Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 251000

Area Forecast Discussion
National Weather Service Birmingham AL
400 AM CST Sun Feb 25 2018

Today and Tonight.

Extensive line of showers along a cold front/outflow boundary
continues to sink southeastward. Convection has weakened
considerably with no lightning being observed. The main shortwave
trough and surface low continue to quickly lift northeastward
through the Lake Superior region. Overall flow is parallel to the
line, and embedded supercells have been unable to remain
along/ahead of the outflow boundary. Measured gusts have been well
below severe limits. We have removed the marginal risk of severe
storms from the forecast. CAMs are in good agreement that the
outflow boundary/cold front should clear the southeast counties
prior to noon and prior to peak heating. While the surface
boundary will clear the area, the front in the 850-700mb layer
will stall over the area as it will be parallel to the southwest
flow aloft ahead of the longwave trough over the Central CONUS. A
bit of a lull in the precipitation may occur around midday as the
stratiform region of the QLCS dries up, but expect one or more
bands of light rain and scattered showers to re-develop by
afternoon due to frontogenesis and over-running. Will indicate
slightly higher PoPs in the morning vs. the afternoon as the
afternoon activity will be lighter, but still expect fairly good
coverage in the afternoon. Future updates may need to increase
afternoon PoPs in some areas once model agreement on placement of
rain bands improves. Temperatures behind the front will be closer
to seasonal averages, much cooler than previous days.

Tonight a shortwave trough currently over the Four Corners will
move through the Mid-South region. A wave of low pressure will
develop along the front. Strengthening 700 mb flow and associated
isentropic lift should result in widespread rain re-developing by
midnight. A convective complex moving along the Gulf Coast may
enhance rainfall totals across the southern counties. A few
thunderstorms are possible there due to elevated instability, but
any surface-based instability is expected to remain near the Gulf
Coast. Area-average rainfall totals have trended downward, and are
now in the 1-1.5 inch range. So the overall risk of flooding is
low, but can`t rule out some minor flooding in poor drainage


Monday through Saturday.

Rain chances along and east of I-59 will end through the late
morning and early afternoon hours on Monday as the cold front gets
booted eastward by an upper-level shortwave. Drier air will move
into the region Monday night, and temperatures should drop to near
40F in the North to the lower 50s in the Southeast on Tuesday

Rain-free conditions should continue into Tuesday afternoon, but
the progressive weather pattern will make for a busy period
Tuesday night through Thursday. A shortwave in the Southwest U.S.
is expected to eject eastward, and a surging warm front should
cause increasing rain chances Tuesday night into Wednesday. At
some point between early Thursday morning and late Thursday night,
a frontal passage is expected along with the potential for
showers and storms, some of which could be severe.

Models are currently not showing much instability, but this should
not be taken at face value. Surface temperatures and dewpoints in
the warm sector of the ongoing system have been underestimated,
and trends over the Gulf of Mexico and inland areas showed an
increase of 2-5F in the lead up to the current system. Even with
these trends, model temperatures and dewpoints are still too low
when compared to current observations. This will be critically
important for our next system arriving Wednesday night into
Thursday, and CAPE values appear to be underestimated by all model
guidance. With the potential for very strong wind shear over the
region, it will not take much of a change in the quality of the
low-level airmass for this system to become an "overachiever"
compared to expectations. Since models are having serious problems
with run-to-run consistency regarding the evolution of the upper
air pattern, a severe risk will not be mentioned in the HWO for
this forecast package. Once better agreement is reached, a threat
is likely to be introduced for late Wednesday night into Thursday.



06Z TAF Discussion.

Slow moving front approaching Central Alabama. The trend from the
previous forecast was generally accepted. The front will move
slowly east and south and reach MGM/TOI near daybreak. Ahead of
the front, southerly winds remain breezy with some lower clouds
developing south and advecting northward. So northern terminals
will experience a pinch between the moisture advection and the
front for reduced ceilings. Thunder seems much less likely and
have removed its mention for the time being. Overall, a
predominant MVFR ceiling and light rain forecast.

Winds will be southerly 10-20kts ahead of the front. As the front
moves in, winds will swing initially west and then north
northwest around 10kts. The northern sites will experience the
veering winds behind the front that eventually make it back around
to the east southeast by periods end. A wave will move along the
front and pull the front back northward with more rain Sunday




High RH values and rain chances will continue through Monday
morning. Dry weather is expected for Monday night and Tuesday, but
high rain chances will return Tuesday night through Thursday.
There are no fire weather concerns.


Gadsden     60  53  66  41  69 / 100 100  40   0  10
Anniston    63  54  68  44  71 / 100 100  60   0  10
Birmingham  61  53  67  45  72 / 100 100  30   0  10
Tuscaloosa  60  54  70  45  73 / 100 100  10   0  10
Calera      60  54  68  46  72 / 100 100  40   0  10
Auburn      66  57  68  51  69 /  80 100  80   0  10
Montgomery  67  58  71  50  74 /  80 100  70   0  10
Troy        72  60  71  52  73 /  80  90  80   0  10




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