Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 250502

Area Forecast Discussion
National Weather Service Birmingham AL
1202 AM CDT Mon Sep 25 2017

For 06Z Aviation.



Updates made this morning appear to still be on track as we are
beginning to see isolated showers in the northeast. Will hold onto
activity through 10 pm, but it will likely be done earlier due to
the isolated nature this afternoon. This isolated nature is due to
2 reason. The first we are seeing a somewhat drier airmass working
into the region from the east. Secondly we are being cutoff from
the rich airmass with all of the convection associated with a meso
low near the Alabama coastline. This low has also increased the
dry airmass as a general subsidence zone has set over the
southeast. There may be some patchy fog again tonight, but again
will be generally thin and should not cause any major issues.


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.



06Z TAF Discussion.

Kept the trend of VFR going for this period. There will be
increase mid and upper level moisture as a rather weak upper level
low drifts northwestward across southwest Alabama and into
Mississippi through Monday. At this time, will keep fog mention to
a minimum due to the increased cloud cover and will also hold any
ceilings 10k ft or above. Some of the newest guidance coming in is
hinting that there may be a few clouds in the 025-040 range, but
have not seen any develop yet and the 00z raob was still dry.
Launching a balloon now in support of Maria, and if it indicates
anything new, it make some adjustments. Any showers that develop
should remain south and west of terminals. Otherwise, close to
persistence with east winds around 7kts day and light at night.




Shower and thunderstorm chances are more confined to the
southwest counties of Central Alabama through 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     64  87  64  88  65 /  10  10   0   0   0
Anniston    66  87  65  87  66 /  10  10   0   0   0
Birmingham  68  88  68  90  67 /  10  10   0  10   0
Tuscaloosa  68  90  68  92  68 /  10  10   0  10   0
Calera      68  88  67  89  67 /  10  10   0  10   0
Auburn      67  87  66  88  68 /  10  10   0   0   0
Montgomery  70  90  68  91  69 /  10  20  10  10   0
Troy        66  88  67  89  67 /  10  20  10  10   0




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