Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 180923

Area Forecast Discussion
National Weather Service Birmingham AL
323 AM CST Sat Nov 18 2017

Today and Tonight.

A wide range of temperatures early this morning with lower 60s
across west Alabama and mid 40s across east Alabama.  Warm air
advection has increased significantly for areas along and west of
Interstate 65 where southerly surface winds have been running 5 to
10 mph. Low clouds have also developed for areas across east
Mississippi and western Alabama. No rain was currently indicated
across the region as the air mass is still relatively dry with
precipitable water values around one inch. A few showers may develop
by late morning across northwest Alabama due to strong warm air
advection and increasing low level moisture. Most of the shower and
thunderstorm activity will be confined to near the surface front due
to a strong pre-frontal upper level inversion. The surface cold
front will enter Marion and Lamar counties late this afternoon and
quickly track southeast, reaching Troy near midnight tonight.
Thunderstorm activity looks very limited ahead of the surface front
due to weak mid level lapse rates and CAPE values struggling to
reach 500 J/KG. Any thunderstorms that may develop will likely be
north of I-59. The rain will end within one or two hours after
fropa, so rainfall amounts will be mostly under one-quarter of an


Sunday through Saturday.

The cold front is well to our southeast by Sunday morning, so expect
clear skies as high pressure builds in with the colder air mass. The
center of the surface high slides eastward through the ArkLaTex and
into the TN River Valley Sunday night into Monday morning. Models
are in pretty good agreement with the center moving across North-
Central AL by sunrise Monday morning. This will lead to clear skies
and calm winds across the area, allowing for very efficient
radiational cooling. Expect much of the area to drop to near or
below freezing early Monday morning with the coldest temperatures
(mid to upper 20s) in the northern counties. With increasing model
agreement in the setup, I will continue mention cold weather impacts
in the HWO and increase the confidence. After discussions with
neighboring offices, will hold off any freeze watch or warning
products for now, as we`re still 48 hours out.

The high pressure shifts eastward towards the Carolinas during the
day Monday. Another longwave trough deepens across the Eastern
Plains Monday night into Tuesday. Both the GFS and ECMWF pick up on
a weak impulse ejecting out of the base of the trough Tuesday
morning. This impulse moves through Central AL fairly quickly, and
without much moisture return, I`m not convince we`ll see any rain
activity out of it. However, have included at least a slight chance
of showers south and east of the I-85 corridor Tuesday just in case.

As we get into the second half of the week, the forecast becomes
somewhat of a headache. Overall, the synoptic setup is a highly
amplified pattern with a deep trough over the Eastern US and a ridge
over the Western US. An impulse moves through the trough into the
Southeast US on Wednesday, and the models have not handled the
evolution of the trough and this impulse very well. The GFS, which
had been showing a phased trough moving through now shows a cutoff
low developing in the northern Gulf of Mexico and sliding eastward
into Southern GA. The ECMWF, which had been showing something
similar now shows a phased trough moving through with high pressure
quickly building in behind. Now, remember that most of this week
we`re under northerly or northeasterly flow, which limits our
moisture return. Under normal circumstances, this would make me lean
on a dry forecast. The GFS solution, however, has a fairly strong
surface low developing in the Eastern Gulf of Mexico, which as it
slides into Southern GA, could lead to enough wrap-around moisture
to cause light shower activity in our southeastern counties.
Essentially, the forecast comes down to (1.) if (or where) a low
develops along the coast and (2.) how strong the low is if it forms.
If a surface low does not form, or if it is weak, expect mostly dry
conditions through the holiday weekend. If a surface low does form
and can pull in enough moisture, expect some light rain showers
Wednesday night through Thursday before high pressure again builds
in Friday into Saturday leading to a dry start to the weekend. For
my forecast this morning, I have edged drier given the lack of
substantial moisture return, but kept a slight chance of showers
south and east of the I-85 corridor on Thanksgiving just in case.
Confidence remains low in the forecast from Wednesday night through
Saturday, so expect changes as models hopefully become more



06Z TAF Discussion.

Sct to bkn high cigs across the area to start the period with light
southeasterly winds at most terminals. Expect MVFR cigs to spread
across the area between 10-12Z, with a chance of IFR cigs at TCL.
Southerly winds will also increase early Saturday morning, to 6-
10kts across the north. Do not anticipate fog development overnight
due to increase surface and boundary layer winds.

Cig heights will improve through the morning tomorrow, but will
remain at MVFR criteria through the day. Winds will increase by mid
morning tomorrow, with sustained winds of 12-14kts and gusts up to
24kt expected through the day.

A cold front will approach the area tomorrow evening. Ahead of the
front, scattered showers are possible across the north, before a
more organized line of showers/rain along and behind the front.
expect an increase in wind speeds as the front quickly crosses the
area. Have included initial timing of the shower activity, rainfall
along the front and a wind shift to the west.




Windy conditions today ahead of fast-moving cold front. Rain
chances increase Saturday afternoon and evening as the front moves
through the area. There are no fire weather concerns at this
time. Dry and colder air sets up for the Sunday and Monday.


Gadsden     69  39  52  27  57 /  50  80   0   0   0
Anniston    70  40  53  29  59 /  30  70   0   0   0
Birmingham  72  42  53  32  58 /  40  80   0   0   0
Tuscaloosa  73  41  56  30  59 /  40  80   0   0   0
Calera      72  40  54  30  58 /  30  70   0   0   0
Auburn      70  43  55  32  59 /  10  50   0   0   0
Montgomery  75  44  58  33  60 /  10  60   0   0   0
Troy        74  45  59  33  61 /  10  50   0   0   0




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