Area Forecast Discussion
National Weather Service Birmingham AL
356 AM CST Tue Jan 3 2023
...New SHORT TERM, LONG TERM,
HYDROLOGY...
.SHORT TERM...
(Today through Wednesday)
Issued at 345 AM CST TUE JAN 3 2023
* Strong to severe thunderstorms
likely across Central Alabama today
and tonight. All severe hazards are possible, in addition to
localized
flash flooding.
A potent upper-level system is currently moving east across The
Plains this morning with a surface low set to move northeast across
the mid/upper Mississippi Valley today. Meanwhile, a
warm sector
continues to establish across the Deep South as persistent southerly
low-level
flow advects warm, moist air northward from the Gulf Coast
through the Ohio Valley. Dewpoints are already in the mid to upper
60s across portions of Central Alabama as of 3 AM this morning.
Throughout today, an ongoing
mesoscale convective system will move
east across Tennessee and Mississippi, generally nearing our western
counties by late morning. Increasing temperatures ahead of this
MCS
will foster 1,000-1,500
J/kg MLCAPE with modest 6-7
C/km mid-level
lapse rates. Enhanced mid- to upper-level
flow will support eff.
bulk
shear <55
kts. This
parameter space will support organized
thunderstorms with a strongest updrafts becoming bowing segments or
supercells. These stronger storms will pose a threat of damaging
wind gusts,
hail, and even a few tornadoes.
The evolution of convective activity still remains in question
today, and there are no correct answers regardless how long you
stare at model guidance. Wind
shear,
instability, and
moisture will
be in place. The focusing mechanism (or lift) has been more
subtle/weak and harder to assess up to now. Primary synoptic support
aloft is
progged to lift well to the north, and this is why
convection is forecast to outrun the cold
front stalling to our
west. Most guidance also suggests
veering 925-850
mb winds to occur
during this time (late afternoon & early evening), shrinking
hodograph curvature. However, kinematic/thermodynamic parameters are
sufficient to support severe weather threats as early as 11 AM in
the west, to 7 AM Wednesday morning in the southeast. And despite
shrinking hodographs, 100-300
m2/s2 0-1 km
SRH is
progged by most
hi-
res guidance.
It appears more probable that we deal with two rounds of severe
weather. The first round should arrive late this morning into the
early afternoon as clusters of strong to severe thunderstorms move
into, or develop, in Central Alabama in association with initial
height falls.
These storms appear to have the best access to quality
warm sector conditions into the mid to late afternoon, including
better 0-3 km CAPE, enhancing tornado potential south of I-20. Most
hi-res guidance suggests this
convection will congeal into loosely
aggregated clusters or lines as they outrun the lagging cold
front
in the late afternoon/early evening. This will cause slower
progression and/or training
convection to occur in areas south of I-
20, leading to
flash flooding potential. A second wave of strong to
severe thunderstorms will be possible later this evening as a
shortwave rotates around the closed upper low toward the Tennessee
Valley. This will provide better
dynamics/support aloft,
accelerating the cold
front eastward into Alabama with thunderstorms
focused along its axis. Any airmass recovery/lingering
SBCAPE will
allow these storms to pose additional severe threats into Wednesday
morning. Severe threats will end as frontal passage occurs into
early Wednesday morning. Any flooding issues (from both rounds of
storms) may persist into late Wednesday morning.