Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 231145

Area Forecast Discussion
National Weather Service Birmingham AL
645 AM CDT Wed Aug 23 2017

For 12Z Aviation.


Today and Tonight.

Currently, a weak cold front is stretched through Northern Alabama.
This weak boundary is expected to move southward through Central AL
today. Models are in pretty decent agreement that the boundary will
push just south of the I-20 corridor before peak heating of the day
helps initiate convection along the front. Have therefore lowered
PoPs in the north and gone with chance PoPs south of I-20 in the
afternoon hours. Models don`t really want to develop much
rain/storms with the boundary today, but given the moist airmass
and source of extra lift, I would expect at least scattered
storms. PWATs are forecast to be in the 1.90 to 2.00" range today,
leading to the possibility of localized heavy rainfall with any
storms that do develop. As we get into the overnight hours tonight
into Thursday morning, the frontal boundary is stretched across
the southern portions of Central AL. However, with the loss of
heating, would expect most of the storms to diminish, so I have
lowered PoPs after midnight.


Thursday through Tuesday.

The cold front is expected to be south of the forecast area by
Thursday morning as surface high pressure builds in. Will only
include small rain chances in southern Central Alabama due to
lingering moisture. However, most areas should remain dry due to
dry northwest flow aloft caused by a deep trough over the
northeast CONUS. Highs should still reach the upper 80s north to
low 90s south due to a lack of significant cold air advection, but
dew points will fall into the 60s in most areas allowing it to
feel more comfortable. Dry conditions are expected Thursday night
with lows in the 60s except for some locations in southeast
Central Alabama. 500 mb heights will rise on Friday as the
northeast CONUS trough lifts away from the area and 500 mb ridging
develops over the northeastern Gulf. Meanwhile, Harvey, which NHC
indicates will likely be a re-generated tropical cyclone, will be
moving towards Texas. Low-level flow will become easterly around
low-level ridging centered near the Great Lakes and a separate
possible weak low pressure area near Florida. This easterly flow
may allow just enough moisture to return to the area for a couple
isolated to scattered showers/storms in far southern Central
Alabama near the stalled front on Friday, but most areas will
remain dry.

On Saturday moisture begins to return northward, allowing for the
possibility of isolated to scattered, mainly diurnally driven
showers and storms. Best chances look to be across the south where
better deep layer moisture will be present, but coverage will
probably be limited by ridging and lingering dry air aloft. The
GFS appears overdone with activity due to questionable vorticity
originating from convection associated with Harvey.

The forecast for Sunday through Tuesday will depend on the
evolution of Harvey. After sitting over the Texas Gulf Coast
region for a couple days due to a strong ridge near the Four
Corners, it should eventually lift northeastward due to troughing
developing over the Central CONUS. The 00Z GFS is much quicker
than the ECMWF in allowing this to happen, allowing an initial
shortwave dropping into the Midwest to capture Harvey. However,
the GFS has varied widely from run to run, on the order of
hundreds of miles, with yesterday`s 18z run indicating a slower
solution. The ECMWF has better ensemble support with a slower
solution, though it did trend slightly faster with its latest run.
The low pressure center has not even formed yet which will cause
problems with the models as well. Meanwhile, high pressure moving
into New England and low pressure developing in the Atlantic
(being monitored by NHC for possible tropical/subtropical
development) will cause a CAD wedge to develop along the East
Coast. The competing factors of a southerly moist flow around
Harvey, and relatively cooler and drier easterly flow due to the
wedge will determine precipitation and temperatures across the
area. Will continue to go with the slower solution which WPC is
going with as well. Rain chances will increase for Monday and
Tuesday, but any possible impacts (e.g. flooding/wind/severe) from
Harvey as either a tropical or extratropical system, look to be
mainly beyond the 7 day forecast period (especially given the
stabilizing effects of the wedge), with plenty of time for things
to change.



12Z TAF Discussion.

Currently, a weak cold front is stretched through Northern Alabama.
This weak boundary is expected to move southward through Central AL
today. Best chances will be across the northern sites from 21 to
00Z. MGM may see some activity after 22 to 01Z. Not sure it will
make it to TOI before the loss of daytime heating dissipates so
did not include any TS at this time there.




Scattered showers and storms are expected today as a cold front
moves through the area. Drier air returns to the area for the end
of the week. There are no fire weather concerns at this time.


Gadsden     86  65  86  62  87 /  40  20   0  10  10
Anniston    87  67  87  64  88 /  50  20  10  10  10
Birmingham  87  69  87  66  88 /  50  20   0   0  10
Tuscaloosa  90  71  88  67  89 /  50  20  10  10  10
Calera      88  70  87  66  88 /  50  20  10  10  10
Auburn      89  72  88  69  88 /  40  30  20  10  20
Montgomery  91  73  92  69  91 /  40  30  20  10  20
Troy        91  74  90  70  90 /  40  30  30  10  30




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