Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 111203 AAA

Area Forecast Discussion...UPDATED
National Weather Service Birmingham AL
603 AM CST Sun Dec 11 2016

For 12Z Aviation.


Today and Tonight.

Currently our surface ridge center is now over the Mid Atlantic
Coast with easterly flow ensuing across the state. Return flow
moisture from the retreating ridge is allowing some stratus clouds
to develop and show up on 11u-3.9u satellite across the southeast
third of Georgia extending into the southeast tip of Alabama and
into the Central Panhandle of Florida. The stratus clouds are slowly
drifting/developing further to the west/north and that westward
spreading of cloud cover should continue through today as well as
an increase in clouds from the northwest. The northwest increase
is thanks to a front digging across the Central Plains early this
AM that will spread toward the Mid Mississippi River Valley today
as the parent low moves northeast toward the Great Lakes. Rain
should hold off until this evening at the earliest. Temperatures
will be milder during the day today with not a huge diurnal dip
tonight as southeast/southerly flow takes over ahead of our
approaching front.


Monday through Saturday.

The front will continue to slowly sag southward during the day on
Monday as it becomes parallel to the zonal upper-level flow,
stalling near the southern border of the forecast area late Monday
afternoon. A weakening band of showers will accompany the front,
maintained by a narrow axis of 1.3 inch PWATs and high 1000-500 mb
mean RH values. Cloudcover will keep highs in the low 60s in most
locations but southerly flow ahead of the front will push highs
into the low 70s in the far south. Instability looks very weak due
to dewpoints struggling to reach 60 in the far south.

Low-level southwesterly flow overrunning the front/isentropic lift
is expected to result in a band of light rain redeveloping Monday
night and lifting northward. Rain chances have been adjusted
accordingly. This will keep low temperatures on the mild side.
Models are coming into better agreement on a weak surface wave
moving along the front Tuesday, nudging the front northward
slightly as a weak shortwave moves through in the fast zonal flow
aloft. This will result in continued likely rain chances across
portions of the area with sufficient deep layer moisture present.
Dew points do creep up into the 62-65F range across the southern
forecast area on Tuesday, with some weak CAPE values supportive of
a couple thunderstorms. A strong storm may be possible with
increased mid-level lapse rates and 45-50 kts of 0-6 km bulk
shear. However, helicity remains weak and low-level shear very
modest with clouds and precipitation making the development of any
appreciable instability questionable.

Rain chances will slide southeastward Tuesday night into
Wednesday. Despite the likely rain chances through the first half
of the week, most areas will only see between a half and three
quarters of an inch of rain. Another arctic front arrives on
Wednesday in response to a deep upper low of Arctic origin located
south of Hudson Bay. Models are trending quicker in moving the
moisture out of the area, well before the colder air arrives. The
Arctic air mass will mainly push eastward towards New England
rather than southward towards Central Alabama, and overnight lows
Wednesday night and Thursday night will mainly just be in the
upper 20s to low 30s, with mid 20s in the colder spots. Thursday
will be the coldest day with highs in the low 40s across the

A longwave trough will amplify over the central and western US
Friday into the weekend, with a shortwave embedded in the trough
resulting in strong surface cyclogenesis over the Central High
Plains. This strong surface low will track northeastward into
Ontario by Saturday night. Strengthening southerly flow over-
running the initially cold air mass will result in isentropic
lift-associated precipitation developing as early as Friday
afternoon. The air mass will be very dry initially and will take
some time to saturate, and models vary on how quickly this will
occur. Still expecting temperatures to warm up enough to support
only liquid precipitation, but will need to monitor the strength
of a wedge developing over Georgia and the Carolinas Friday night.

Some potential for severe weather may also evolve with this system
as well given the relatively strong wind fields. The models are
in remarkably good agreement with the synoptic features this far
out but there are still a few differences. The ECMWF is currently
more aggressive with moisture return (dewpoints in the mid to
upper 60s) yet also faster with the progression of the cold front
compared to the GFS. There are still too many limiting factors to
add a low confidence tornado threat to the HWO including weak
upper-level forcing/height falls at this latitude and 500 mb
temperatures only around -10C resulting in weak mid-level lapse
rates, due to the amplified and positively tilted nature of the
trough. However, we will continue to closely monitor this system.



12Z TAF Discussion.

MVFR stratus clouds are creeping their way into Central Alabama
from the south/east around the base of the retreating surface
ridge. They are already at TOI and approaching MGM. Think that
these CIGs will make it as far north as EET this AM. By sunset we
should see further progression across the state of lower CIGS.
Toward the end of the TAF period, -RA will approach from the
northwest ahead of a front. All sites except MGM/TOI will have a
PROB30 group for -RA by around 9-10Z.




Moisture begins to increase today with rain chances returning
late tonight into Monday. Rain chances will continue through the
middle of next week as a front stalls over the area. Relative
humidity values will be well above critical thresholds.


Gadsden     54  45  61  51  62 /   0  60  80  40  40
Anniston    53  46  62  54  63 /   0  50  80  60  50
Birmingham  57  51  62  54  63 /   0  50  80  60  40
Tuscaloosa  58  52  65  55  64 /   0  40  80  60  40
Calera      57  50  64  56  65 /   0  40  80  60  50
Auburn      55  48  68  58  66 /  10  30  60  60  60
Montgomery  59  52  72  60  69 /  10  30  60  60  60
Troy        58  49  74  61  70 /  10  30  40  50  60





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