Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 231821

Area Forecast Discussion
National Weather Service Birmingham AL
121 PM CDT Fri Apr 23 2021

Midday Update.


/Updated at 121 PM CDT Fri Apr 23 2021/

Today through Saturday.

Two periods of severe weather potential will affect the forecast
area from the pre-dawn hours on Saturday through Saturday evening.


Zonal flow prevails aloft as flattening riding over the area will
move east of the area tonight while a deep trough over West Texas
approaches from the west. Expect the trough to move across the
Tennessee Valley region later in the day on Saturday.

Surface high pressure positioned over the Carolinas will move
southeast offshore into the Southwest Atlantic Basin late tonight.
To our west, Surface low pressure is gradually becoming better
organized from Eastern New Mexico into Northwest Texas. Expect the
surface low to become positioned over far Southwest Oklahoma this
evening with a surface warm front extending from the low center
southeast across Western Louisiana. Towards midnight tonight,
expect the surface low to be centered just north of Dallas Texas
with the warm front extending across Southern Louisiana just
offshore of the Mississippi and Alabama Gulf Coast. Towards
sunrise on Saturday, expect the surface low to be approaching
Shreveport Louisiana with the warm front extending east into
Central Mississippi where another surface low looks to form. At
this time, the effective warm front should be positioned from near
Tuscaloosa southeast to near Dothan. The developing surface low
will quickly track east across our northern and central counties,
moving into the Carolinas by early evening while the surface cold
front approaches from the west, becoming positioned near the
Interstate 59 corridor by Saturday evening.

Severe Weather Concerns.

A storm system will develop to the west of the forecast area
tonight with very impressive dynamics. A robust low-level jet with
speeds over 60 kts along with storm-relative helicity values over
600 m2/s2 will provide a highly sheared environment that will be
supportive of organized severe storms. Primary concerns will be
damaging winds and a few tornadoes as directional shear will be
maximized with the warm front as it moves northeast into the
forecast area during the very early morning hours on Saturday.
Surface-based instability values are largely the limiting factor,
but severe storms are expected given the strong dynamical support.

The early morning activity will quickly move east across the area
and by late morning we expect a decrease in convective activity.
However, the surface cold front remains well to the west of the
state at this time and some breaks in the clouds along with
continued warm air and moisture advection in the lower levels will
promote quickly destabilizing conditions in the lower levels.
More unidirectional wind shear profiles are forecast from the
southwest through the afternoon that will help to minimize
directional shear, but speed shear through the vertical profile
will persist and support organized severe convection. While drier
air will be gradually infiltrating the area from the southwest,
expect drying to materialize in the mid and upper levels through
time during the afternoon hours. Height falls aloft with the
approaching upper level low will result in steepening lapse rates
through the afternoon and into the evening hours. This scenario
will be supportive of isolated to scattered thunderstorm activity
with severe storm potential due to very strong instability due to
the combination of high low-level Theta-E values and cooling
temperatures aloft due to approaching height falls. Afternoon and
evening convection will pose a damaging wind and large hail
threat though an isolated tornado will remain possible, especially
if convection becomes involved with any lower-level boundary
interactions. This risk will begin to decrease as the lower levels
stabilize with the arrival of the surface cold front during the
late afternoon into early evening. As we get past sunset, loss of
daytime heating will allow low-level instability values to wane
with time, decreasing the coverage and intensity of convection as
we progress further into the evening hours.


Clouds will continue to increase today from the west with low
chances for a few showers across portions of our northern and
western counties this afternoon. Winds will increase form the
southeast 7-14 mph and may be breezy at times. Temperatures will
reach into the lower 70s north with highs in the mid 70s south.


Expect further increasing clouds tonight with rain and
thunderstorms arriving from the west and southwest late, expanding
east across much of the area through the early to mid morning
hours. Damaging winds and a few tornadoes will be the primary
risks during this time frame. Winds will become more southerly
with time with speeds from 6-12 mph. Lows will range from near 50
far north to near 60 southeast.


Morning rain and thunderstorms will move east across the area
during the morning hours posing a risk for damaging winds and a
few tornadoes. Isolated to scattered showers showers and
thunderstorms are expected to develop through the afternoon and
into the evening hours as the actual cold front approaches from
the west. Thunderstorms in the afternoon will be capable of
producing damaging winds, large hail and a few tornadoes.
Improving conditions are expected from west to east through the
evening hours as the cold front moves east across the area. Winds
will be from the southwest 10-20 mph and breezy at times. Highs
will range from the mid 70s north to near 80 far south.


/Updated at 0350 AM CDT Fri Apr 23 2021/

Saturday night through Thursday.

Rain exits the area stage right shortly around Midnight and then we
will see a fairly dry pattern through midweek. Look for highs to
modify some during the first of the week with mid to even some upper
80s by Wednesday. The next system will move into the area sometime
between Wednesday night and Friday. The long term models are quite
divergent on overall timing, thus reducing confidence on timing. As
we get the system closer the models should become better in
agreement. It does appear that we will at least get strong storms if
the earlier scenario pans out. The slower scenario allows much more
moisture to advect into the region and would increase the severe
potential by the end of the week. Again too much model spread to
have any confidence of threats at this time.



18Z TAF Discussion.

VFR conditions will prevail this afternoon.

Upper level ridging overhead today will give way to a deep trough
currently over the Southern Plains that will approach the area
from the west tonight into Saturday. Surface high pressure over
the Carolinas will move further to the east as a storm system
organizes across the Southern Plains tonight and approaches the
area tonight into Saturday.

Expect clouds to continue to increase this afternoon with a small
chance of showers north and west. Flying conditions will continue
to deteriorate tonight with lower and thicker cloud cover
arriving from the west through the overnight hours with increasing
chances of showers and some thunderstorm activity. Conditions
will become MVFR tonight followed by IFR potential as we progress
into the morning hours on Saturday. Showers and thunderstorms will
expand from west to east across much of the area through the
morning hours on Saturday with severe storms possible. More
isolated to scattered convection is forecast to develop during the
afternoon hours as the surface cold front approaches from the

Expect low-level winds 6-12 kts today from the southeast followed
by southerly winds 6-12 kts tonight. Winds will become out of the
southwest 8-16 kts outside of convection on Saturday.




Look for increasing clouds and rain chances during the late
afternoon into the evening hours. Winds will become southeasterly
around 5 to 10 mph today with moisture gradually increasing ahead
of this next weather system tonight. This system will carry a
widespread chance of soaking rains and the possibility of strong
to severe thunderstorms on Saturday. Dry conditions return by
Sunday and continue through at least midweek.


Gadsden     50  75  48  74  45 /  90  90  30   0   0
Anniston    53  75  52  74  47 /  90  90  30   0   0
Birmingham  53  78  52  75  50 /  90  90  20   0   0
Tuscaloosa  55  79  52  77  50 /  90  70  10   0   0
Calera      53  76  53  74  50 / 100  90  20   0   0
Auburn      55  74  54  75  50 /  90  90  40   0   0
Montgomery  58  79  56  79  52 /  90  90  30   0   0
Troy        59  79  56  79  52 /  90  90  30   0   0




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