Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 051057

Area Forecast Discussion
National Weather Service Birmingham AL
457 AM CST Mon Dec 5 2016

For 12Z Aviation.


Today and Tonight.

Showers have largely diminished across the area as mid and upper-
levels have dried out and the low level jet has weakened. This was
in response to a northern stream shortwave trough and a surface wave
along the stationary front near the Gulf Coast exiting the region.
Plentiful low-level moisture in and under the frontal inversion has
resulted in very low clouds and drizzle across the area. Will add a
mention of visibilities less than 1 mile to the HWO with a couple
sites reporting visibilities below a mile at times, and low
visibilities will also be likely where the very low clouds intercept

Widespread showers are expected to re-develop by late morning across
West Alabama and spread eastward and northward through the day. This
will happen due to isentropic lift re-strengthening and the 925-700
mb warm front aloft lifting northward in the diffluent flow ahead of
a southern-stream shortwave lifting northeastward from northern
Mexico into Texas. Weak elevated instability will allow for the
possibility of a few lightning strikes across southern portions of
the area, but the surface front will remain near the Gulf Coast with
no surface-based convection expected during the day today.

Overnight the compact shortwave trough will lift quickly
northeastward and take on a small negative tilt as it moves over the
area. A small 80+ kt mid-level jet streak will move over the area
with a 45-50 kt low level jet while a surface low moves from
Louisiana to northwest Alabama. The main question regarding severe
weather potential will be how far north the warm front and surface-
based instability south of it can lift northward. High-res models
mainly do not lift the warm front as far north as the global models,
and would lean towards that solution as precipitation falling north
of the warm front should reinforce the cooler air mass. Cold air
damming will also begin to develop this evening as a surface ridge
moves eastward from the Ohio Valley to the Mid-Atlantic, resulting
in easterly flow strengthening ahead of the warm front.

Keeping that in mind, the previous limited threat area still looks
reasonable based on the general consensus of model guidance and SREF
instability probabilities. Confidence is higher in the southern
portion of this area, with the northern extent of the limited threat
more uncertain. Please stay tuned for further updates, and make sure
to have a way to get warnings while asleep given the nocturnal
nature of the threat. The main threat will be the potential for
isolated brief tornadoes given the shear/helicity and weak
instability in place, as well as damaging winds. The start time
looks to be slightly later than previously forecast, with the warm
front not expected to enter the southern forecast area until after
10-11PM at the earliest. Storms should exit the area by around 6 AM


Tuesday through Sunday.

A fast moving short wave trof will pass to the north of Alabama
Tuesday morning. Ongoing showers and thunderstorms are likely for
areas along and east of I-65 early Tuesday morning, but rapid
drying aloft will bring an end to the rain for areas west of I-65
by 9am Tuesday and for the remainder of central Alabama by 12
noon. After several days of cloudy skies, there will likely a
return of sunshine across areas south of I-20 on Tuesday. Cold air
advection north of I-20 will likely keep skies mostly cloudy
throughout the day Tuesday and into Tuesday night. More sunshine
likely on Wednesday as warm air advection increases ahead of an
approaching cold front. There is a chance of rain across the
northwest counties Wednesday night as pre-frontal band of showers
develops. The showers will spread southeast across central Alabama
during the day Thursday, with strong cold air advection behind
frontal passage. The models do indicate rapid drying behind the
front so there so should clearing from north to south during the
day. The showers should exit the southeast counties before sunset
Thursday. The coldest air mass of the season will spread across
Alabama Thursday night through Saturday. Highs on Friday will only
be in the 40s, with some upper 30s across the far northern
counties. Lows will dip down into the lower to middle 20s Friday
and Saturday mornings. Moderating temperatures on Sunday as the
surface high shifts east of Alabama and surface winds become
southeast. A weak cold front will move into western Tennessee on
Sunday and there is a small chance for showers for areas north of
I-59 Sunday afternoon.



12Z TAF Discussion.

Surface warm front will remain south of the terminals through
much of this TAF cycle, before lifting through the southern TAF
sites late in the cycle. A strong frontal inversion, plentiful
low-level moisture, and isentropic lift will maintain low cigs
throughout the TAF cycle. Cigs are LIFR at most sites currently
and will mainly remain that way, possibly lifting just into the
IFR category by afternoon. Rain showers will re-develop later this
morning and becomes widespread by afternoon, continuing into
tonight. Some thunderstorms will be possible late this afternoon
and tonight, but confidence/coverage is too low to include in the
TAF at this time. Visibilities will be more variable during the TAF
 period due to drizzle/fog early and rain later on.




Light to moderate rain will continue across the area this morning.
Heavier rainfall will occur this afternoon and tonight as the main
short wave trof approaches the area from the west. A few thunderstorms
could be severe across the far southern counties late tonight. Forecast
rainfall amounts will average 1.0 to 2.0 inches through Tuesday. A
strong cold front will bring a chance of showers on Thursday, then
turning much colder Thursday night through Saturday.


Gadsden     56  53  62  41  57 / 100 100  60  10  10
Anniston    58  55  64  44  59 / 100 100  60  10  10
Birmingham  57  55  63  43  57 / 100 100  50  10  10
Tuscaloosa  58  57  62  44  58 / 100 100  30  10  10
Calera      58  57  64  44  59 / 100 100  40  10  10
Auburn      61  57  68  47  61 / 100 100  60  10   0
Montgomery  63  60  67  47  64 / 100 100  30  10   0
Troy        65  62  69  46  64 / 100 100  30  10   0




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