Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 180011

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

Aviation Discussion.



Ridging aloft with surface high pressure over the Mid Atlantic
region will continue to support dry conditions with fair skies
this afternoon and evening. Expect a gradual increase in clouds
overnight into early Saturday as a longwave trough swings east
toward the area. Winds will veer to the south this evening and
will start to increase in response to an increasing surface
pressure gradient.

For the extended period made some adjustments with breezy
conditions expected on Saturday with the approach of a sharp cold
front from the northwest. Wind advisory criteria is not expected
to be met for a large enough area to warrant an advisory but
winds will be gusty at times, particularly across our far northern
counties and along the higher elevations in central and eastern
Alabama. Rain showers with the potential for a few thunderstorms
will increase during the day Saturday and into the overnight
hours. The lack of surface-based instability and relatively weak
lapse rates will discourage strong and severe thunderstorm
activity despite favorable dynamics aloft. Behind the front a
freeze is becoming increasingly likely Monday morning across much
of our north and central counties. Both the gusty winds and
potential freezing temperatures Monday have been updated in the
hazardous weather outlook.


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.



00Z TAF Discussion.

Increasing cirrus clouds expected overnight with initially calm to
light winds. Expect MVFR cigs to spread across the area between 10-
12Z, coinciding with an increase in southerly winds of 6-8kts across
northern terminals. Do not anticipate fog development tonight due to
increase surface and boundary later winds.

Winds will increase by mid morning tomorrow, with sustained winds of
12-14kts and gusts up to 22kt expected through the day. Cig heights
will improve through the morning, but will remain at MVFR criteria
through the day.

A cold front will approach the area just beyond the forecast period,
with much of the expected rainfall along and behind the front. Rain
will need to be added to the forecast for most if not all terminals
with the 18/06Z forecast.




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


Gadsden     49  71  39  51  27 /   0  50  80   0   0
Anniston    49  71  40  53  29 /   0  40  70   0   0
Birmingham  55  72  41  53  32 /   0  50  80   0   0
Tuscaloosa  56  75  40  56  30 /  10  60  80   0   0
Calera      53  74  38  54  30 /   0  40  70   0   0
Auburn      49  72  43  55  32 /   0  20  60   0   0
Montgomery  50  77  44  58  33 /   0  20  60   0   0
Troy        49  76  44  58  33 /   0  10  60   0   0




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