Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 212131

Area Forecast Discussion
National Weather Service Birmingham AL
331 PM CST Wed Feb 21 2018

Through Tonight.

Upper level pattern remains unchanged this afternoon, with a large
trough across the western conus and ridge centered off the
Atlantic coast. A surface front stretches across Central TN,
clipping northwest Alabama, and through Central MS. The front will
stall across the northwest part of the forecast area this evening,
unable to progress further eastward due to the ridge, and lift
back to the north later tonight.

Scattered showers and a few thunderstorms across the local area
will diminish with loss of daytime heating this evening. The
highest rain chances overnight will shift to the northwest,
closer to the front. With increased low level moisture and
dewpoints in the 60s tonight, low clouds and fog are possible.
Overnight lows remain muggy, with temps in the 60s.


Thursday through Wednesday.

The stationary front over North MS and into West TN will slowly
begin to lift northward as a warm front on Thursday. There could
still be some showers and isolated thunderstorms in the far
northwestern counties early Thursday, but this should start to
clear out as we get into Thursday night. The rest of Central AL
will again see unseasonably warm temperatures Thursday and Friday
as the high pressure to our east continues to usher in warm/moist
air off the Gulf of Mexico.

On Friday, an upper trough swings through the Great Basin and a
surface low develops on the lee side of the Rockies late Friday
night into Saturday morning. This low quickly moves northeastward
through the day on Saturday and stretches a cold front down
through the Mississippi River Valley. Models have come into
slightly better agreement, but the GFS still deepens the low more
than the ECMWF does and moves the cold front through Central AL
quicker during the overnight hours Saturday night into Sunday
morning. This system features a fairly strong low level jet
(50+kts at 850mb) and a 100-115kt upper level jet begins to nose
into Northern AL.

Here in Central AL, dewpoints are running high, and daytime
heating builds up quite a bit of instability. However, timing of
this front brings it in during the overnight hours, so instability
might be on the lower end (300-500 J/kg), but still sufficient to
support thunderstorms. With the strong low level jet across the
area along and ahead of this front, there will be quite a bit of
vertical shear in place to support organized storm development.
Therefore, expect a line of storms to develop along the cold front
and push into Central AL. The severity of these storms is still
uncertain with several limiting factors including marginal lapse
rates and lower instability. Will continue to monitor for model
trends in the next few days. For now, I don`t have enough
confidence to include mention of severe in the HWO.

Meanwhile, another shortwave trough swings through the larger
upper level trough and will bring an impulse through the area
just behind the departing front. This will lead to another round
of rain to move through during the day on Monday, mainly south of
the I-59 corridor.

Ridging builds in across the area Tuesday under more zonal flow
aloft, leading to drier conditions.



18Z TAF Discussion.

A mix of VFR and MVFR cigs at mid day as a sct to bkn cu field
streams northward across the area. Southerly winds will be gusty in
the 15-20kt range through the afternoon, due to enhanced daytime
mixing. Winds will subside this evening, remaining out of the south
at 4-7kts overnight. Expect cigs to lower, with widespread MVFR
conditions, and localized IFR and LIFR, most likely at MGM and TOI
early Thursday morning.

Isolated to scattered showers are possible this afternoon, mainly
north and west. Confidence of activity at any terminal is only great
enough to mention VCSH at TCL.




Unseasonably high RH values continue for the next several days.
With the higher relative humidity values and good overnight
recovery, do not expect there to be any fire weather concerns.


Gadsden     65  78  59  79  58 /  30  10  10  10  20
Anniston    65  80  60  81  59 /  20  10  10  10  10
Birmingham  66  80  62  81  62 /  30  10  10  10  10
Tuscaloosa  68  81  62  81  62 /  40  20  10  10  20
Calera      65  80  61  80  62 /  20  10  10  10  10
Auburn      63  80  62  79  61 /  10  10  10  10  10
Montgomery  64  83  61  82  62 /  20  10  10  10  10
Troy        63  82  60  81  60 /  10  10  10  10  10




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