Area Forecast Discussion
Issued by NWS Birmingham, AL

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817
FXUS64 KBMX 211132
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
632 AM CDT Mon Aug 21 2017

.UPDATE...
For 12Z Aviation.

&&

.SHORT TERM...
Today and tonight.

Looking for a typical Summer day, with generally just isolated
showers and storms this afternoon. Of course this is not a typical
day as we will be able to see an eclipse of the sun this afternoon.
So with rain in the forecast, are my chances at seeing the eclipse
done? The answer is no. Stated earlier it will be a typical summer
time day, with hit or miss showers/storms that for the most part
will be brief. Clouds should be isolated to scattered as well, so
there should be breaks in there as well. Rain chances dwindle after
10 pm in the south and 7 pm in the north, with the loss of daytime
heating.

16

.LONG TERM...
Tuesday through Sunday.

Mid-level ridging will remain in place on Tuesday but will be in
the process of weakening as a deep trough digs into the Great
Lakes. The combination of the weakening of the ridge and
increasing PWATs should result in an increase in coverage of
scattered summertime convection. Coverage will be more isolated
across the far north where PWATs will be a bit lower. Hard to pin
down exactly where the highest rain chances will be; perhaps
somewhere between the I-20 and I-85 corridors where 850-700 mean
RH will be a bit greater. A cold front will be sagging southward
towards the area, due to a sub-1000 mb surface low over Quebec. It
will be well north of the area during the day on Tuesday, but
some scattered remnants of convection that develops along the
front could reach our far northern counties after midnight.

The cold front will move into the area Wednesday/Wednesday night.
Main upper-level forcing will be well removed from the area but
there will be a weak mid-level vorticity axis. There is some
question as to whether the highest coverage will be across the
north near the front or across the south along a lee trough/pre-
frontal trough and possible outflow from Tuesday night`s
convection. There are also some model differences with how far
south the upper level jet will extend and whether the old TUTT low
over the northwest Gulf will contribute any energy. But overall
Wednesday will see the highest rain chances of the forecast
period. Nothing severe is expected due to typical late August weak
lapse rates/bulk shear.

The ECMWF is more amplified with the trough over the eastern
CONUS and faster with the progression of the front, keeping almost
all of the forecast area dry on Thursday, while the GFS has
trended slower. Will stick close to the previous forecast which is
somewhere in the middle with PoPs for Thursday. For Friday
through the weekend, models are in agreement with the upper-level
pattern, indicating northwest flow developing aloft between
ridging over the central CONUS and the trough over New England.
They also agree in taking potential re-generated TC Harvey
westward through the Bay of Campeche towards Mexico, while a large
surface high moves eastward across the CONUS and causes a CAD
wedge to develop along the East Coast. However, they differ with
the placement of the old frontal boundary and timing of a backdoor
wedge front moving into the area. Some of this is related to the
interaction of the old front and a tropical wave located near the
Bahamas and any low pressure development in the Atlantic. But
locally the main impact of these differences is just the timing of
the westward-moving backdoor front, with isolated to scattered
showers/storms possible along and ahead of it, and a dry/stable
easterly flow developing behind it. Will keep PoPs in the 20-30%
range during this period.

32/Davis


&&

.AVIATION...
12Z TAF Discussion.

Some passing cirrus this morning, will give way to few to
scattered cumulus late morning, early afternoon. Looks like TOI
and MGM will see the highest chances at seeing rain, with VCTS at
TOI after 21Z. After looking at the latest models removed VCTS at
MGM as confidence is too low to include at this time, but
continued at TOI through 3Z. Otherwise VFR conditions expected.

16

&&

.FIRE WEATHER...

Moisture and chances rain increase gradually through the week,
ahead of a cold front that will move through the area Wednesday.
There are no fire weather concerns at this time.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     93  72  92  72  89 /  10  10  20  20  60
Anniston    93  73  91  74  89 /  10  10  30  20  60
Birmingham  94  76  92  76  90 /  10  10  30  20  60
Tuscaloosa  95  75  93  75  91 /  10  10  30  20  50
Calera      94  75  92  75  90 /  10  10  30  20  50
Auburn      91  74  90  74  89 /  20  10  40  20  50
Montgomery  95  76  94  76  93 /  20  20  30  20  50
Troy        93  73  92  73  91 /  30  20  30  20  50

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
None.

&&

$$



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