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

Update...
aviation discussion.

&&

Short term...
today and tonight.

The Arctic front has moved into northwest Alabama and will be making
its way through the forecast area this morning and afternoon. A band
of heavy snow is currently ongoing in eastern Arkansas and western
Tennessee. The eastern edge of this band should reach our
northwestern County Warning Area border around 12z, likely with less intensity than
currently observed. Models are in general agreement with the idea of
a weakening band in the quantitative precipitation forecast output, but a swath of 700mb relative humidity values
near 100% appears healthy across the entire County Warning Area along with
saturation through the column. This system has been overperforming
across the Memphis and Paducah areas, and with that in mind, we have
raised accumulations across the entire area to the 1 to 1.5 inch
range. A relative minimum seems likely somewhere in the County Warning Area as the
isentropic lift takes a pause before lift increases in association
with the upper-level trough. This minimum may actually occur
across the northwestern part of the area, and the focus could
shift to a second wave farther east in the afternoon and evening.

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.

87/Grantham

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 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.

32/Davis

&&

Aviation...
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.

14

&&

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 34 12 29 14 41 / 70 40 0 0 0
Anniston 41 12 29 17 42 / 70 70 0 0 0
Birmingham 33 12 30 15 42 / 70 40 0 0 0
Tuscaloosa 35 13 32 16 43 / 70 20 0 0 0
Calera 37 13 30 18 43 / 70 70 0 0 0
Auburn 52 16 31 18 43 / 20 70 10 0 0
Montgomery 50 16 33 18 44 / 30 70 10 0 0
Troy 55 17 33 18 44 / 10 70 10 0 0

&&

Bmx watches/warnings/advisories/...
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.

Hard freeze watch from this evening through Thursday morning 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...

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