Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 091745

Area Forecast Discussion
National Weather Service Birmingham AL
1145 AM CST Fri Dec 9 2016

For 18Z Aviation.


Today and Tonight.

After a cold start to the day, most of Central AL will stay in the
low to mid 40s, with the northern counties struggling to reach 40
degrees. Surface ridge has started to build across the Southeast,
clearing out any mid to high level clouds that were lingering in the
southeastern counties.

Winds will remain breezy for most of the morning hours, creating
wind chills around freezing. By the afternoon, the winds begin to
weaken some, bringing wind chills closer in line with actual
temperatures, which will still be cold. With the ridge in place by
the evening, the clearing skies and the weaker winds will allow a
little more radiative cooling to take place overnight. This will
result in colder temperatures Saturday morning than we`re seeing
this morning. Overnight lows will drop into the low to mid 20s for
most of the area. I wouldn`t be surprised to see some valley
locations drop below 20.


Saturday through Thursday

Models are in fairly good agreement through Monday, before some
placement and timing differences occur regarding a frontal
boundary that will be stalled across the region during the first
half of the workweek before moving out late in the week. These
differences are due to the low-amplitude, quasi-zonal progressive upper
level pattern in place at this latitude, south of a deep cold
upper low moving across Canada.

Temperatures will begin to moderate on Saturday, but only slightly
as weak 850mb warm advection will not occur until late in the day
as the sprawling surface high gradually shifts eastward. Highs
will be in the mid 40s to low 50s across the area. Low
temperatures will not be as cold Saturday night under light
southeast winds, but still make it into the upper 20s to low 30s.
A low stratus deck may approach the southeast counties by Sunday
morning as low-level moisture begins to return to the area.
Strengthening southerly flow will bring milder temperatures on
Sunday but also an increase in cloud cover due to moist
isentropic lift. Precipitation with the approaching system will
remain west of the area through the day on Sunday, but wouldn`t be
surprised if isentropic lift squeezes out a stray shower in the
southeast Sunday afternoon.

Low temperatures Sunday night will probably occur in the early
evening before temperatures gradually rise through the night as
dew points increase. A shortwave trough will move through the
Midwest and Great Lakes Sunday night into Monday as a surface low
moves from Kansas to Ontario. Meanwhile a trailing cold front will
reach the area by Monday morning. The main upper-level forcing
will remain north of the area, but there should be enough moisture
pooling along the front courtesy of a 45 kt LLJ, as well as
isentropic lift and glancing influence from the right entrance
region of an upper-level jet streak, for showers to be likely
across the north late Sunday night into Monday. Amounts will not
be heavy, however. With dewpoints remaining in the 50s instability
looks to be negligible through Monday morning. Models do indicate
some instability developing across the southern counties south of
the front by Monday afternoon which could allow for a few
thunderstorms. The LLJ and the upper-level dynamics will have left
the area by then, so am not concerned with severe weather at this

The front will stall near the southern counties by late Monday
afternoon. The ECMWF develops a surface wave and lifts the front a
bit further north Monday night before pushing back to the south
Tuesday night, while the GFS waits until Tuesday night into
Wednesday to lift it back north before pushing it back to the
south Wednesday night. These differences would be expected given
the low-amplitude pattern with spread present in the ensembles as
well. Some adjustments will need to be made to rain chances and
temperatures when models come into better agreement. Another shot
of cold air will move in behind the front Wednesday night, but
should lag behind any remaining moisture. Lows will probably need
to be lowered either Wednesday night or Thursday night when
models come into agreement on timing of the coldest air. Another
rain-maker will arrive just beyond the end of this forecast



18Z TAF Discussion.

Surface high pressure slowly builds into Central Alabama this
period. VFR conditions will prevail. Little in the way of cloud
cover anticipated. North winds around 10kts today, becoming light
overnight. Winds veer northeast on Saturday and lighter around




Relative humidity values will drop into the low 30s this afternoon
and below 25 percent in portions of the area Saturday afternoon.
Winds will be light, however. Moisture begins to increase Sunday
with rain chances returning late Sunday night into Monday. While the
best chances will be late Sunday night into Monday, rain chances
will remain a possibility through the middle of next week as a front
stalls over the area.


Gadsden     41  18  46  25  54 /   0   0   0   0   0
Anniston    42  22  48  28  55 /   0   0   0   0   0
Birmingham  41  24  48  31  58 /   0   0   0   0   0
Tuscaloosa  43  22  49  29  59 /   0   0   0   0  10
Calera      43  23  49  30  57 /   0   0   0   0   0
Auburn      44  27  49  32  56 /   0   0   0   0  10
Montgomery  48  26  52  30  60 /   0   0   0   0  10
Troy        47  23  51  30  61 /   0   0   0   0  10




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