Area Forecast Discussion
Issued by NWS Birmingham, AL
FXUS64 KBMX 300505
Area Forecast Discussion
National Weather Service Birmingham AL
1205 AM CDT Thu Mar 30 2017
For 06Z Aviation.
Tonight...Clouds may lessen across the area this evening, only to
increase again after midnight. Southerly flow will keep things
mild and somewhat humid overnight. Patchy fog is possible along
with some lower clouds mainly for the western two thirds of the
area after midnight. Some blow off higher cloudiness is also
possible from the storms to our west.
Believe much of the rain activity will remain to our west
overnight. So at this time, have removed any mention of rain. Will
watch the evolution of the storms upstream for a potential
adjustment later tonight.
Thursday through Tuesday.
A complex forecast is setting up for Thursday`s potential severe
weather event. Synoptically, a longwave neutrally tilted trough
will be advancing into the area, taking on a negative tilt by
Thursday evening. Within this trough, an upper low will be moving
through the Ozarks while a pronounced southern stream shortwave
will be moving along the Gulf Coast/northern Gulf, extending well
south into the Gulf in association with a strong subtropical jet.
Strong upper-level forcing will overspread the Deep South and Gulf
Coast due to vorticity advection and diffluent flow aloft. A
surface low will be located near St. Louis with a cold
front/surface trough extending southward through the Mississippi
Favorable dynamics, shear, lapse rates, and moisture will
be present for severe weather, but there are still two main
uncertain mesoscale details that will determine whether
significant destabilization occurs. The first question is if the
line of storms to our west this evening over the Arklatex region
will hold together and move into the area Thursday morning in
association with a lead shortwave and low level jet. If it were to
hold together then a scenario similar to last Saturday could
evolve where later activity is inhibited. However, the majority of
the model guidance including the CAMs and the typically reliable
WRF- ARW and NSSL WRF currently indicates this activity will
either dissipate or become scattered. So therefore, believe this
will be a non factor west but may have some implications east.
The second question is whether potential convection developing
along the LA/MS Gulf Coast associated with the southern stream
wave/subtropical jet will cause convection further north to be
"cut off"/suppressed. This will be something to watch in future
CAM runs. Currently, however, CAMs seem to suggest that this
activity may develop too late, be too far south, or be too
scattered to prevent strong destabilization/supercell development
ahead of the front over northern Mississippi. Additionally,
forcing with the upper low and higher dew points already being in
place may prevent coastal convection from being an inhibiting
factor. But this activity should spread some rain/storms chances
over southeastern areas and limit the destabilization there.
In that case, a favorable environment for supercell development
will be in place over northern Mississippi with 1000-2000 J/kg of
CAPE and 50+ kts of southwesterly shear helping to keep storms
ahead of the slow-moving front. These supercells would then have
the potential to move into West Alabama during the late
afternoon/early evening, with sufficient helicity/backed surface
winds for a couple tornadoes and large hail. Keeping in mind the
above uncertainties, the enhanced risk area from SPC in our far
western/northwestern counties depicts the potential threat well,
with a threat gradually decreasing off to the east. In addition,
southwesterly Corfidi vectors indicate that some of the
convection associated with the southern stream wave will move into
mainly our southern counties Thursday afternoon and pose a threat
for mainly damaging winds and hail.
In summary, our advertised Enhanced area will remain the same with
the potential of supercells. Tornadoes, damaging winds and large
hail are all possible. These cells may organize into clusters and
affect areas generally northwest of I-59 after 3pm starting west.
The remnant outflow and potential development moving in from the
south earlier in the day are much less certain until mesoscale
details can be analyzed. But a strong to severe storm can not be
ruled out southeast but the timing is also in question. That is
why this area will remain in a Marginal risk.
Models are depicting a quicker exit to this system and the
forecast area should dry out quickly Friday morning. Ridging will
make a brief appearance and result in a pleasant weekend. Yet
another upper-level system will begin to impact the area late
Sunday night into Monday while taking on a negative tilt. Fairly
strong dynamics/wind fields will be present with this system with
a sub-1000 mb surface low developing over Arkansas. Will introduce
a low confidence severe threat into the HWO for Monday, with a
small chance that a severe threat could evolve as early as late
Sunday night. Still a lot of uncertainty with this system with
widespread convection/MCS activity that will have the potential to
inhibit instability from spreading too far inland from the Gulf.
06Z TAF Discussion.
Intently watching ongoing (as of 05z) convection to the west of
central Alabama. Looks like two areas of convection may survive
the night, but those two areas may very well split our area to the
north and south. Will continue to watch it, and will make updates
as needed. Otherwise, 06z TAFs will continue the trend established
with the previous forecasts with lower ceilings developing in
most areas around sunrise. Still some lower confidence on the
exact timing, but feeling better about the general idea of MVFR
category ceilings developing at some point in the next 6 to 9
Central Alabama will remain in a warm and moist pattern through the
week, with the highest rain chances Thursday and then again on
Monday. Critical fire weather conditions are not expected through
the next 7 days.