Area Forecast Discussion
Issued by NWS Birmingham, AL

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS64 KBMX 201057

Area Forecast Discussion
National Weather Service Birmingham AL
557 AM CDT Tue Apr 20 2021

For 12Z Aviation.


/Updated at 0332 AM CDT Tue Apr 20 2021/

Today through Wednesday.

A relatively dry airmass will remain in place over the region ahead
of an approaching upper-level trough and strong cold front. Warm
advection associated with southwesterly low-level flow will push
temperatures into the mid to upper 70s this afternoon. Due to the
limited moisture return ahead of the cold front, only a very slight
chance of showers remains in our forecast for the northern fringes
of the forecast area after midnight through daybreak. Anomalously
strong cold advection will accompany the frontal passage Wednesday
morning, and temperatures are expected to fall into the upper 30s
across our northwest counties. Breezy conditions will drive wind
chill values into the lower 30s for those locations. Though sunshine
will be abundant, Wednesday afternoon temperatures will be well
below normal, only reaching the upper 50s to mid 60s for many


/Updated at 0332 AM CDT Tue Apr 20 2021/

Wednesday night through Monday.

With the cold front well to our east by Wednesday night,
northwesterly flow will advect cold, dry air into the area while a
surface high centers over the Ozarks. Temperatures will drop
steadily throughout the night unrestricted by clear skies and
calm winds. Lows in the 34 to 36 degree range are expected north
of I-20 Thursday morning with temperatures near 40 degrees across
the south. Patchy frost may develop in the colder areas after
midnight, but there are some questions on how widespread the frost
will be given the dry air and dewpoint depressions of 4 to 8
degrees. Regardless, Thursday will be sunny and cool with highs in
the 60s with a light, northwesterly breeze. High pressure will
settle over the Mid-Atlantic coast on Friday and low-level winds
will become southerly with westerly flow above 850mb. There`s a
small chance for a shower in the southwest late Friday afternoon
as deep layer moisture increases ahead of an upper-level trough
which will be situated over the Central Plains Friday night, but
most of the day will be dry with increasing mid to high level

Downstream of the trough, model guidance continues to develop a
surface low over the Southern Plains which will track into the
ArkLaMiss Saturday morning and produce widespread showers and
thunderstorms across Central AL during the day as a warm front lifts
north from the Gulf Coast towards the Tennessee Valley. There are
still questions regarding the strength and track of the surface low,
the amplitude of the positively tilted trough, and the northward
extent of the warm sector. Nonetheless, strong deep-layer shear will
accompany this system and provide a means for organized
thunderstorms, some strong to severe, capable of producing
damaging winds up to 60 mph and large hail if mid to upper 60s
dewpoints along the Gulf coast can advect far enough inland and
establish a favorable warm sector for severe storms. This scenario
will be dependent on the track of the low which is still in
question, of which a more northern track across the Tennessee
Valley would favor a more uncontaminated warm sector and greater
destabilization. There is also a conditional threat for a few
brief tornadoes but would become less likely with time as surface
winds begin veering during the afternoon, so straight-line winds
would be the primary threat. At the very least, the last several
model runs have been consistent with a low-end threat in our
southern counties, mainly near and south of US Hwy 80. Will go
ahead and add a low confidence mention of severe into the HWO for
the area mentioned. Trends will be monitored closely over the next
several days to fine-tune the details and see if a threat could
materialize further north.

The upper-level trough quickly ejects towards the Mid-Atlantic
states Saturday night. Meanwhile, the center of a subtropical ridge
will move eastward from the Mexican Plateau and build across the
Southeast at the start of next week. This will return dry conditions
with gradually warming temperatures. Highs on Monday will rise back
into the low 80s for most of the forecast area.



12Z TAF Discussion.

VFR conditions will continue through this period with high clouds
exiting west to east through the day. Some fair weather cu may
develop this afternoon, but will not have impacts on flight
criteria. Light southwesterly surface winds will begin to shift
overnight as a dry cold front passes through the area becoming
northwesterly at all sites by 10-12Z. An occasional gust up to 20
kts may occur as the front moves through before sunrise.




Rain free conditions through Friday. Light southwesterly winds
today around 5-8 mph. Minimum RH values 30 to 35 percent this
afternoon. A strong cold front will move through Alabama tonight,
but only a very small chance of a brief shower in the northwest.
Surface winds will become northwest 9-13 mph behind the front on
Wednesday. Much cooler and drier Wednesday and Thursday with
patchy frost possible Thursday morning. RH values Wednesday
afternoon will fall as low as 30 percent and even slightly lower
Thursday afternoon.


Gadsden     76  41  59  35  66 /   0  10   0   0   0
Anniston    77  45  61  36  67 /   0  10   0   0   0
Birmingham  77  44  61  39  67 /   0  10   0   0   0
Tuscaloosa  77  44  63  39  69 /   0  10   0   0   0
Calera      75  45  61  39  67 /   0  10   0   0   0
Auburn      75  50  65  40  68 /   0  10   0   0   0
Montgomery  77  50  67  41  71 /  10  10   0   0   0
Troy        78  53  69  40  72 /  10  10   0   0   0




$$ is the U.S. government's official web portal to all federal, state and local government web resources and services.