Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 171822

Area Forecast Discussion
National Weather Service Birmingham AL
1222 PM CST Fri Nov 17 2017

For 18Z Aviation.


Today and Tonight.

Clear skies and calm winds have allowed temperatures to drop into
the middle 30s across areas north of I-20 early this morning, with
upper 30s to lower 40s elsewhere. Patchy fog has also developed,
mainly in the valleys and along the rivers. Some of the fog has been
locally dense across the southern counties. Visibilities have been
variable, and will continue to highlight in the HWO, but no dense
fog advisory anticipated at this time. Some mid to high clouds will
impact mainly western counties later this morning and afternoon as a
weak short wave trof passes to the north of Alabama.

Temperatures will climb quickly this morning as low level winds
become south and southeast. Highs today will be close to the values
on Thursday, with upper 60s to lower 70s.  Southwest low level flow
will increase tonight ahead of a strong cold front. Much warmer
temperatures expected along and west of I-65 where lows will be in
the lower to middle 50s, with mid 40s east of I-65.  The air mass
above 850mb will remain too dry for any precipitation.


Saturday through Friday.

A longwave trough stretches down through the Central US Saturday
morning and develops a surface low that quickly moves northeastward
into the Great Lakes, stretching a strong cold front southward
through the Mississippi River Valley. The trough digs southward
through the day pushing the front quickly across Central AL. Ahead
of the front, the pressure gradient tightens, which will lead to
breezy conditions across the area during the day Saturday. Models
are not handling the timing of the front exceptionally well,
likely due to its quick forward speed. Expect the front to move
from our northwestern counties to south of the I-85 corridor in
6-8 hours Saturday evening and night. Just along and behind the
front, expect a narrow band of rain showers. At any one location,
I wouldn`t expect rain to last for very long. By midnight Saturday
night, the front has pushed south of our area, ending the best
rain chances.

The system for Saturday has substantial upper level support with a
50kt jet at 850mb and a 120kt+ jet at 250mb. Models don`t have the
jets overlapping across our area, but still show 0-6km bulk shear
magnitude exceeding 50kts. Instability is minimal given the low
dewpoints and a temperature inversion aloft. Even with the minimal
CAPE, I can`t rule out an isolated thunderstorm given the favorable
dynamics. However, I will not be adding mention for severe weather
in the forecast due to the very marginal setup overall.

High pressure builds in with the colder air mass behind the front
SUnday. Expect clear skies and cooler than normal high
temperatures. Overnight Sunday into Monday morning, the center of
the high pressure shifts over North-Central AL, leading to
synoptic subsidence, clear skies, and calm winds. This should
allow effective radiational cooling by early Monday morning.
Expect most areas across Central AL to be near or below freezing,
with the northern- most counties dipping into the upper 20s. Will
therefore continue mention of cold weather impacts in the HWO and
increase the confidence a little more due to models coming into
better agreement on the track of the high pressure center.

Despite the cold start Monday, afternoon highs should rebound into
the upper 50s to near 60 as the high pressure shifts to our east,
veering our winds to the east-southeast. A secondary upper-level
impulse moves through the area Tuesday ahead of deepening trough
sliding down from the Pacific NW.  It`s at this point in the
extended (early Wednesday) that models have been having problems
resolving the evolution of this trough. The 00z GFS and ECMWF have
come into better agreement with the trough digging into the northern
Gulf of Mexico, though the EC still tries to cut-off a low and slide
it eastward from there. Given both scenarios bring in dry northerly
flow for Central AL, I have edged the forecast on the drier side
Wednesday through Friday. It will be worth watching how models trend
with this trough as solutions have been flipping on a wet vs. dry
end of the week- so while my current forecast is dry, confidence
isn`t overly high due to model inconsistency.



18Z TAF Discussion.

Mostly clear skies are expected through the first 12-18 hours.
MVFR ceilings entering the picture from the northwest after 11z.
Strong warm air advection pattern becoming more noticable with
time. South winds will remain at 4-7kts overnight eliminating the
chances for any fog development.

A cold front will approach the area just beyond the forecast
period. Winds ahead of the front will increase at and above the
surface. It appears that winds will remain just below the LLWS
criteria at this time. Will most likely add some higher winds and
some gust on the next package for the tail end of this period and




Dry weather continues for today. Expect winds to increase ahead of
a fast-moving cold front on Saturday. Rain chances increase
Saturday evening as the front moves through the area. There are no
fire weather concerns at this time.


Gadsden     49  71  39  51  28 /   0  50  70   0   0
Anniston    49  71  40  53  29 /   0  40  70   0   0
Birmingham  55  72  41  53  32 /   0  50  70   0   0
Tuscaloosa  56  73  40  56  31 /  10  60  70   0   0
Calera      53  73  38  54  31 /   0  40  70   0   0
Auburn      49  72  43  55  33 /   0  20  60   0   0
Montgomery  50  76  44  58  32 /   0  20  60   0   0
Troy        49  75  44  58  34 /   0  10  60   0   0




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