Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 201119
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
619 AM CDT Wed Sep 20 2017

.UPDATE...
For 12Z Aviation.

&&

.SHORT TERM...
Today and Tonight.

Fairly widespread convection occurred yesterday, resulting in an
outflow boundary sweeping through much of the forecast area. Clear
skies and calm winds in the cold pool behind this outflow
boundary has resulted in an environment favorable for dense fog
development. With the clear skies, the experimental GOES-16 fog
product nicely shows where the fog has developed. It is most
widespread from Winston and Walker Counties eastward to Cherokee
and Calhoun Counties. Patchy areas extend southward to Pike and
Barbour Counties, especially near bodies of water and where
pockets of heavier rainfall occurred. The dense fog advisory looks
on track, with some of the far western counties excluded where
less rainfall occurred.

As mentioned above, a long composite outflow boundary from
yesterday`s convection currently appears to extend from near
Starkville MS southeastward towards Meridian MS and down to the
Florida panhandle. It then extends northeastward into Georgia.
This suggests that the air mass over much of the forecast area is
fairly stable. Meanwhile at upper levels, water vapor imagery
shows that the Ohio Valley shortwave trough and accompanying
midlevel westerly speed max on its southern flank, responsible for
yesterday`s convection, continues to push east of the area,
undercutting the eastern CONUS ridge. It will make a turn to the
southeast into the Carolinas/Georgia today as it`s blocked by
Tropical Storm Jose located south of the New England coast,
causing the flow over the forecast area to change from westerly to
north-northwesterly. Meanwhile, a southern stream shortwave will
push into the Lower Mississippi Valley, shunting the strongest
part of the subtropical jet south of the forecast area. A deep
broad trough remains over the western CONUS. Water vapor imagery
and experimental GOES-16 total precipitable water data shows dry
air aloft pushing into northern Alabama, with PWATs as low as 1.2
inches.

Have limited mentionable PoPs to the southern and western
counties, as dry air aloft should suppress diurnal convection
elsewhere, also factoring in yesterday`s convective overturning
and the time needed to mix out the fog. 500 mb heights will also
be rising. Best chances of scattered afternoon showers/storms
looks to be in the far southwest counties, in proximity to the
outflow boundary, better moisture, and approaching southern stream
shortwave. This boundary seems ill-defined on its western side,
but some adjustments may be necessary with later mesoscale
satellite analyses. Latest CAMs agree with the above assessments.
Will keep small rain chances going for a couple hours after sunset
in the far south/southwest given the southern stream wave.

32/Davis

.LONG TERM...
Thursday through Wednesday.

A retrograding upper-low will bring rain chances to the area for
Thursday through Saturday as it moves westward across the Deep
South. Moisture quality appears to be lacking, so only 20-40 POPs
seem reasonable with the highest chances across the southern half.
Drier air will move in from the east on Sunday with only a slight
chance of convection across the western half of the forecast area.
Monday through Wednesday appear dry for now with drier air aloft
moving into the region and rising heights taking place.

87/Grantham


&&

.AVIATION...
12Z TAF Discussion.

Shallow ground fog is primarily affecting ANB/ASN/TOI this
morning. It is close to but not currently affecting MGM/EET at the
moment, with low confidence as to whether those sites will be
affected. Fog should mix out by 15-16z, followed by VFR
conditions. Chances of an afternoon shower/storm are too low to
mention in any TAF due to dry air moving in aloft; will monitor
for any possible development near TCL or TOI closer to the better
moisture. Drier air should prevent widespread fog development
tonight, but still expect some development near bodies of water,
which will be assessed in future TAF issuance.

32/Davis

&&

.FIRE WEATHER...

Isolated to scattered showers and storms and storms are expected
today mostly across the southern part of the area. A similar
pattern will continue for the next several days. There are no fire
weather concerns.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     87  66  88  67  86 /  10  10  20  20  30
Anniston    87  68  88  67  87 /  10   0  20  20  30
Birmingham  88  70  89  70  89 /  10  10  20  20  30
Tuscaloosa  91  70  92  71  91 /  20  10  30  20  30
Calera      88  70  89  70  88 /  10  10  20  20  30
Auburn      87  69  88  69  86 /  20  10  20  20  40
Montgomery  90  72  91  71  90 /  20  10  30  20  40
Troy        88  69  89  69  87 /  30  20  30  20  40

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Dense Fog Advisory until 10 AM CDT this morning for the
following counties: Autauga...Barbour...Bibb...Blount...
Bullock...Calhoun...Chambers...Cherokee...Chilton...Clay...
Cleburne...Coosa...Elmore...Etowah...Fayette...Jefferson...Lee...
Lowndes...Macon...Montgomery...Pike...Randolph...Russell...
Shelby...St. Clair...Talladega...Tallapoosa...Tuscaloosa...
Walker...Winston.

&&

$$



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