Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 281120
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
520 AM CST Tue Feb 28 2017

.UPDATE...
For 12Z Aviation.

&&

.SHORT TERM...
Today and Tonight.

Dense fog advisory continues through 9am this morning. Several sites
have been showing 1/4 to 1/2 mile visibility through most of the
overnight hours. This should mix out by late morning, improving
visibilities.

A surface low develops in the Central Plains this morning under
generally zonal upper level flow. A warm front lifts northward
through Central AL today, sparking a chance for showers and
thunderstorms generally north of the I-20 corridor. Temperatures
in the warm sector (behind the front) will warm into the mid to
upper 70s. Areas north of the I-20 corridor will likely reach into
the low to mid 70s as the precip will keep afternoon temps from
reaching as high. As far as severe potential with the warm front,
instability is pretty limited due to the morning low clouds and
fog. Just not sure areas in the northern counties will be able to
destabilize enough ahead of the front. Forecast soundings show a
mid-level cap in place through the afternoon before being eroded
by the evening. However, the low levels dry out by this time,
making any convection more elevated in nature rather than surface-
based. Forecast hodographs do show a strong backing of the winds,
so wouldn`t be surprised to see a potential for a few stronger
storms with smaller hail. However, with such low instability, the
elevated nature, and limited buoyancy in the hail growth zone, not
expecting a widespread potential for severe storms across Central
AL.

By this evening, rain chances diminish as warm front lifts north and
east of the area ahead of the approaching cold front. Have kept
chance PoPs in for the overnight hours tonight into Wednesday for
our northwestern counties to account for any scattered showers ahead
of the front as it approaches Central AL. Have also included patchy
fog in for the overnight hours tonight for our southeastern
counties. Forecast soundings show a "nearly" saturated surface
layer, but it`s difficult to say whether this will materialize as
low clouds or fog at this point in time.

25/Owen

.LONG TERM...
Wednesday and Wednesday Night.

Very little change to the forecast thinking regarding Wednesday`s
severe threat. The tornado threat continues to look very low-end
with only a brief tornado or two possible, mainly with any initial
storms earlier in the event. The main threat continues to be
damaging winds. The 850 mb LLJ will lifting out of the area and
veering before the front arrives, as the surface low lifts
northeastwards through Ontario. Helicity values will be around 150-
200 m2/s2 initially before quickly decreasing through the afternoon.
Did consider lowering the tornado confidence value in the HWO to a
1, but will leave it as a 2 to avoid too much confusion as the
damaging wind threat remains elevated.

Some storms will be ongoing near the Mississippi River at the start
of period but the more robust part of the squall line will be
further north in the Ohio Valley. A initial weak shortwave in the SW
flow aloft may move through during the morning hours and result in
the development of scattered showers and a few thunderstorms across
the north. Helicity values are better during this time period but
CAMs do not show anything robust developing during this time with
instability remaining lower during the morning hours.

By afternoon, temperatures will warm into the mid to upper 70s with
some low 80s across the south. Combined with steep mid-level lapse
rates this will result in CAPE values of 1000 J/kg or more. A broken
line of storms with some embedded supercells will begin to move into
the northwest counties shortly after noon near the front and move
east-southeastward through the afternoon and into the evening. A
brief tornado or two will be possible with the initial cellular
activity before helicity values decrease. Severe hail will also be
possible with the stronger cells given steep mid-level lapse rates,
strong deep-layer shear, and sufficient WBZ heights. The main threat
will be damaging winds, aided by strong winds and some dry air
aloft. Whether or not a widespread damaging wind event occurs will
hinge on whether convection stays more cellular or congeals into a
line, which is less certain given the weaker upper-level forcing and
orientation of the deep layer shear, but damaging wind will be
likely in any bowing segments that develop regardless. The current
elevated threat area looks good based on where the storms are
expected to be during the time of best instability during the
afternoon.

32/Davis

Thursday through Monday.

A large dome of high pressure will build across Central Conus on
Thursday behind the front with decent cold air advection/northerly
flow around it into Alabama. This will be reinforced by northwest
flow aloft to help bring some Canadian air down into Conus
Thursday. Winter is not done with us just yet with 30s expected
for both Thursday and Friday nights. Some areas will likely dip
below freezing on Friday night as the large 1032-1036 mb surface
ridge slowly shifts into the eastern 1/3 of Conus.

During the day on Saturday, our airmass will begin to moderate as
the surface ridge moves to the Atlantic Seaboard by Saturday
night. Our cold air advection is cut off and the flow turns back
around to receive warmer moist air off of the gulf. Sunday looks
to be our transition day with milder temperatures and slowly
rising dew points as southerly winds return. Low rain chance
return by Sunday night in the northwest with rain chances
increasing some by the end of our forecast period by late Monday
into Tuesday as another front approaches. While models are all
trending a little slower with this next frontal system they are
in disagreement as to how slow its arrival will be and thus rain
chances are low to round out the end of the forecast.

08


&&

.AVIATION...
12Z TAF Discussion.

Foggy start to the morning across most terminals. Combination of
lower VIS and lower CIGs across Central AL are creating IFR and
LIFR conditions for most sites except BHM. By late morning, a warm
front moves through the area and should help the daytime heating
to mix out the lower VIS and CIGs. Northern sites will see a
chance for rain in the afternoon, so have left mention of VCTS in
the TAFS for those sites. Towards the end of the forecast period,
another round of fog is possible, especially in the southern TAF
sites. However, winds should remain up (around 8kts), which might
result in lower CIGs rather than fog. For now, have mention 4SM
and OVC007. If winds calm locally, could see VIS drop more.

25/Owen

&&

.FIRE WEATHER...

A wet pattern will continue through Wednesday with several rounds
of showers and thunderstorms. Some strong to severe storms are
possible Wednesday afternoon and evening. A cold front will bring
an end to the rain Wednesday night. There are no fire weather
concerns.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     70  60  73  41  59 /  50  20  90  40   0
Anniston    75  61  76  42  60 /  30  20  80  60   0
Birmingham  74  64  75  42  61 /  40  20  90  40   0
Tuscaloosa  78  66  77  44  63 /  20  20  80  30   0
Calera      73  63  77  44  62 /  20  20  80  50   0
Auburn      76  61  77  47  61 /  10  10  40  80  10
Montgomery  78  64  82  48  64 /  10  10  50  80  10
Troy        77  63  81  50  64 /  10  10  30  70  10

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Dense Fog Advisory until 9 AM CST this 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.

&&

$$


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