Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 161149

Area Forecast Discussion
National Weather Service Birmingham AL
549 AM CST Sat Dec 16 2017

For 12Z Aviation.


Today and Tonight.

The short term remains fairly uneventful with all the big changes
and weather impacts in the extended forecast. As for the impacts
today, we are starting out much colder and will slow to warm up
during the day. Throw in the expectation of some thicker high to mid
level clouds this afternoon and upper 40s to low 50s can be
expected. We will continue to have clouds increase this afternoon
and tonight. Temperatures will likely bottom out between Midnight
and 3 am before beginning to increase ahead of the next rain maker
on Sunday. For the most part Central Alabama will remain rain free
through sunrise Sunday, with only a few isolated showers in the far
northwest after 3 am.


Sunday through Saturday.


The cutoff low currently over northwest Mexico will eject
northeastward across Texas today as a negatively tilted shortwave
trough. Showers and elevated thunderstorms will blossom across
Texas later today and tonight as the shortwave intercepts Gulf
moisture and a developing low-level jet. This will occur north of
a warm front along the Texas Gulf Coast where a weak surface low
will move northeastward. This large complex of showers and a few
thunderstorms will push eastward tonight and be located over
Mississippi early Sunday morning in association with a 40-50 kt
LLJ. Models are in good agreement in bringing this activity into
the western counties Sunday morning but in a weakened state as it
encounters a stable air mass over Alabama. The shortwave will lift
quickly northeast away from the area into the Ohio Valley by
midday. This will result in continued weakening/decrease in
coverage with eastward extent, but moist isentropic lift in the
weakening low level jet will maintain shower chances through the
day. Models differ on how far east this activity will make it, and
will indicate decreasing chances further east. Some drier air
aloft will work in from the west behind this system, resulting in
decreased rain chances in the northwest during the afternoon.
With very little in the way of MUCAPE will not mention thunder
during this period, and rainfall amounts will be on the lighter
side. Rain falling into the initially dry air mass at the surface
will result in cool temperatures being in the 40s as the rain is
falling, and have continued a lowering trend in high temperatures.
This rain will also slow the progress of a warm front trying to
lift northward from the coast.

Sunday night through Monday night:

Low confidence forecast for rain chances during this period with
model disagreement continuing. Southwest to west-southwest flow
with potential weak waves will remain in place aloft between
another cutoff low over the Desert Southwest and a strong
subtropical ridge near Cuba and the Bahamas. Models differ
regarding how much drier air moves in aloft behind the departing
shortwave and how quickly moisture lifts back into the area. A
precipitation-reinforced front will also be located over the area.
The ECMWF continues to be the driest model during this period,
with more pronounced anti-cyclonic flow aloft due to a stronger
subtropical ridge, and the Canadian has also trended much drier.
The NAM and GFS continue to have a wetter solution. PoPs have
generally trended downward especially across the north during this
period. For Sunday night, will indicate slightly higher PoPs after
midnight versus Sunday evening, with CAMs looking drier at the
tail end of the run at 0z Sunday. The European ensemble is wetter
than the deterministic ECMWF across the southeast after midnight
Sunday night and Monday morning, so will keep some likely PoPs in.
Overall QPF has trended downward, and the flooding concern looks
to at least be delayed beyond this period. All models are looking
fairly dry for Monday evening before ramping up rain chances
either after midnight Monday night or Tuesday morning.
Temperatures will be dependent on the position of the frontal
boundary, which now looks to be further south. Enough elevated
instability will be present for the potential for isolated thunder
but any surface-based instability will probably stay south of the
forecast area.


Models have all trended slower with the ejection of the next
southern stream cutoff out of the Desert Southwest, and now
indicate it moving across the area as a compact shortwave trough
on Wednesday. Increasing moisture ahead of this system will ramp
up rain chances beginning Tuesday. The lingering west-to-east
oriented frontal boundary looks to serve as the potential for
training activity with locally heavy rainfall possible due to an
unseasonably moist air-mass. Models differ on the placement of the
front with the GFS giving it an extra push south from a northern
stream shortwave. Have trended more towards the ECMWF solution
which is in agreement with its ensemble mean on a frontal
position across North Alabama. This places much of the forecast
area in the warm sector as the shortwave passes with the ECMWF
indicating a surface low tracking just to the northwest of the
area. This would be of concern for a potential threat of
supercells and isolated tornadoes as the ECMWF indicates 65
dewpoints with a 50 kt LLJ, 500-750 J/kg of CAPE, and 60 kts of
0-6 km shear. However, all models have been struggling with run to
run consistency issues lately and the GFS keeps the front mainly
south of the area. Will hold off on mentioning anything in the HWO
at this time until model agreement and consistency improves.

Thursday through Saturday:

Mainly dry conditions are expected Thursday with westerly flow
behind the Wednesday system. Another trough moves into the western
CONUS towards the end of the week as the complicated split flow
pattern continues. This results in another cold frontal passage on
Friday. Sufficient moisture return for appreciable instability to
develop ahead of this front in the wake of the previous system is
uncertain. Cold air will be lurking behind this front with
southwest flow aloft continuing. A threat of wintry precipitation
may develop somewhere across the southern CONUS towards Saturday,
but it`s too early to say where. Ensembles favor this threat
remaining northwest of the forecast area, and will keep
precipitation all liquid in the forecast at this time but continue
to monitor.



12Z TAF Discussion.

VFR conditions for the most part this morning. Some river moisture
has developed in the form of low clouds/ground fog at TCL and will
remain there through 14Z. Otherwise high level clouds will thicken
for the day, before retreating tonight. Then we will see some mid
level clouds begin to work in after 9Z across the north. Any rain
remains west of the TAF sites through 12Z.




No rain is expected until early Sunday morning. Relative humidity
values will remain well above critical thresholds. A wet pattern is
expected beginning Sunday through the first half of next week.


Gadsden     52  32  49  47  62 /   0   0  80  50  30
Anniston    53  34  53  50  63 /   0   0  60  50  40
Birmingham  52  37  51  49  62 /   0   0  80  50  40
Tuscaloosa  54  39  52  50  61 /   0  10  90  50  40
Calera      52  38  54  51  63 /   0   0  80  60  50
Auburn      54  37  59  56  67 /   0   0  30  70  60
Montgomery  54  39  60  58  67 /   0   0  50  70  60
Troy        55  40  64  60  69 /   0   0  30  60  60




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