Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 210551

Area Forecast Discussion
National Weather Service Birmingham AL
1151 PM CST Tue Feb 20 2018

For 06Z Aviation.


Rest of the Afternoon through Wednesday.

Deep layer southerly flow is in place this afternoon between a
longwave trough over western North America and the anomalous 595
decameter mid-level ridge centered off the Southeast Atlantic
Coast. Temperatures rose quickly earlier today but have leveled
off somewhat due to a large cumulus field associated with moist
onshore flow. Stay tuned for the climate reports and record high
information. Isolated to scattered showers have also developed in
this onshore flow. Greatest coverage for the rest of the
afternoon and into the early evening will be in the southeast and
far east-central counties due to a zone of enhanced lift and
moisture in the 850-700mb layer rotating around the ridge. A
couple lightning strikes cannot be ruled out in the far southeast
with MLCAPE values around 500 J/kg. Showers should mainly
dissipate with the loss of daytime heating tonight, except in the
moisture axis in the far eastern counties where isolated to
scattered activity may linger overnight. Elsewhere, a stray shower
overnight can`t be completely ruled out due to isentropic lift,
but mid-levels will be too dry to mention in the forecast. There
should be enough of a pressure gradient for winds to not go calm
overnight, largely preventing fog formation, but a low stratus
deck is expected to develop instead. Cloud cover, southerly
winds, and anomalously high dew points will keep lows very mild,
in the mid to even upper 60s.

A fairly similar upper-level pattern remains in place on
Wednesday, though the winds aloft will be more southwesterly
rather than southerly. A strong cold front and associated zone of
showers and storms will sink southeastward tonight and tomorrow.
It will be in the process of stalling as it becomes parallel to
the southwest flow aloft and a wave of low pressure develops over
east Texas. While the front may not quite reach it to our far
northwest counties, CAMs indicate that outflow from convection
along the front likely will, resulting in enhanced rain chances in
these areas. Not expecting anything strong to severe given limited
instability/forcing. Elsewhere, isolated to widely scattered
diurnally driven showers will be possible, but become increasingly
spotty the farther southeast you go due to closer proximity to the
ridge and dry air aloft. The anomalously warm air mass will remain
in place, with near-record or record highs continuing to be a good


Wednesday through Tuesday.

Made a few tweaks to PoPs in the long term based on model trends.
We continue to monitor the cold front expected to move in late
Saturday night into Sunday morning. The GFS continues to remain an
outlier with a much stronger 500mb wave and surface low than
other guidance. But either way there will be plenty of Gulf
moisture and shear to work with. But the late night/early morning
timing will keep CAPE values generally 500 J/kg or less. With
this being very late in the Day 5/early in the Day 6 timeframe and
inherent uncertainties with high shear low CAPE events, will hold
off on any severe mention in the HWO at least for this run.


Previous discussion:

The unseasonably warm weather continues through the end of the week
for Central AL. The anomalous ridge to our east is leading to a deep
southerly flow bringing in warm/moist air from the Gulf of Mexico. A
cold front is expected to slowly approach the area from the
Northwest, but models continue the trend of stalling it just to our
north and west during the day on Wednesday and into Thursday. By
Thursday night, the southerly flow lifts the cold front northward as
an effective warm front. I have decreased rain chances considerably
Wednesday through Friday as I expect most of the lift to remain
north of Central AL. There could be enough lift in the far north and
northwest counties to have some slight chance to chance PoPs, but
I`m keeping those below 50%. Due to the cold front never truly
making it into Central AL, expect high temperatures to near or
exceed record highs Wednesday, Thursday, and Friday.

As we go into the weekend, another upper level shortwave develops
a surface low on the lee side of the Rockies early Saturday
morning. This surface low quickly moves northeastward into the
Great Lakes region and stretches a cold front down through the MS
River Valley that will push into Central AL late Saturday into
Sunday. Here`s where things get interesting/uncertain with regards
to the forecast. The GFS rapidly intensifies this surface low as
it moves through the Midwest, which increases the 850mb low level
jet and thus the overall 0-6km shear values. The intensified
surface low is able to weaken the anomalous ridge to our east
enough to push the cold front through our area completely a little
quicker compared to other guidance. The ECMWF keeps the surface
low weaker, which leads to a weaker low level jet, albeit still
present, but isn`t able to weaken the ridge as much. This makes
the ECMWF a little slower on the progression and even stalls the
front out over south-central AL Sunday evening. The Canadian is
more in line with the ECMWF and the NAEFS maintains the anomalous
ridge through at least Monday morning. The question then becomes,
"what is this going to mean for our weather" - There`s plenty of
instability in the area due to several days of above average
temperatures and the high dewpoints. The GFS solution with added
shear and a stronger front could lead to the possibility of severe
weather late Saturday night into Sunday, especially in the
western portions of Central AL. A limiting factor with this is the
lower mid-level lapse rates, which run 5.7-6 C. The ECMWF still
has sufficient shear in place with the weaker surface low, but
this would likely be more of a strong storm and heavy rain setup.
Plus, the ECMWF brings the front through later and begins to stall
it out on Sunday, which will decrease the shear as the surface
low lifts further away. At this time, I have more confidence in
the weaker ECMWF solution due to the support it has with the
Canadian and NAEFs in the extended. Considering the uncertainty
with strength of the surface low and the timing of the front, in
addition to the marginal mid-level lapse rates, I`m going to hold
off adding any mention of severe (or tornadoes) in the HWO at this
time, and will continue to monitor the trends in future model
runs. The SPC outlook, which clips our northwest counties, also
mentions these uncertainties. Instead, will carry 60-75% chance
of rain moving through Central AL late Saturday night through
Sunday with thunderstorms mentioned.

Rain chances decrease across the area Monday into Tuesday, but
confidence is low until models get a better handle on the weekend`s



06Z TAF Discussion.

A north/south line of light/moderate showers will proceed over TCL
over the next couple of hours. Low chances of -SHRA are possible
elsewhere through the late night hours but will leave out for now.
Cigs are beginning to lower with MVFR already starting at some
sits. Expecting lower MVFR cigs to continue to develop and spread
northward with the low level onshore flow. A few spots could see
some IFR develop toward 8-10z for a few hours. Most cigs should be
MVFR. While some patchy fog is possible, think that pressure
gradients should help winds stay up overnight (and be gusty during
the afternoon Wed) and keep much of the fog from occurring and
most low level moisture will be in the way of stratus. Conditions
are not expected to go VFR until toward midday on Wednesday. Large
area of rain currently extending from the Great Lakes to Texas is
not expected to affect the TAFs during the forecast period as
system should stall before reaching here. Will be monitoring for
any outflow activity, but will leave out of TAFs for now.
Otherwise, just low diurnally driven afternoon pops are expected
on Wednesday and will forgo mention in terminals as well for now.




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  79  62  77  59 /  30  20  30  20  10
Anniston    65  79  64  79  60 /  30  20  20  20  10
Birmingham  66  80  65  79  62 /  20  20  20  20  10
Tuscaloosa  67  82  66  80  62 /  20  30  40  20  10
Calera      66  81  65  79  61 /  20  20  20  20  10
Auburn      65  79  65  80  62 /  40  20  10  10  10
Montgomery  66  83  65  83  61 /  20  20  10  10  10
Troy        65  81  64  82  60 /  20  20  10  10  10





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