Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 230957

Area Forecast Discussion
National Weather Service Birmingham AL
457 AM CDT Sat Sep 23 2017

For 12Z Aviation.


Today and Tonight.

The trend of less fog each night continues, with only Jasper,
Haleyville, and Pell City reporting any visibility reductions. The
Southeast upper trough is in the process of breaking off into a
retrograding cutoff upper low today. At upper-levels, the low
remains elongated and will be centered over Alabama today, while
at mid-levels the low will be centered over the northern Gulf with
an inverted trough extending northward into Alabama. This upper
low is part of a rex block pattern over the eastern CONUS with a
northern stream ridge centered over the Great Lakes, and a
southern stream trough over the Gulf/Yucatan. To the east,
Hurricane Maria will continue to drift northward northeast of the
Bahamas, while the deep trough remains over the western CONUS.
Light easterlies remain in place at low-levels, south of a low-
level ridge centered over the Great Lakes and Ohio Valley.

Layer mean RH values remain modest with experimental GOES-16 derived
PWATs mainly around 1.5 to 1.6 inches. Daytime heating and remnant
boundaries remain the main forcing mechanisms for development of
summer showers/storms. Raised PoPs from isolated to scattered for
the afternoon across the far northern and northwestern counties.
This is where the best moisture appears to be, with lift aided by
weak vort maxes rotating around the northern side of the upper low,
and advection of potential vorticity along the dynamic tropopause
due to the westward moving upper low/+PV anomaly. Experimental GOES-
16 derived PWATs shows an wedge of drier air (1.3 inch PWATs) nosing
into the east-central counties (e.g. Randolph, Chambers, Clay,
Tallapoosa) from Georgia. This will continue to knife its way
westward into the middle of the forecast area, and could suppress
convective development across the middle/central third
(latitudinally speaking) of the area. But will leave an isolated
mention again given the low-level moisture present. A separate area
of showers/storms may develop in far southeast AL south of this dry
wedge with another vort max working in, but not confident enough in
coverage there to go above isolated.

A mid-level cold pool (500 mb temperatures around -10 to -11C) will
be moving over the area due to the upper low, contributing to
halfway decent lapse rates. This combined with plenty of dry air
aloft/DCAPE will result in a threat once again of a couple strong
storms with sub-severe hail and wind gusts of 40-50 mph, as well as
frequent lightning. Can`t completely rule out an isolated marginally
severe storm as happened yesterday, but forecast microburst
composite parameter values remain in the lower end of the moderate
category. Activity should diminish after sunset. Also bumped up high
temperatures which have been over-performing recently, with little
change in 1000-850 mb thickness values from yesterday.


Sunday through Friday.

By Sunday, the upper energy from what was Jose continues to
deteriorate further over the Atlantic east of New England while
the low over the Northern Gulf retrogrades toward MS/LA, and Maria
moves further northward further constricting the weak ridging to
NE US while the West Coast system progresses eastward to over the
Rockies. On Monday, we see any upper level signature from Jose
gone, absorbed into Maria. The system over the Rockies opens up
pulling the energy over the southern states into it as an open
shortwave. The NE US ridge breaks down as Maria continues
northward. The result for C AL Sunday into Monday will be only low
pops south/west with better chances near the Gulf Coast.

Tuesday into Wednesday, the main upper low associated with the
Rockies system moves northeast toward Manitoba. At the same time,
a piece of energy breaks off from the trough forming a closed low
over the Desert Southwest. Ridging fills in over the Gulf behind
Maria and starts to creep around into E Conus. 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 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. This is a
little faster than yesterday`s run. Each run has trended drier
ahead of the front. So, I will only carry some low pops across
the northern counties on Thursday with drier air filling in
quickly behind the front. A second reinforcing surface front
moves in behind it on Friday escorting a 1028 MB ridge across the
Upper Plains into the Mid West States with tight gradients and a
northerly flow across Alabama to insure cooler weather a nice
taste of fall and absolutely beautiful weather as we end the work
week and head into next weekend.



12Z TAF Discussion.

Other than a brief period of MVFR BR around sunrise, VFR
conditions should be predominate through much of the period.
Daytime heating, remnant heating, and an upper low will result in
isolated afternoon showers and storms near the TAF sites. But due
to limited moisture, coverage is too low to mention in the TAFs
at this time with better coverage remaining north of the




Isolated to scattered showers and storms are expected today across
the area. Rain chances are more confined to the southwest half of
Central Alabama for Sunday and Monday. Relative humidity values
only drop into the upper 40s and 50s. Transport winds remain
rather light which will keep dispersion values down too. A change
in the pattern is expected by Thursday with the arrival of a cold
front along with cooler and drier weather expected as we head
into next weekend.


Gadsden     87  65  85  65  86 /  30  20  10  10  10
Anniston    89  66  85  65  85 /  20  20  10  10  10
Birmingham  90  69  86  67  86 /  20  20  10  10  10
Tuscaloosa  91  69  88  69  88 /  20  20  20  10  20
Calera      90  69  86  67  86 /  20  10  10  10  10
Auburn      85  67  85  67  86 /  20  10  10  10  10
Montgomery  93  69  88  69  89 /  20  10  20  10  20
Troy        88  67  86  67  87 /  20  10  20  10  20




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