Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 212351

Area Forecast Discussion
National Weather Service Birmingham AL
651 PM CDT Mon Aug 21 2017

For 00Z Aviation.


Through Tonight.

A few showers and storms are ongoing across southeast Central
Alabama and far west Central Alabama this afternoon. Expect a
slight uptick in coverage over the next couple hours as the cu
field recovers from the slight decrease that occurred as the
eclipse crossed the region. Shower/storm activity will diminish
this evening around sunset.

Dewpoints continue to run in the low to mid 70s, which will lead
to light, localized fog overnight. Temperatures will be in the low
to mid 70s.


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.



00Z TAF Discussion.

VFR conditions expected much of the period. Very isolated
convection will be ongoing at 00z, but this activity will quickly
diminish in the next hour or so. Once such area was right near
Dannelly Field in Montgomery and added a tempo group for an hour.
Scattered thunderstorms are expected area wide on Tuesday and have
mention a prob30 accordingly. A nice low level inversion sets up
over night with slightly elevated winds above it. Due to the
nearly saturated near ground conditions, patchy fog is possible.
It should not get too restrictive but may cause some MVFR vis for
a few hours around sunrise. Winds will be light overnight and
south southwest at 7 kts or less after 15z.




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


Gadsden     72  92  72  89  69 /  10  20  20  60  20
Anniston    73  91  74  89  71 /  10  30  20  60  30
Birmingham  76  92  76  90  73 /  10  30  20  60  30
Tuscaloosa  75  93  75  91  72 /  10  30  20  50  30
Calera      75  92  75  90  73 /  10  30  20  50  30
Auburn      74  90  74  89  73 /  20  40  20  50  30
Montgomery  76  94  76  93  75 /  20  30  20  50  30
Troy        73  92  73  91  73 /  20  30  20  50  30




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