Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 051145

Area Forecast Discussion
National Weather Service Birmingham AL
645 AM CDT Thu Aug 5 2021

For 12Z Aviation.


/Updated at 0322 AM CDT Thu Aug 05 2021/

Today through Friday.

The upper trof that has been parked over Alabama the past few days
will flatten out today as an upstream trof digs into the Southern
Plains States. The most notable weather feature today is an inverted
low level trof that has pushed into west Alabama. East of the trof
axis, surface winds have become easterly and surface dewpoints have
risen into the upper 60s. West of the trof axis, surface dewpoints
were still in the lower to middle 60s. The inverted trof will be
somewhere in the vicinity of the I-59 corridor. The trof will be the
focus for convection today, and raised rain chances to 30 percent
near the trof axis. No cloud to ground lightning was observed
yesterday across Central Alabama due to a strong warm layer in the
700-500mb layer. This warm layer will persist into this afternoon,
and only forecasted showers. A few showers will linger into the
evening hours. On Friday, the aforementioned upstream short wave
trof will get close enough to Alabama to produce more diurnal
showers and thunderstorms. A feature of note is a low level trof
that is forecast to track inland over south Alabama. This will
enhance rain chances for areas south of I-85.


/Updated at 0230 AM CDT Thu Aug 05 2021/

Friday night through Wednesday.

The shortwave trough will continue to move across the area Friday
night, causing a weak area of low pressure along the old frontal
boundary (most evident around 850mb) to lift northeastward across
far SE Alabama and Georgia. This will continue to result in at least
isolated to scattered showers and storms with the highest chances in
the far east where the best moisture associated with the area of low
pressure will be. Will keep a chance of thunder in overnight with
500mb temps dropping to -10C in the trough axis. The best rain
chances on Saturday may actually be during the morning hours due to
height rises/subsidence behind the shortwave, but could still get
isolated to widely scattered redevelopment in the afternoon with
daytime heating and a weak vort max possibly moving across the
northeast counties in northwest flow aloft.

On Sunday a mid-level ridge axis will move overhead ahead of another
shortwave moving through the Midwest, and subtropical ridging will
also begin to strengthen over the Gulf. This will keep rain chances
isolated at most, with temperatures back in the low 90s for most
areas. Broad ridging will remain in place for the rest of the period
with very weak flow aloft. Moisture will increase to sufficient
levels for scattered diurnal convection, but not confident in
placement of weaknesses in the ridge to go any higher than 40 PoPs
for now. Will have to monitor heat indices during this period, but
models do show dew points mixing out some each afternoon. Therefore
have leaned towards the NBM 10th percentile for dew points as some
of the bias-corrected guidance seems to high, and confidence in 105
heat indices is too low to mention in the HWO at this time.



12Z TAF Discussion.

An inverted low level trof over west Alabama will be the focus
for scattered showers today. Sct-bkn clouds with bases arnd
6000ft agl will occur along and east of the trof axis. A warm
layer aloft will limit thunderstorm potential and only included
showers in the forecast. The showers will peak in coverage between
18z and 23z.




Isolated to scattered showers will be possible today. Showers and
storms will increase in coverage tomorrow into Saturday but
remain scattered in nature. RH values will remain above 45
percent through the forecast period.


Gadsden     88  69  88  67  89 /  30  20  30  30  30
Anniston    88  69  88  70  89 /  30  20  30  30  40
Birmingham  88  71  88  70  90 /  30  20  30  20  30
Tuscaloosa  90  72  90  70  91 /  30  20  30  20  20
Calera      87  71  87  70  88 /  30  20  30  20  30
Auburn      86  71  86  70  87 /  20  10  40  40  40
Montgomery  90  71  90  71  91 /  20  10  40  20  30
Troy        89  71  89  71  90 /  20  10  40  30  40




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