damaging wind gusts and
tornadoes.  This threat will persist as activity moves eastward due
to strong southerly low level winds just ahead of the storms
maintaining enhanced low level shear and helicity values.  Some
breaks in the clouds and modest afternoon heating may further
destabilize the area and maintain the Enhanced risk of severe
storms.

...AR/LA/MS This Afternoon and Evening...
Water vapor loop shows a lead shortwave trough moving eastward
across the Gulf Coast states.  Subsidence behind this system will
help to suppress deep convection over the ArkLaTex region for a few
more hours.  However, another shortwave trough over NM and West TX
will lead to increasing lift and cooling aloft by mid/late
afternoon.  Present indications are that scattered thunderstorms
will develop over southeast OK and northeast TX around peak heating
and track eastward across parts of AR/LA and into MS this evening.
Forecast soundings show steep lapse rates, cold mid level
temperatures, and favorable deep layer shear values to promote a
widespread risk of severe hail.  CAM solutions also indicate
discrete supercells will be favored, further increasing the risk of
very large hail and a few tornadoes.  Therefore have upgraded
portions of this area to MDT risk for the hail threat.

...LA/MS/AL/GA This Evening and Tonight...
Considerable uncertainty exists regarding the evolution of the
remnant outflow boundary from this morning`s convection and the
re-development of thunderstorms tonight over this area.  Forecast
soundings indicate that the combination of sufficient instability,
ample low level moisture, and significant low level vertical shear
will promote a tornado risk in any storms that form.  A few models
suggest another active round of severe storms affecting areas
similar to this mornings storms.  Therefore will maintain the
Enhanced Risk area with minimal changes.

$$



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