Area Forecast Discussion
Issued by NWS Birmingham, AL

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS64 KBMX 211251
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
751 AM CDT Mon Oct 21 2019

.UPDATE...
For 12Z Aviation.

&&

.SHORT TERM...
/Updated at 0344 AM CDT Mon Oct 21 2019/

Today and Tonight.

Areas of patchy fog have once again developed overnight and will
persist through the early hours this morning, especially in the
eastern and southeastern counties. By mid-morning, an increase in
southerly winds should help mix out any lingering fog.

Just to our west, a cold front is moving through the MS River
Valley and will move through Mississippi during the afternoon.
Meanwhile, a pre-frontal shortwave lifts out of the Western Gulf
and moves through South MS and into Central AL. This will lead to
widespread rain and thunderstorms across our area. We could see
some destabilization, but just how much remains uncertain. For
now, I think there could be enough instability to lead to some
isolated stronger storms during afternoon, especially in the west.
Early in the afternoon, the dynamics are initially fairly good in
the northwest, with a low level jet providing some speed shear.
However, the surface winds veer quickly as the pre-frontal trough
moves through. We end up with uni-directional shear, which limits
any tornado threat initially. Therefore, I think the main concern
will be isolated damaging winds, but I suppose there`s still a
non-zero chance of a brief tornado. Then, as we go through the
afternoon, the low level jet fizzles out and lifts to our north,
well ahead of the upper level jet. High resolution models are
picking this transition up and showing more stratiform rain vs.
convection with a focused lifting mechanism. Therefore, I think
there will be a break in the mid-afternoon/early evening as the
rain/storms associated with the pre-frontal shortwave slowly
weaken due to lack of dynamical support and push eastward. This
will also act to stabilize the thermodynamic profiles before the
main cold front is able to push into the area, limiting any
further severe threat for much of the area. The exception might be
the southern and southeastern areas, where models are picking up
on a quick pulse of the low level jet intensifying in the evening
and into the night (generally before midnight). This could provide
enough lift for convection south and east of I-85 that could once
again have some stronger storms embedded in it. It`s in these
areas that the surface winds remain backed for longer, so we might
see some better directional shear that could lead to a tornado
threat, but confidence is still low because there`s uncertainty in
the models as to whether this intensification of the LLJ will
even occur. Overall, the severe weather threat for Central AL
remains highly conditional and marginal. At different times and
locations we have the thermodynamic and dynamic conditions, which
might overlap, but the main cold front looks to remain to the west
of our area at the times we could see this overlap, so we may end
up not having a clear focus for the convection.

The main cold front moves into Central AL in the evening, but the
area it moves into has been stabilized, so I expect weaker storms
along the front itself. By midnight, the LLJ has completely moved
out of the area, decreasing the low level shear. The cold front
will continue to move through the area during the overnight
hours. Any rain/storms along the front will mainly be supported by
the frontal convergence and the divergence aloft with the upper
level jet, but the thermodynamics do not support strong to severe
storms by this point.

25/Owen

.LONG TERM...
/Updated at 0344 AM CDT Mon Oct 21 2019/

Tuesday through Sunday.

High pressure will move into the region on Tuesday behind a cold
front. Although the airmass behind the front will be quite dry,
mild afternoon temperatures are expected. A strong pressure
gradient should lead to breezy conditions Tuesday afternoon with
highs in the upper 60s to lower 70s. The center of the high
pressure will move close to us on Wednesday morning, allowing for
light winds and strong radiational cooling. Some locations in the
north could drop into the upper 30s with 40s elsewhere. Dry and
tranquil conditions will continue through Thursday afternoon as
the high pressure shifts toward the Carolinas.

There is significant model disagreement regarding the evolution of
the upper air pattern Thursday night through Sunday. The GFS is
much more progressive with the ejection of the shortwave and also
more aggressive with onset of southerly flow Thursday night. This
would result in rain becoming widespread after midnight. The
ECMWF shows a significant portion of the trough becoming cut off
over Texas, with much less coverage of rainfall through Friday.
This solution seems a little too slow while the GFS may be too
fast. High rain chances will be maintained for Thursday night
through Friday night with a potential for widespread beneficial
rainfall. The timeline for a frontal passage and arrival of drier
conditions remains unclear for Saturday and Sunday.

87/Grantham


&&

.AVIATION...
12Z TAF Discussion.

Showers and thunderstorms currently in MS will continue across AL
through the afternoon and evening. Showers ahead of the main line
will begin to impact each site...with thunder along the line
reaching each site one to two hours after showers begin. Have
included a TEMPO for now due to uncertainty in coverage of
thunder as they progress east. In each thunderstorm...conditions
are expected at IFR or lower...though enough moisture and winds
will keep at least MVFR with each shower. Overnight...winds will
remain brisk...so not expecting fog development. Moisture will be
plentiful with IFR or lower ceilings through the early morning.

24

&&

.FIRE WEATHER...

Showers and thunderstorms will be possible ahead of a cold front
this afternoon and tonight. Cooler and drier conditions will
follow for Tuesday through Thursday. Minimum afternoon RH values
for Tuesday and Wednesday are expected to range from 30 to 35
percent.

The Alabama Forestry Commission continues to have a fire alert in
effect that includes all of Central Alabama. For more information
about restrictions, visit their web site at forestry.alabama.gov.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     76  53  70  39  69 /  60  80   0   0   0
Anniston    77  56  70  42  69 /  60  80   0   0   0
Birmingham  77  55  70  43  71 /  70  80   0   0   0
Tuscaloosa  78  53  71  43  73 /  80  80   0   0   0
Calera      77  54  71  43  70 /  60  80   0   0   0
Auburn      77  60  71  45  69 /  60  80  10   0   0
Montgomery  81  60  74  45  72 /  60  80   0   0   0
Troy        81  60  73  43  71 /  50  80  10   0   0

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
None.

&&

$$


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.