Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS02 KWNS 201728
SPC AC 201727
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1127 AM CST Fri Jan 20 2017
Valid 211200Z - 221200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SATURDAY
THROUGH SATURDAY NIGHT ACROSS PARTS OF SOUTHEASTERN ALABAMA...THE
FLORIDA PANHANDLE AND ADJACENT SOUTHWESTERN GEORGIA...
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
NORTH CENTRAL MISSISSIPPI AND ADJACENT PORTIONS OF ALABAMA...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING
AREAS OF THE LOWER MISSISSIPPI VALLEY THROUGH MUCH OF THE EASTERN
GULF COAST STATES...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS WITHIN A
CORRIDOR IMMEDIATELY SURROUNDING THE SLIGHT RISK AREA...
Severe thunderstorms are expected across parts of the lower
Mississippi Valley through much of the eastern Gulf Coast states
Saturday through Saturday night. This includes the risk for large
hail, potentially damaging wind gusts and a couple of tornadoes.
One or two strong tornadoes are possible, particularly across parts
of the western Florida Panhandle into southeastern Alabama and
Models indicate that a strong zonal mid/upper jet will continue to
emerge from the mid-latitude Pacific and undergo amplification as it
develops generally eastward across the southern tier of the nation.
One significant embedded short wave impulse is forecast to progress
across the southern Rockies, before amplifying and digging
southeastward across parts of the southern Plains into the lower
Mississippi Valley by the end of the period. In association with
this feature, a 110-130 kt 500 mb jet streak is expected to
redevelop eastward from near the southern California coast through
the northwestern Gulf of Mexico and adjacent Gulf coastal areas.
In lower levels, within broader surface troughing east of the
Rockies, and likely encompassing much of the Plains and
Mississippi/lower Ohio Valleys by 12Z Saturday, one significant
cyclone may form across the south central Plains before migrating
across the Red River Valley into the Ozark Plateau by late Saturday
night. In response to a preceding short wave impulse, stronger
south southwesterly 850 mb flow, and associated low-level moisture
return, appears likely to be initially focused across the north
central Gulf Coast, into parts of Mississippi and Alabama early
Saturday. During the day Saturday, models suggest that this jet
will weaken while shifting eastward through the eastern Gulf states.
There probably will be a period of veering to a more westerly
component, before 850 mb flow begins to back and strengthen in
response to the stronger upstream impulse. Guidance suggest that
this will generally occur from the northeastern Gulf of Mexico
through parts of the Florida Panhandle, southeastern Alabama and
adjacent southern Georgia by late Saturday evening.
...lower Mississippi Valley through the Southeast...
Primarily driven by low-level warm advection beneath
difluent/divergent upper flow associated with the lead short wave,
considerable thunderstorm development may be ongoing at 12Z Saturday
across parts of Mississippi and Alabama, and in the process of
developing eastward into/through much of Georgia. In the presence
of a moistening lower levels, including the boundary layer, and a
residual strong south southwesterly 850 mb jet (40-50 kt), the
environment seems likely to be conducive to the risk for severe
storms. This includes the potential for tornadoes in stronger cells
within an evolving organized storm cluster, and in discrete
supercells near or ahead of it, until the stronger low-level jet
weakens/veers and shifts to the east of better boundary layer
instability during the day.
Thereafter, potential convective evolution becomes a bit more
unclear. Variability among the models on smaller scale developments
is considerable. This includes residual moisture and potential for
significant destabilization near/east of the Mississippi Valley,
with the approach of the amplifying impulse. It still appears
possible that cooling aloft within the exit region of the
approaching mid-level jet could support an area of vigorous
thunderstorm development. This may not be until Saturday evening,
or later, but it could still be accompanied by an appreciable risk
for severe hail.
It currently seems more probable that the more prominent convective
development will occur in association with forcing accompanying the
strengthening low-level jet across parts of southeastern Alabama,
the western Florida Panhandle, and southwestern Georgia late
Saturday evening through Saturday night. Mid-level cooling will not
be as strong across this region as areas to the north and west.
However, considerably higher boundary layer/low-level moisture
content, in the presence of large, clockwise-curved low-level
hodographs (supported by 50+ kt 850 mb flow), probably will
contribute to an environment conducive to organized convection and
discrete supercells capable of producing damaging wind gusts and a
few tornadoes. This may include the risk for one or two strong
Although it currently seems a lesser probability, it is possible
that this activity could spread through much of northern Florida and
the Savannah River Valley by 12Z Sunday, with a continuing severe
weather threat. This will need to continue to be monitored.