Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 261145

Area Forecast Discussion
National Weather Service Birmingham AL
545 AM CST Sun Feb 26 2017

For 12Z Aviation.


Today and Tonight.

The ridge will begin to weaken today across the Southeast U.S. with
the center pushing off the Carolina coast by late in the day.
Temperatures should be a tad milder for most this afternoon (except
the far southeast as they had a later frontal passage yesterday) as
winds shift around in response to the ridge departure along with
plenty of sunshine. However, readings should be right about normal
for late February. By this evening, we should see the upper zonal
flow turn southwest as an upper trough digs in the Pacific Northwest
and helps to kick some weak disturbances from the Desert Southwest
moving through this southwest flow late tonight into Monday across
Eastern Conus. At the same time, onshore/low level flow increase.
Precipitation could begin right before sunrise on Monday across
the far west counties, but most should hold off reaching Central
Alabama until after sunrise.


Monday through Saturday.

A shortwave trough will eject into Texas this afternoon and
interact with return flow out of the Gulf, resulting in the
development of showers and thunderstorms over Texas. Models are
in fairly decent agreement on upscale growth near the Arklatex
region this evening into a few organized clusters or potentially
an MCS that will move eastward across the Lower Mississippi Valley
tonight aided by a 35 kt LLJ and the shearing out shortwave in
west-southwest flow aloft. Most model guidance (WRF- ARW, ECMWF,
SREF, European ensembles, and the global/regional Canadian models)
indicate a weak MCS or its remnants entering the western counties
around/shortly after sunrise Monday morning and sweeping across
the state during the day. A 35 kt LLJ, isentropic lift over a warm
front attempting to lift northward, and PWATs increasing to 1.3
inches in the southwest counties will help maintain this activity.
However, there is still some model spread as would be expected
due to typical predictability issues with upstream convection.
This activity will be in a weakened state as it reaches the drier,
stable air mass over Central Alabama, with mainly showers and a
few rumbles of thunder (mainly south) with some weak elevated
instability. Nothing strong is expected. A decent gradient in high
temperatures is expected with highs reaching the 70s in the far
south in southerly flow ahead of the rain, while evaporative
cooling effects will keep temperatures cooler across the north.
Some areas may not get out of the 50s there depending on how long
the rain hangs around.

The main area of precipitation should be out of the area by Monday
evening. The warm front, which may be impeded by rain during the
day, should then lift northward through the area. Height rises
will occur due to the influence of upper-ridging located near the
Florida Straits, indicating some subsidence. Moist southerly flow
and isentropic lift will continue, however, so a few showers and
perhaps an isolated thunderstorm will continue to be possible,
with the focus shifting to the far northern counties near a low-
level moisture axis. This will continue to be the case on Tuesday,
with some rain chances along the northern periphery of the upper
ridge while some drier air aloft begins to work in from the south.
CAPE values will reach 1000 J/kg on Tuesday with dew points
around 65 and around 45 kts of deep layer shear. Given steep mid-
level lapse rates in place, if a storm developed it could be
strong or even severe. However, due to the lack of a trigger
mechanism and subsidence aloft this still seems like a low chance.

Confidence is increasing in the severe weather potential for
Wednesday. While the primary upper-level forcing will remain north
of the area with the shortwave trough, 500 mb height falls will
occur across the area as a 80+ kt mid-level jet streak and 120 kt
upper-level streak move in. A quality warm sector will be in place
ahead of the front with highs in the mid to upper 70s (around 80
in some locations), dewpoints in the mid 60s, and mid-level lapse
rates of 7+ C/km allowing for CAPE values in excess of 1000 J/kg.
Models have varied a bit on LLJ strength, now indicated to be
around 45 kts, and winds will be veered, however sufficient
helicity and low-level shear will be present with curved
hodographs indicating the potential for a couple tornadoes.
Additionally, the deep layer shear will have enough of a
perpendicular component for supercells to be present ahead of a
squall line along the front. Damaging winds will be a threat with
a squall line, and steep mid-level lapse rates will result in a
threat for hail with supercells. The timeframe looks to be
Wednesday afternoon and evening with the relatively greatest
threat being across the northwest half. Behind the front, dry
conditions will move in Thursday through the rest of the forecast



12Z TAF Discussion.

VFR forecast for the next 24 hours. For the most part, winds
should remain under 10 KTS. However, winds will begin to turn
today as the ridge moves off to the east. North/northeast winds
will become southeast today. Although overall flow is set to
change and a system will be approaching, we should not see any
shower or tstorm activity in this forecast. We should see the
cirrus shield ahead of a complex to move in this evening. Expect
SHRA to likely be added to the latter part of the TAFs in the next
set for some with isentropic lift helping moisture/activity
advect north of an approaching warm front near the coast.




Relative humidity values will fall below 25 percent is most areas
this afternoon. However, 20 foot winds will only be around 5 mph,
and KBDI values remain low due to recent rainfall. Therefore
critical fire weather conditions are not expected. Moisture and
rain chances return on Monday, with a moist pattern remaining in
place for the first half of the week.


Gadsden     59  40  59  56  73 /   0  10  80  50  60
Anniston    60  41  62  56  75 /   0  10  70  40  40
Birmingham  62  44  63  57  75 /   0  10  80  50  50
Tuscaloosa  63  44  66  61  79 /   0  20  80  50  40
Calera      63  44  64  59  77 /   0  10  80  30  40
Auburn      63  44  67  58  75 /   0   0  60  20  20
Montgomery  65  44  71  61  81 /   0   0  60  20  30
Troy        65  45  72  61  79 /   0   0  50  20  20





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