Area Forecast Discussion
Issued by NWS Birmingham, AL

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782
FXUS64 KBMX 141742
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
1142 AM CST Thu Dec 14 2017

.UPDATE...
For 18Z Aviation.

&&

.SHORT TERM...
Today and Tonight.

This AM in the upper levels we are transitioning to zonal flow
across the Deep South. The surface front remains held back some to
our northwest with the associated surface low currently over Eastern
Ohio. The low should continue to the east and weaken today, but
could drag the front a little further to the south bringing it into
Central Alabama before fizzling out, but there will not be much of a
push in the upper levels. We continue to look at a moisture starved
front with no rain expected, only some scattered clouds. For
temperatures, we should see a little warmer conditions across the
south with a north/south gradient due to possible sagging
southward of the front.

08

.LONG TERM...
Friday through Wednesday.

Friday/Friday night:

Strong west-southwest flow aloft will be in place over much of
the eastern CONUS on Friday between troughing centered over
eastern Canada and the northeastern CONUS and subtropical ridging
centered near the Greater Antilles. A shortwave will be moving
through the northeastern CONUS trough with a shearing axis of
vorticity extending southwestward to a cutoff low centered over
the Gulf of California and northwest Mexico. This shortwave and
a surface high moving into the Southern Plains will push the cold
front south of the area. Northwesterly low-level flow behind the
front will maintain a dry air mass below 600mb and prevent any
measurable precipitation despite strong upper-level forcing for
ascent. Expect just an increase in mid and high level clouds, with
some virga across the far southern counties. Couldn`t rule out a
stray sprinkle there, but chances are too low to mention in the
forecast at this time. Low-level cold air advection will bring in
925mb temperatures around 0C across the north. This will result in
a cool day with highs only in the low to mid 40s across the north,
with upper 40s to low 50s south. Skies should clear out Friday
night with the exception of some cirrus from the subtropical jet.
With calm winds expected as surface high pressure slides eastward
across the Deep South, and a dry air mass, expect strong
radiational cooling with lows in the upper 20s to around 30, and
some low to mid 20s in the typically colder locations.

Saturday/Saturday night:

A trough digging into the western CONUS will serve as a kicker to
cause the cutoff low over Mexico to eject across Texas as a
negatively tilted trough Saturday and Saturday night. Ridging will
build in aloft across the Deep South ahead of this feature. Flow
above the surface will become southwesterly, but an easterly
component to winds at the surface due to high pressure moving into
Georgia will keep high temperatures in check, remaining in the
50s. Dry air will remain in place at low-levels, with only mid and
high level clouds expected during the day and evening. A 40-50 kt
LLJ will strengthen over the Lower Mississippi Valley after
midnight Saturday night. A weak surface low that develops along
the old frontal boundary in the northwest Gulf due to forcing
associated with the trough will also lift northward across this
area. Models have trended slower with the onset of precipitation
in West Alabama late Saturday night, and it may hold off until
Sunday morning. However, will keep in a chance of scattered
showers in the west for now given strong warm air
advection/isentropic lift if the dry air at the surface can be
overcome. Lows will be in the upper 30s to low 40s, with low to
mid 30s possible in the northeast. Many areas will probably see
their low temperatures in the evening before rising after
midnight. Strong warm air advection will prevent any wintry
precipitation concerns.

Sunday/Sunday night:

The shortwave trough will be in the process of shearing/weakening
as it lifts northward towards the Ohio Valley, while deep layer
moisture will continue to increase. There are some model
discrepancies in QPF that begin to appear due to the decrease in
upper-level forcing behind the shortwave and the potential for
convection along the Gulf Coast. Will focus the highest PoPs
generally west of I-65 where the best LLJ will be. At the surface,
a warm front will try to lift northward into our southern
counties, but its northward progression will be restricted
initially by evaporative cooling as rain falls into the lingering
dry air mass across the area. This will limit high temperatures
and instability during the day on Sunday, and have removed the
slight chance of thunder from the southern counties during this
time.

Moist deep layer southwesterly flow remains in place on Sunday
night ahead of a trough over the western CONUS. The ECMWF has
trended towards the GFS with a wetter solution during this time.
PWATs will increase to 1.5 to 1.8 inches, which is at or above the
95th percentile for December. Dewpoints will climb into the 60s
across the southern counties which could allow for some weak
instability and thunderstorms to be possible. But the LLJ will
have weakened by this time with little upper-level forcing and
weak lapse rates, so severe storms are not expected at this time.

Monday through Wednesday:

Forecast confidence becomes low by this time period. The GFS
continues to indicate a wetter solution, keeping a more phased
trough to the west of the area. Meanwhile the ECMWF pushes the
northern portion of this trough quickly eastward while indicating
a cutoff low in the southern stream over northwest Mexico. Overall
it seems the ECMWF has trended slightly more to the GFS, and the
EPS ensemble mean/a fair number of EPS members are wetter than the
deterministic ECMWF. Therefore have tended towards a wetter
solution for Monday and Tuesday, but kept PoPs in the chance
range. The GFS keeps high PWATs around during this time, with
unidirectional flow indicating the potential for training and the
possibility of a couple rounds of heavier rainfall. Flash flood
guidance is high across the area given recent dry conditions, so
the ground could handle 1 or 2 rounds of rain but localized
flooding could develop if multiple rounds develop. Will hold off
on mention any flooding threat in the HWO at this time given
inherent uncertainty with QPF and the model differences.

The GFS dries out the area by Wednesday, while the ECMWF brings
some precipitation across the Gulf Coast associated with the
remnants of the cutoff low. Will keep a dry forecast going during
this period.

32/Davis

&&

.AVIATION...
18Z TAF Discussion.

VFR forecast for the next 24 hours with only high cirrus expected.
Winds will be variable, but become consistently out of the west to
northwest at 5-7kts this afternoon as a front moves southward
through the area. Winds diminish tonight, before increasing Friday
morning from the northwest to north behind the cold front.

No rainfall is expected with this system with only meager upper
moisture.

14

&&

.FIRE WEATHER...

No rain is expected until late Saturday night or Sunday. Dew points
increase today, and relative humidity values remain well above
critical thresholds through the weekend.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     54  30  44  26  53 /   0   0   0   0   0
Anniston    56  32  44  27  54 /   0   0   0   0   0
Birmingham  56  32  45  29  55 /   0   0   0   0   0
Tuscaloosa  58  33  47  29  56 /   0   0   0   0   0
Calera      57  33  45  28  55 /   0   0   0   0   0
Auburn      58  37  49  31  56 /   0   0  10   0   0
Montgomery  60  37  50  30  57 /   0   0  10   0   0
Troy        61  39  50  31  57 /   0   0  10   0   0

&&

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

&&

$$

08/32



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