Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 180725

Area Forecast Discussion
National Weather Service Birmingham AL
225 AM CDT Fri Aug 18 2017

Today and Tonight.

A 998 mb surface low is currently located over the Upper Peninsula
of Michigan, with a trailing cold front extending southwestward into
far SE Missouri and northern Arkansas. A pre-frontal trough is
located ahead of the front extending down into Tennessee. At upper-
levels, Central Alabama is located between a trough over the Great
Lakes and a ridge along the Gulf Coast. Showers and isolated
thunderstorms are ongoing across Tennessee into north Alabama due to
the pre-frontal trough, a weak LLJ, and a weak shortwave aloft. In
Central Alabama, shower development has been very isolated due to
less mid-level moisture/weaker forcing. The pressure gradient/weak
LLJ should be enough to prevent much in the way of fog formation
this morning.

The front will become more ill-defined today, but winds will shift
to the northwest and daytime mixing down of drier air aloft will
allow the front to effectively pass through the area. Despite the
front and weak shortwave, models (especially CAMs) do not show more
than isolated showers today. This is due to capping aloft (caused by
700 mb temperatures around +11 C), weak forcing (only slight height
falls present as the ridge center retrogrades to near Houston), and
drying through the column (caused by advection of drier air aloft.
So, PoPs have been lowered considerably, especially in the afternoon
where they have been limited to areas south of US 80/I-85 while
PWATs fall to around 1.25 inches across the north. Highs will be in
the upper 80s to low 90s.

Dry conditions are expected tonight. Low temperatures will depend on
how much dew points can drop due to the competing factors of
advection and evapotranspiration. They should range from the upper
60s in the typically cooler spots to the low 70s. Drier low-level
air should inhibit fog formation.


Saturday through Friday.

An upper trough moves eastward through the Midwestern states on
Saturday as a broad upper-level low retrogrades westward through the
Gulf of Mexico. Low-level ridging is in place across the
Southeastern US with northerly flow making its way into Central AL.
This dry northerly flow will keep us mostly dry Saturday and Sunday.
I`ll keep mention of slight chances for rain/thunderstorms across
the southern half of Central AL both days as I can`t rule out some
afternoon development with peak heating.

Upper level ridging builds in Monday and southerly flow returns.
Expect Monday and Tuesday to be typical summer days with diurnal
thunderstorm development. PWATs increase to near 2" with lower
values in the northwestern counties Monday. Have therefore kept 30-
40% PoPs in the forecast for areas south and east of the I20/59
corridor Monday, becoming area-wide on Tuesday.

Another surface low pressure system develops in the Upper Midwest on
Tuesday and moves eastward with the base of a synoptic scale trough
Wednesday. A cold front stretches through the Ohio River Valley and
pushes southward into the Tennessee Valley Wednesday night into
Thursday. Expect increased chances of showers/storms towards this
end of next week as the front passes through. Exact coverage of
storms along the frontal boundary is still uncertain, so will keep
30-40% in the forecast at this time.



06Z TAF Discussion.

A weak cold front will move through the area overnight and Friday.
MVFR cigs will be possible at the northern sites for a couple
hours after sunrise due to the front. Couldn`t rule out a brief
drop to IFR cigs at BHM. Confidence is too low to add a tempo but
will leave in the SCT mention. Meanwhile, vis already dropped to
MVFR at TOI and could briefly drop to IFR around sunrise due to
BR. Isolated showers are possible near the northern sites
overnight into Friday morning, and near the southern sites Friday
afternoon, but chances are too low to mention in the TAFs.




Drier weather today and through the weekend. Isolated to scattered
afternoon showers possible for southern parts of the Central
Alabama. There are no fire weather concerns at this time due to
the abundant moisture across the area.


Gadsden     89  68  91  71  93 /  20   0  10  10  10
Anniston    89  70  91  71  93 /  20   0  10  10  20
Birmingham  90  72  93  74  94 /  20   0  10  10  10
Tuscaloosa  91  72  94  74  96 /  10   0  10  10  20
Calera      90  72  92  73  94 /  20   0  20  10  20
Auburn      90  72  91  73  92 /  20  10  20  10  20
Montgomery  94  74  94  74  95 /  20   0  20  10  20
Troy        92  72  93  74  94 /  30  10  20  10  20




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