Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 162136
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
336 PM CST Tue Jan 16 2018

.SHORT TERM...

Snow is starting to move into the Birmingham Metro here just
before 2:00pm. A stronger band of heavier snow is stretching
through Pickens and Northern Tuscaloosa County now and should
start moving into the Tuscaloosa area in the next hour. Soundings
using any of the well-initialized models show completely saturated
profiles well into the dendritic growth zone. Temperatures aloft
remain well below freezing, which will not allow any melting of
the snowflakes. This combined with the saturated profiles not
allowing much evaporation is resulting in some fairly large
snowflakes that exhibit the "classic" dendrite snowflake shape.
One impressive feature is just how quickly the temperatures are
dropping as this band of snow approaches. In the last 40 minutes
or so, we`ve dropped 5 degrees here at the Shelby County Airport.
Birmingham has already dropped into the upper 20s while Anniston
is sitting at 41 degrees. We have gotten some reports of a brief
winter mix at the onset of precipitation, but this will quickly
transition to all snow as the colder air filter in.

The concern continues to be the icing of roads and bridges as the
initial snow melts then re-freezes when temperatures plummet.
We`ve already seen this occur in the northwestern counties as the
temperature in Hamilton is sitting at 20 degrees now. Travel is
highly discouraged throughout central AL this afternoon and
evening with some roads in the northwest already being deemed
impassable.

Overall, the current forecast seems to be on track with the
timings across Central AL. Some high resolution guidance still
hinting at re-development of the snow bands in the southeastern
counties tonight, so will continue mention of impacts through this
evening as the system moves south and east of the I-85 corridor.
Problem with increasing to wording is two-fold. One the
temperatures in this area did get up to the 50s and even near 60
degrees. This likely allowed the ground temperatures to warm up
and could possibly melt the initial amount of snow, so confidence
to increase to a Winter Storm Warning is not as high as needed at
this time, but we will be monitoring the trends very closely to
see if the upgrade is needed.

Did go ahead and break the Winter Weather Advisory into two
sections to discuss the differences in accumulation. The north
will see generally an inch with upwards of 1.5, whereas the south
could see 1.5 to 2 inches with possibly higher totals.

25/16

.LONG TERM...
Wednesday through Monday.

Wednesday/Thursday:
Strong cold air advection will be in place on Wednesday, on the
back side of the upper trough and to the east of a 1042mb surface
high centered over the Southern Plains/Ozarks. Clear skies will
be present due to the dry Arctic air mass, but midday 925 mb
temperatures around -12C and potential snowcover will keep
temperatures below freezing across the majority of the area
(generally north of US80/I-85). Highs will range from the upper
20s to low 30s north and be in the low 30s south. If the expected
band of snow across the southeastern counties materializes then
high temperatures there may need to be lowered. With breezy
northwest winds wind chills will still be in the upper teens to
low 20s even in the afternoon. Another hard freeze is expected
Wednesday night as the surface high moves eastward across the Deep
South with lows in the teens areawide.

The deep layer flow will become westerly on Thursday. This will
allow cold air advection to cease. But the air mass being advected
in from the west will still be cool initially, coming off an
additional area of snowcover over the ArkLaMiss. This combined
with shallow mixing due to high pressure overhead will only allow
highs to reach the low 40s. Temperatures will rise above freezing
by midday. Another cold night is expected Thursday night as high
pressure overhead results in ideal radiational cooling conditions.
All but the coldest spots in the northeast will remain above hard
freeze criteria (15), with lows ranging from the upper teens far
north to low 20s elsewhere.

Friday/Saturday:
The trough currently moving into the Pacific Northwest will split
into a couple pieces as it moves eastward, with the southern
piece forming a partially cutoff low over the Rio Grande region.
These waves will remain moisture starved over our area with just
mid/high level clouds at times. Warm air advection kicks in for
Friday/Saturday with deep layer west-southwesterly flow, allowing
highs to reach the 50s Friday and low 60s by Saturday.

Sunday/Monday:
A stronger Pacific trough will move across the CONUS during this
time period. This allows a Colorado low to deepen as it moves
northeast into the Great Lakes with a trailing cold front moving
through the area. There could be enough moisture ahead of the
front for some scattered showers Sunday. Models/ensembles are
showing some usual spread in the timing of the front, but expect a
line of widespread showers and at least isolated thunderstorms to
move across the area sometime in the Sunday night/early Monday
timeframe. Models show 60 dewpoints trying to creep up into our
southern counties, but dewpoints could easily underachieve given
this week`s cold front scouring out moisture across the Gulf, and
a southeasterly component to the winds across our area. This will
be the main limiting factor for the development of any instability
across Central Alabama ahead of a possible QLCS. Will continue to
monitor given the shear and forcing, and mid-level lapse rates
around 6-6.5 C/km. But any low-end severe threat looks far too
marginal/questionable to mention in the HWO at this time.

32/Davis

&&

.AVIATION...
18Z TAF Discussion.

MVFR to IFR conditions expected this afternoon and evening with
light snow at most terminals. Improving conditions expected on
Wednesday.

An arctic front extended from near Mobile northeast to near
Montgomery to near Alexander City at this hour. Winds will shift
abruptly to the north upon passage of the front at MGM/TOI in the
next few hours. Expect lowering ceilings at our northern sites
from west to east as more snow arrives from the west. Our southern
sites will experience lowering ceilings later this afternoon and
evening as more precipitation develops over southwest Alabama and
moves to the northeast. Low level winds will remain out of the
north with some LLWS expected in a shallow layer just off of the
surface through the next few hours at ANB/ASN/EET.

Precipitation and coverage of BKN/OVC skies will subside after 06Z
tonight followed by continued improvement of CIG`s through the end
of the cycle.

05

&&

.FIRE WEATHER...

An Arctic front will slide south of the area tonight with light
to moderate snow likely. Dry and very cold conditions are expected
Wednesday.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     12  29  14  41  17 /  80   0   0   0   0
Anniston    12  29  17  42  20 / 100   0   0   0   0
Birmingham  12  30  15  42  21 /  70   0   0   0   0
Tuscaloosa  13  32  16  43  22 /  70   0   0   0   0
Calera      13  30  18  43  22 / 100   0   0   0   0
Auburn      16  31  18  43  24 / 100  10   0   0   0
Montgomery  16  33  18  44  22 / 100  10   0   0   0
Troy        17  33  18  44  22 /  90  10   0   0   0

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Hard Freeze Warning from 9 PM this evening to 9 AM CST Thursday
for the following counties: Autauga...Barbour...Bibb...Blount...
Bullock...Calhoun...Chambers...Cherokee...Chilton...Clay...
Cleburne...Coosa...Dallas...Elmore...Etowah...Fayette...Greene...
Hale...Jefferson...Lamar...Lee...Lowndes...Macon...Marengo...
Marion...Montgomery...Perry...Pickens...Pike...Randolph...
Russell...Shelby...St. Clair...Sumter...Talladega...Tallapoosa...
Tuscaloosa...Walker...Winston.

Winter Weather Advisory until 6 AM CST Wednesday for the
following counties: Autauga...Barbour...Bibb...Blount...
Bullock...Calhoun...Chambers...Cherokee...Chilton...Clay...
Cleburne...Coosa...Dallas...Elmore...Etowah...Fayette...Greene...
Hale...Jefferson...Lamar...Lee...Lowndes...Macon...Marengo...
Marion...Montgomery...Perry...Pickens...Pike...Randolph...
Russell...Shelby...St. Clair...Sumter...Talladega...Tallapoosa...
Tuscaloosa...Walker...Winston.

Wind Chill Advisory from 9 PM this evening to 9 AM CST Wednesday
for the following counties: Autauga...Barbour...Bibb...Blount...
Bullock...Calhoun...Chambers...Cherokee...Chilton...Clay...
Cleburne...Coosa...Dallas...Elmore...Etowah...Fayette...Greene...
Hale...Jefferson...Lamar...Lee...Lowndes...Macon...Marengo...
Marion...Montgomery...Perry...Pickens...Pike...Randolph...
Russell...Shelby...St. Clair...Sumter...Talladega...Tallapoosa...
Tuscaloosa...Walker...Winston.

&&

$$



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