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Area forecast discussion
National Weather Service Birmingham Alabama
1138 PM CST Wed Feb 20 2019

Update...
for 06z aviation.

&&

Short term...
through tonight.

Showers and thunderstorms continue to track across central Alabama
this afternoon, but thankfully the deeper moisture and higher dew
points have stayed to our south today along with our warm front.
Thus, we did not reach the severe potential that we initially
through we would reach. Dew points ranged today from the upper
40s northeast (with a cad wedge helping to keep that area cooler)
to the lower 60s in the south. The deeper moisture (higher 60s dew
points) did not come to materialize. We also kept socked in with
low stratus that helped to keep temperatures down. We did have
enough moisture and lift for some elevated convection today, but
only a few strong storms accompanied it. The main threats have
been heavy rain and lightning. The heavy rain should taper down
during the evening hours, but there still should remain the need
for a higher band of rain chances. This will be due to our surface
cold frontal boundary remaining across the area. It is expected
to stall and then meander back to the north as we head into
Thursday as a warm front. Overnight low temperatures for tonight
will range greatly from the northwest to the southeast with
respect to which side of the boundary you lie. Readings will vary
little in the southeast with lows expected in the middle 60s
(generally southeast of the I-85 corridor) as they will remain in
the warmer airmass. While readings in the far northwest are
expected to range through the 40s behind the front. In between we
will see a large temperature gradient with the moving boundary.

08

Long term...
/previous discussion issued at 404 am CST Wed Feb 20 2019/
Thursday through Tuesday.

Height rises will occur aloft on Thursday as ridging temporarily
builds in over the eastern Continental U.S. Ahead of a closed low near Las
Vegas. The surface front will remained stalled out near the I-85
corridor during the day. However, southwest flow returns aloft
ahead of the trough out west, causing the 925/850mb front to lift
northward. Associated frontogenesis/isentropic lift/overrunning
will maintain the Post-frontal precipitation area and cause it to
begin to shift northward. While overall rainfall amounts should be
lower, the presence of some weak elevated instability suggests
any ongoing flooding could be aggravated, and the Flood Watch will
continue to be in effect. Cams also suggest scattered
thunderstorm development south of the front with daytime heating
of an unstable air mass. This could pose a conditional
strong/isolated severe storm risk over our far southeast counties
where hail/gusty winds would be possible given sufficient deep
layer shear, while the stronger low-level shear stays north of the
boundary.

On Friday, continued southerly low-level flow should cause the
surface front northward as a warm front. The front will continue to
serve as a focus for precipitation along and north of it. The
NAM/European model (ecmwf) push the front north of the area while the GFS is slower
to push it north and is wetter. If the wetter solution wins out,
then an extension may be needed to the watch. A similar
conditional risk for a strong/isolated severe risk to Thursday
will exist over a greater portion of the area, but if the
NAM/European model (ecmwf) solution wins out then we may remain capped on Friday
afternoon with warm/dry air aloft. Also of note will be the
warming trend in temperatures with readings near 80 in our
southern counties.

Attention then turns to the potent shortwave trough and associated
deepening surface low tracking from the Southern Plains to the Great
Lakes, and the associated cold front. There remains some model-to-
model and run to run differences in the tilt/amplitude/timing of
this shortwave that will be key in determining the magnitude of the
event. The stronger upper-level forcing remain well northwest of the
area but there are some decent height falls. The somewhat weaker
upper-level forcing could help storms stay discrete longer, but
could also limit the areal coverage of severe storms or prevent them
from occurring at all this far south given capping concerns/warmer
700mb temperatures. A Stout warm sector will be present with this
system, with mid to upper 60s dew points and temperatures in the mid
to upper 70s, resulting in potential cape values of 1000-1500 j/kg
with favorable lapse rates. 70kt southwesterlies at 500mb and a 50
kt 850 mb jet would suggest the potential for supercells to form
ahead of a qlcs. Latest model trends are a bit quicker with this
system, suggesting storms may move through at a time when the
atmosphere is more unstable. Srh is certainly supportive of a
tornado threat. Storm Prediction Center has expanded the 15%/slight risk southeastward
to include areas along and north of I-59, and we will follow suit
in our graphics and begin to trend the severe weather potential statement confidence number
upward. Again, the possible forcing issues remain one potential
limiting factor, but looking at the moisture and quantitative precipitation forecast depicted by
the models suggests thunderstorms should develop this far south.
This rainfall could also cause additional flooding concerns.

Conditions finally dry out Sunday afternoon through Monday. Rain
chances return to the forecast Monday night and Tuesday, however.
Rainfall amounts currently look to be on the lighter side with an
overrunning situation as a weak wave of low pressure moves along the
Gulf Coast.

32/Davis



&&

Aviation...
06z taf discussion.

Even with the complex frontal structure we have across central
Alabama tonight, will hold on to the IFR ceilings/vis in many
locations overnight. The boundary stalls in south central Alabama
and only moves very slowly northward. Therefore, many locations
north will remain IFR throughout. The southern locations may
touch MVFR and then drop back down after 00z.

Mixture of LIFR/dense fog to start off several locations.
Additionally, the next upper impulse and associated upward motion
has already started back in Mississippi. Therefore, showers and
thunderstorms will be in the terminals overnight and into
Thursday. The southern terminals may get a reprieve before more
activity develops on Thursday. At any rate, restrictions rule this
package with a mix of ceilings/vis/rain/thunder. Wind direction
will be an issue also as the current frontal structure has north
winds in the north and south winds in the south. This appears to
remain the directions. Luckily, winds at most sites are light.

75

&&

Fire weather...

A prolonged period of rainfall will continue through the end of the
week, with a strong storm system expected over the weekend. Very wet
conditions are expected, especially across the northern half of the
area. There are no fire weather concerns at this time.

&&

Preliminary point temps/pops...
Gadsden 51 56 51 67 57 / 90 90 80 80 80
Anniston 55 57 54 72 58 / 100 80 80 70 70
Birmingham 53 57 55 71 63 / 90 90 80 70 70
Tuscaloosa 54 57 55 72 63 / 90 90 80 70 60
Calera 56 60 56 73 62 / 90 70 80 70 70
Auburn 58 74 61 75 61 / 80 60 70 40 50
Montgomery 63 75 63 80 64 / 100 60 70 50 30
Troy 67 76 65 80 63 / 50 60 50 30 20

&&

Bmx watches/warnings/advisories/...
dense fog advisory until 9 am CST Thursday for the following
counties: Autauga...Barbour...Bibb...Bullock...Chambers...
Chilton...Clay...Cleburne...Coosa...Dallas...Elmore...Hale...
Lee...Lowndes...Macon...Marengo...Montgomery...Perry...Pike...
Randolph...Russell...Shelby...Talladega...Tallapoosa.

Flood Watch through late Thursday night for the following
counties: Bibb...Blount...Calhoun...Cherokee...Clay...Cleburne...
Etowah...Fayette...Jefferson...Lamar...Marion...Pickens...
Shelby...St. Clair...Talladega...Tuscaloosa...Walker...Winston.

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