FXUS64 KBMX 232324

Area Forecast Discussion
National Weather Service Birmingham AL
624 PM CDT Wed Aug 23 2017

For 00Z Aviation.


Through Tonight.

A surface cold front extends from near Demopolis, AL to Lagrange,
GA this afternoon with a broken line of showers and thunderstorms
ahead of/along this front. Storms are expected to stay below
severe limits, but a few strong storms with brief heavy rain and
gusty winds will continue to be possible with heating this
afternoon. Not really expecting any "cold" air behind the front.
However, slightly lower temperatures and lower dew points are
expected to filter into the area which will help with the humidity
for Thursday.


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.



00Z TAF Discussion.

Large majority of the time appears VFR this period. The outflow
propagation last night ended up pushing the pre-frontal trough a
bit farther south. This move added lesser focus for storms and it
ended up with only some scattered storms late this afternoon. TOI
may experience some in the next hour or so, so have VCTS mentioned
there. The front was moving through MGM and will end up past TOI
in a hour or so. Much drier air was located behind the front.
Therefore, most areas with little lower clouds overnight. MGM/TOI
will have a brief period of potential MVFR fog in the morning, but
removed the mention elsewhere due to the dew point depressions.
Winds will be around 5 kts or so overnight swinging to northwest
to northeast. On Thursday, northerly winds around 7kts are
expected. The front slows just south of the area, and most of the
convection will remain off to the south.




Scattered showers and storms will continue to sag southward this
afternoon into early evening 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     65  86  62  87  66 /  10   0  10  10  10
Anniston    67  87  64  88  69 /  10   0  10  10  10
Birmingham  69  87  66  88  69 /  10   0   0  10  10
Tuscaloosa  70  88  67  89  70 /  10   0  10  10  10
Calera      70  87  66  88  70 /  10   0  10  10  10
Auburn      71  88  69  88  71 /  20  10  10  20  10
Montgomery  73  92  69  91  72 /  20  20  10  20  10
Troy        72  90  70  90  71 /  20  20  10  30  20





NWS BMX Office Area Forecast Discussion