Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 131222

Area Forecast Discussion
National Weather Service Birmingham AL
622 AM CST Wed Dec 13 2017

For 12Z Aviation.


Today and Tonight.

This AM in the upper levels an Eastern Conus trough will swing
around northeastward toward New England from the Atlantic Seaboard
and toward Quebec. This will change our flow today across Alabama
from the northwest that we had yesterday to west-northwest today and
more zonal tonight. This change will have an effect on the
progression of the next surface front. The front will progress
southeast from the Dakotas toward the Deep South today as the
associated low moves from Minnesota toward Ohio. The front will be
aided by an upper shortwave trailing behind the exiting upper
trough. By this evening though as our upper flow changes to more
zonal our surface low should scoot eastward and off the coast with
our front flattening/stalling/fizzling out as it moves into Central
Alabama with no decent push from the upper levels. This front does
not appear to bring us any rain or even much in the way of cloud
cover as there is little moisture associated with it, only a
shifting around of the winds today and tonight.


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



12Z TAF Discussion.

VFR forecast for the next 24 hours with mainly a wind forecast.
Tight pressure gradients this afternoon will be present ahead of
the next approaching front. Some gusty winds will be possible.
Front looks to stall/fizzle out across the mid section of Alabama
tonight. No rain or clouds are expected with this system. The
atmosphere is too dry overall.




No rain is expected until Saturday night. Relative humidity
values will fall into the lower 30s with 20 foot winds around 10
mph this afternoon, but will remain above critical values. Minimum
relative humidity values on Thursday and Friday will be higher
than today as a dry cold front moves through.


Gadsden     49  32  52  30  43 /   0   0   0   0   0
Anniston    49  34  54  32  45 /   0   0   0   0   0
Birmingham  51  37  54  32  44 /   0   0   0   0   0
Tuscaloosa  55  36  56  33  47 /   0   0   0   0  10
Calera      51  35  56  32  45 /   0   0   0   0   0
Auburn      50  36  56  37  49 /   0   0   0   0  10
Montgomery  54  36  58  36  49 /   0   0   0   0  10
Troy        54  38  60  40  51 /   0   0   0   0  10





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