Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 132332

Area Forecast Discussion
National Weather Service Birmingham AL
532 PM CST Wed Dec 13 2017

For 00Z Aviation.



A cold front is approaching the area from the northwest this
afternoon. Not much cloud cover (and no precip) associated with
the front as it is pushing across northern Arkansas and Tennessee.
The surface low associated with the front will push eastward
across the Central Appalachians and Delmarva tonight, with the
upper level pattern becoming more zonal. The front will slow
without a stronger upper level push and remain north of the area

Winds have been breezy this afternoon due to the tightened
pressure gradient, but will relax this evening. Skies will remain
mostly clear, with temperatures falling into the 30s.


Thursday through Tuesday.

The upper-level pattern will change across the area by tomorrow,
in the wake of today`s clipper system. High amplitude ridging will
remain in place along the West Coast, with troughing over the
northeastern CONUS, but a positively tilted trough axis will set
up over the Desert Southwest and the Plains. Southern portions of
the trough will form a cutoff low centered over northwest Mexico
near the Gulf, while northern portions of the trough will push
eastward across the Ohio and Tennessee Valleys as a shearing out
shortwave trough, in response to a strong upper low digging into
Ontario. The net result for Central Alabama will be northwesterly
upper-level flow shifting to westerly and then southwesterly
through the beginning of next week. At the surface, as mentioned
above, the front associated with today`s clipper system will be
slowing stalling as it becomes parallel to the upper-level flow. A
very dry air mass will remain in place over the area with little
in the way of cloudcover expected along the front, but there will
be some cirrus across the southern counties due to the
strengthening southern stream jet. The front will result in a
temperature gradient across the area, with readings struggling to
get out of the upper 40s along the northern row of counties, while
the far southeastern counties will see highs around 60.

The front will make another push southward Thursday night and make
it all the way through the forecast area this time, as high
pressure builds into the Southern Plains and low pressure develops
over Michigan. Northerly low-level flow behind the front will
prevent any low-level moisture return ahead of the shortwave
trough moving through the Ohio and Tennessee Valleys on Friday.
Therefore the shortwave will mainly just produce increased mid and
high level clouds despite favorable QG forcing for ascent. Some
virga or a stray sprinkle is possible across the far southern
counties, but dry air below 600 mb will prevent any measurable
precipitation. With 925 mb temperatures falling below 0C across
the northern counties, northwest winds, and high clouds, a cool
day is expected with highs in the 40s north of I-85 and low 40s
across the northern counties. A surface ridge axis will move over
the area Friday night. This will result in almost ideal
radiational cooling conditions with the exception being a few
cirrus clouds. Lows will fall into the 20s in many areas with some
lower 20s possible in the typically coldest locations.

The cutoff low over northwest Mexico will eject northeastward as a
shearing shortwave trough over the weekend in response to a kicker
trough moving into the Rockies/High Plains. Southerly flow ahead
of this system will result in milder temperatures on Saturday, but
dry air will remain in place through the daytime hours. A 40-50 kt
LLJ will strengthen over Mississippi Saturday night, resulting in
increasing rain chances after midnight in the west Saturday night
due to increasing moisture/isentropic lift and passage of a warm
front aloft. PWATs increase to around 1.5 to 1.8 inches Sunday,
around the 95th percentile for December. With a 40-50 kt LLJ,
widespread soaking rains are expected. Lack of instability,
antecedent conditions, and the fairly progressive nature of the
system should prevent a flooding threat unless it slows down too
much. There is always the possibility for models to be too quick
with ejecting a southern stream cutoff low, but the strength of
the kicker trough and good model and ensemble agreement increases
confidence in the timing, and PoPs were raised substantially for
Sunday. A weak surface low may develop, while the surface warm
front will try to lift northward into our southern counties,
followed by the passage of a cold front. With rain falling into an
air mass with dew points in the 30s initially, northward
progression of the warm front will likely be hampered by
evaporative cooling processes, and may end up staying just south
of the area. Models indicate some weak elevated instability over
our southern counties, but surface-based instability looks
negligible with dew points struggling to reach 60. Therefore,
severe storms are not expected at this time, but will continue to
monitor just in case due to the strong shear.

Models begin to diverge Sunday night. The ECMWF is quicker with
the exit of precipitation Sunday night, but has trended towards
the GFS in maintaining southwest flow aloft ahead of the trough
over the southern High Plains. It remains quicker with the
ejection of this trough during the first half of the week, on the
fast side of its ensemble members. Will favor a slower solution
similar to the GFS and the EPS mean, given the lack of a strong
kicker in this case and strengthening ridging over the Bahamas,
with the front stalling to the southeast of the area and post-
frontal rains continuing under southwest flow aloft, with the
highest chances in the southeast. Temperatures remain plenty warm
to prevent non-liquid precipitation concerns. Dry air moves in by



00Z TAF Discussion.

VFR conds thru the period with only cirrus clouds expected. A weak
cold front near the Arkansas/Missouri state line will push into
north Alabama on Thursday and become stationary near the I-20
corridor by 00z. Southwest winds thru 12z will become westerly
as the front moves into the area.




No rain is expected until Saturday night. Dewpoints increase
Thursday, and relative humidity values remain well above critical
thresholds through the weekend.


Gadsden     33  52  30  43  26 /   0   0   0   0   0
Anniston    35  54  32  45  28 /   0   0   0   0   0
Birmingham  37  54  32  44  29 /   0   0   0   0   0
Tuscaloosa  35  56  33  47  28 /   0   0   0  10   0
Calera      35  56  32  45  28 /   0   0   0   0   0
Auburn      36  56  37  49  32 /   0   0   0  10   0
Montgomery  35  58  36  49  30 /   0   0   0  10   0
Troy        39  60  40  51  33 /   0   0   0  10   0




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