Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS02 KWNS 211722
SPC AC 211721
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1121 AM CST Sat Jan 21 2017
Valid 221200Z - 231200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SUNDAY ACROSS
MUCH OF NORTHERN FLORIDA...SOUTHEASTERN GEORGIA...SOUTH CAROLINA AND
ADJACENT PORTIONS OF NORTH CAROLINA...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING
AREAS OF THE SOUTH ATLANTIC SEABOARD...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
SURROUNDING PORTIONS OF THE SOUTHEAST...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
CENTRAL CALIFORNIA...INCLUDING THE SAN FRANCISCO BAY AREA...
Severe thunderstorms, accompanied by a risk for potentially damaging
wind gusts and a few tornadoes, are possible across parts of the
Southeast Sunday into Sunday night. Strong thunderstorms, with at
least some risk for potentially damaging wind gusts, are also
possible around the San Francisco Bay area into nearby portions of
the Sacramento and San Joaquin Valleys.
Amplification within a strong belt of westerlies emerging from the
mid-latitude Pacific, and progressing through the southern tier of
the U.S., appears likely to continue through this period. Within
this regime, one significant trough is forecast to gradually pivot
from the southeastern Plains and lower Mississippi Valley through
much of the Southeast, taking on more of neutral to perhaps negative
tilt as it does. Another significant upper trough appears likely to
begin progressing inland across the California coast. Shorter
wavelength developments within both these features remain at least
somewhat unclear, complicating the convective forecast.
In general though, at lower levels, models indicate further
deepening of a broad cyclone associated with the lead feature, as
its center tracks from the Ozark Plateau into the vicinity of the
southern Appalachians Sunday through Sunday night. As it does, a
consolidating trailing cold front may surge across and east of much
of the south Atlantic coast by 12Z Monday. Upstream, while a deep
lower/mid tropospheric cyclone may linger west of the Pacific
Northwest coast, a cold front is expected to advance southeastward
across Oregon, northern and central California and portions of the
Large spread evident in model ensemble output and deterministic
guidance, concerning shorter-wavelength developments associated with
the significant larger-scale lower/mid tropospheric disturbance,
continues to contribute to uncertainty concerning placement and
extent of convective potential for this period. Among other
details, the extent of convective development ongoing at 12Z Sunday
across the Southeast, and its possible influence on subsequent
destabilization remains unclear. It seems most certain that
considerable thunderstorm activity will be ongoing along a
stalled/slow moving frontal zone across parts of the southern
Appalachians through southern South Carolina. Continuing convection
along a remnant southerly low-level jet axis extending southward
across Georgia/northern Florida is more unclear.
Though details in the deterministic models differ, in general, the
ECENS and NCEP SREF both suggest considerable strengthening of
south/southwesterly 850 mb flow (40-50+ kt) across parts of
northern Florida/southeast Georgia, perhaps northward into the
coastal plain of South Carolina, during the 22/18-00Z time frame.
This may occur in tandem with a corridor of boundary layer heating
and destabilization, ahead of the consolidating cold front, which
may provide the focus for the primary convective development.
At this time, perhaps due to spread among the ensemble members,
ensemble output suggests limited potential for the development of
much more than modest CAPE on the order of 500+ J/kg. However,
given favorable large-scale forcing for ascent, this may still be
enough to support considerable organized convective development,
including discrete supercells, with a risk for severe wind gusts and
a few tornadoes.
Deterministic guidance suggests at least pockets of stronger
destabilization and lift are possible, but this may be confined to
either a lingering warm frontal zone across the Carolinas, or ahead
of the cold front, across parts of northern Florida. It remains
unclear, which, if either, scenario is more likely.
...San Francisco Bay area into nearby interior California Valleys..
The exit region of an intense (110-130 kt) westerly 500 mb jet
streak appears likely to gradually spread southward across the
region late Sunday into Sunday evening. Associated cooling aloft
seems likely to contribute to at least weak boundary layer
destabilization in the moist post-frontal onshore flow regime.
Although post-frontal low-level wind fields may not be particularly
strong, deep layer shear should be. And latest model guidance seems
increasingly suggestive that a forced low-topped convective band
could spread inland across the San Francisco Bay area, accompanied
by the risk for strong wind gusts. Other, more discrete, convective
cells could form ahead of this band, across nearby portions of the
interior valleys, perhaps with some risk for a brief/weak tornado.