Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 240958

Area Forecast Discussion
National Weather Service Birmingham AL
458 AM CDT Sun Sep 24 2017

For 12Z Aviation.


Today and Tonight.

The upper-level pattern continues to remain relatively stagnant
across the CONUS. The cutoff relatively moisture starved upper low
overhead will continue to drift westward into Mississippi today,
embedded within a broad southern stream trough. The deep western
CONUS longwave trough is slowly moving eastward, sending the mid-
level ridge center eastward from the Great Lakes into NY/PA.
Hurricane Maria continues to move slowly northward over the
Atlantic. Low-level ridging is centered near Toronto, maintaining
easterly low-level flow over Central Alabama. The surface high will
try to build down the lee of the Appalachians, aided by
northeasterly flow associated with Maria.

Easterly flow will help pull in some dry air at 700 mb, with
experimental GOES-16 derived PWATs indicating values around 1.1 to
1.2 inches over east Alabama and the northern two thirds of Georgia.
The main forcing with the upper low will push into Mississippi with
weak height rises building in behind it. Relatively best moisture
will be over the southwest counties, but don`t think coverage will
be any more than isolated. Some of the CAMs show a westward moving
weak 700mb wave and patch of 700 mb moisture, with some isolated
convection developing around the Birmingham metro area during the
early afternoon. Therefore, added a "finger" of isolated PoPs
extending up into this area from 1-4PM. Overall not expecting
anything that will have a big impact on outdoor plans.

Mid-level lapse rates remain elevated due to the cold core aloft
associated with the upper low, and plenty of DCAPE remains present
due to the dry air. Therefore, if an updraft is able to overcome dry
air entrainment, a couple strong storms are possible mainly in the
southwest counties with wind gusts of 40-50 mph, hail up to dime
size, and frequent lightning. Temperatures will remain warm, and
only be a degree or two cooler than yesterday with only a slight
drop in 1000-850 thicknesses due to the easterly flow. Winds will
not be as light as previous days due to the gradient associated with
the high pressure building in and Maria. Another quiet night is
expected tonight.


Monday through Saturday.

By Monday, we can finally say goodbye to any last upper level
signature from Jose, taken over by Maria. The large upper trough
over the Rockies opens up pulling the energy over the southern
states into it as an open wide shortwave. The NE US ridge breaks
down as Maria continues northward. The result for C AL Monday
will be only low pops in the afternoon with diurnal heating in
the southwest with better chances near the Gulf Coast.

Tuesday into Wednesday, the main upper low associated with the
Rockies system moves northeast toward Manitoba Tuesday and toward
Quebec by Wednesday. At the same time, a piece of energy breaks
off from the trough forming a closed low on Wednesday over the
Desert Southwest. Ridging fills in over the Gulf behind Maria (as
she treks northward along the Atlantic Seaboard over the Atlantic)
and starts to wrap its way around into Texas and the Lower MS
River Valley. We should remain dry for Tuesday into Wednesday
across Central Alabama being on the dry northerly flow side of
Maria along with ridging in place across much of the Deep South.

At the surface, a cold front makes progress across the central
part of the US to near our door step by late Wednesday. Each run
of the guidance looks a little drier ahead of the front. We really
do not get an opportunity for much of any return flow ahead of the
front due to our position in respect to Maria. So, I have removed
the low pops on Thursday that were there. A second reinforcing
surface front moves in behind it on Friday escorting a 1030-32 MB
ridge across the Upper Plains into the Mid West States. Extended
models do agree on the secondary front with a nice ridge behind
it. Some disagreement comes into play with the orientation of said
ridge and exactly the extent of our gradients and northerly flow.
This in turn is somewhat dependent on the exact position of Maria
by Friday. If she is just off the East Coast then we will have
tighter pressure gradients and a more northerly flow across
Alabama with colder temperatures. If Maria ends up a little
further to the east over the Atlantic then we could end up with
more relaxed pressure gradients and a flatter ridge over more of
Conus and thus not as strong of a northerly flow with not as much
cold air advection and thus milder temperatures but still
definitely cooler than we are now.



12Z TAF Discussion.

Other than the possibility of brief MVFR BR around sunrise, VFR
conditions should be predominate through the period. Coverage of
afternoon showers/storms appears too isolated to include in any
TAF due to dry air moving in aloft and weak forcing, but will need
to monitor TCL. East winds today will be around 7-9 kts.




Shower and thunderstorm chances are more confined to the
southwest counties of Central Alabama for this afternoon and
Monday afternoon with daytime heating. Relative humidity values
only drop into the upper 40s and 50s. There are no fire weather
concerns at this time. A change in the pattern is expected by
early Thursday with the arrival of a cold front along with cooler
and drier weather expected as we head into next weekend.


Gadsden     86  64  86  64  88 /  10   0  10   0   0
Anniston    87  66  86  65  87 /  10   0  10   0   0
Birmingham  89  68  87  68  90 /  20   0  10   0  10
Tuscaloosa  90  68  89  68  92 /  20  10  10   0  10
Calera      88  68  87  67  89 /  20   0  10   0  10
Auburn      85  67  86  66  88 /  10   0  10   0   0
Montgomery  91  70  89  68  91 /  10  10  20  10  10
Troy        87  66  87  67  89 /  10  10  20  10  10




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