Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 200857

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

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

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.


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.



06Z TAF Discussion.

Ground fog has already developed at ANB/ASN, and expect it to
develop at other sites as well due to Tuesday`s rainfall, clearing
skies, and light winds. Will take all sites down to IFR vis except
BHM (due to climatology) and TCL (did not receive appreciable
rainfall today); will keep vis MVFR at those two sites. Fog
should mix out by mid-morning on Wednesday. Coverage of afternoon
showers/storms looks too low to mention in any TAF at this time
due to the stabilizing effects of Tuesday`s convection and the
terminals being located between upper level disturbances. TCL and
TOI are the two sites that will have to be monitored the most for
possible showers/storms in the vicinity.




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.


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


Dense Fog Advisory until 10 AM CDT this morning for the
following counties: Autauga...Barbour...Bibb...Blount...
Shelby...St. Clair...Talladega...Tallapoosa...Tuscaloosa...


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