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Area forecast discussion
National Weather Service Birmingham Alabama
1047 am CST Tue Jan 16 2018

morning update.



Water vapor imagery and global model data is in good agreement in
a sharp positively-tilted trough extending from Michigan and
extending southwest across northern Indiana through central
Illinois and into central Missouri and further extends southwest
into central Oklahoma.

Toward the surface, an extensive Arctic front just north of Mobile
to near Prattville further northeast to near Alexander City and
was continuing to slide southeast.

Satellite imagery shows mostly cloudy to cloudy skies across all
but our southeast counties, roughly from Montgomery to Alexander
City and points southeast, where mostly sunny skies were observed.

Radar data shows a large, extensive precipitation shield
consisting mostly of snow that extends from west-central
Mississippi northeast across our west-central, northwest and
northern counties and extends further north and east to include
much of far northern Alabama and into central and eastern



Short term...

Despite surface temperatures that have risen above freezing over
the past few hours roughly along and southeast of the Interstate
59 corridor we continue to observe noticable temperature decreases
across our west-central and northwest counties as the
precipitation has started to fall. The main concern continues to
be the risk of snow melting when it comes into contact with Road
surfaces and then the moisture will freeze onto the pavement as
temperatures fall below freezing, creating icy patches and slick
spots on roadways. A Winter Weather Advisory remains in effect as
snow will continue affect our northwest counties with snow
eventually impacting more of our central counties early this
afternoon along with portions of our south and eastern counties
this afternoon and evening.

Changes in the overall thinking is not huge. The one difference
is that the area of rain that is developing over southwestern
Alabama/southeastern MS will move into the southeast ahead of the front
and may provide a brief window of rain before the snow starts.
This will do two things. One it will moisten the overall column
and two it will begin to cool down the airmass ahead of the Arctic

So we are still expecting a significant impact to the area even with
light amounts (2 inches or less). This a slow moving band and will
take almost sort of a staircase approach through the state.
Currently watching another band developing further south in MS
and this will then slide east into the area.

The approaching vort Max at 500mb appears very impressive this
afternoon into tonight and will have an opportunity to enhance the
snow band as it moves east of I-59. Precipitable water values will surge in
this zone, and additional lift could result in snowfall amounts
exceeding our current forecast. The ECMWF, several European
ensemble members, and the regional Canadian indicate the potential
for 2-4 inch amounts along and east of I-59 all the way to our
eastern and southern borders. This scenario will be monitored for
a possible upgrade to a Winter Storm Warning. Impacts could be
significant across our entire forecast area due to temperatures
falling into the 20s and teens as the snow falls. A Wind Chill
Advisory has been issued for the entire area for tonight and
Wednesday morning for wind chills of 5 above zero to 5 below zero.


Long term...
Wednesday through Monday.

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.

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.

a stronger Pacific trough will move across the Continental U.S. 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 severe weather potential statement at this time.



12z taf discussion.

A strong Arctic front is pushing southward through the area this
morning, with northwesterly to north winds near and north of the I-
20 corridor. Behind this front, MVFR cigs, snow, and gusty winds
will overspread the area through the day today. Have timed out
arrival of light snowfall and MVFR cigs based on current
radar/satellite trends and hi-res guidance. Depending on upstream
snowfall intensity, might have to revise vis downward as heavy snow
bands move through the area.

Expect strong northwesterly to northerly winds to continue after the
snow ends, with cigs improving to VFR.



Fire weather...

An Arctic front will move across the area today with light to
moderate snow likely areawide. Dry and very cold conditions are
expected Wednesday.


Preliminary point temps/pops...
Gadsden 33 12 29 14 41 / 100 40 0 0 0
Anniston 40 12 29 17 42 / 100 90 0 0 0
Birmingham 32 12 30 15 42 / 100 40 0 0 0
Tuscaloosa 34 13 32 16 43 / 100 20 0 0 0
Calera 36 13 30 18 43 / 100 70 0 0 0
Auburn 51 16 31 18 43 / 50 100 10 0 0
Montgomery 49 16 33 18 44 / 70 100 10 0 0
Troy 54 17 33 18 44 / 20 90 10 0 0


Bmx watches/warnings/advisories/...
Winter Weather Advisory until 6 am CST Wednesday for the
following counties: Autauga...Barbour...Bibb...Blount...
Russell...Shelby...St. Clair...Sumter...Talladega...Tallapoosa...

Hard freeze watch from 9 PM CST this evening through Thursday
morning for the following counties: Autauga...Barbour...Bibb...
Randolph...Russell...Shelby...St. Clair...Sumter...Talladega...

Wind Chill Advisory from 9 PM this evening to 9 am CST Wednesday
for the following counties: Autauga...Barbour...Bibb...Blount...
Russell...Shelby...St. Clair...Sumter...Talladega...Tallapoosa...

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