Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 170018
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
618 PM CST Sat Dec 16 2017

.UPDATE...
Evening Update and 00Z Aviation.

&&

.SHORT TERM...

Although surface high pressure was drifting eastward, southerly
flow has not kicked up yet and sounding indicates very dry
boundary layer conditions. Additionally, the higher cloudiness is
not that thick at this time. Therefore, expect temperatures to
drop like a rock in the few hours after sunset. Went ahead and
lowered hourly trends and low temperatures overnight. The overall
trend will remain as clouds will increase late tonight with rain
starting early Sunday west. The temperatures will probably rise
across the western two thirds after midnight with the east
remaining cool.

75

Previous short-term discussion:
Increasing confidence in the forecast overnight and tomorrow as
the relatively weak shortwave ejects into the mean flow. Moisture
recovery looks substantial enough aided by the enhanced LLJ that
most, if not all of the area should at least see some measurable
precip by tomorrow night. Raised PoPs slightly to include most
areas into categorical, leaving the southwest out for now as the
moisture axis rotates to a more east-west orientation and is a bit
slow to transition southward during the day tomorrow.

17/KLAWS

.LONG TERM...
Sunday through Saturday.

Sunday:

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.

Tuesday/Wednesday:

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.

32/Davis

&&

.AVIATION...
00Z TAF Discussion.

The trend of the previous forecast remains. Ceilings will
gradually lower with restrictions starting west around 12-13z.
The MVFR ceilings will spread fairly quick west to east and lower
throughout the day. IFR conditions anticipated at TCL/BHM/EET
toward the end of the period. With the onset of the restrictive
ceilings, light rain will also spread into the state. Winds will
be tricky as this will mainly be an upper level system affecting
the terminals. A weak surface reflection of the upper system moves
northeast into central Alabama on Sunday with a warm front. This
front hangs around in the south. Winds generally will be less 7kts
or less, and most locations will remain north of the front. So
most places will see winds east northeast with TOI getting some
southeast component.

75

&&

.FIRE WEATHER...

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.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     30  49  47  62  48 /   0  90  50  30  30
Anniston    35  53  50  63  51 /   0  70  50  40  30
Birmingham  40  51  49  62  52 /   0  90  50  40  30
Tuscaloosa  38  52  50  61  53 /  10  90  50  40  40
Calera      40  54  51  63  53 /   0  90  60  50  30
Auburn      37  59  56  67  55 /   0  60  70  60  30
Montgomery  41  60  58  67  57 /   0  70  70  60  30
Troy        42  64  60  69  58 /   0  60  60  60  30

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
None.

&&

$$



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