Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 281203
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
703 AM CDT Wed Oct 28 2020

.UPDATE...
For 12Z Aviation.

&&

.SHORT TERM...
/Updated at 0452 AM CDT Wed Oct 28 2020/
Today and Tonight.

All eyes are on the Gulf this morning as Zeta is a hurricane once
more with an impressive satellite presentation especially for this
time of year. There is little change in the overall forecast
thinking as we continue to highlight gusty to damaging winds
capable of downing numerous trees and power lines as the highest
threat. Secondary threats of flooding and brief tornadoes remain.

Forecast:
Zeta will accelerate northward today ahead of a potent upper low
over the southern Plains. Some additional strengthening is
possible today, before shear and cooler waters cause it to level
off or weaken slightly, but there won`t be much time for it to
weaken since it will be moving so fast. A band of showers and
isolated thunderstorms will continue to lift north of the forecast
area along a warm front this morning. There will be a bit of a
lull in activity, before another band over southern Mississippi
moves into West Alabama later this morning. With a very a moist
airmass, expect quite a bit of coverage of showers and embedded
thunderstorms by this afternoon, except in the southeast where
activity will be more scattered in nature. CAPE values will be
around 1000 to 1500 J/kg but mid-level lapse rates will be weak
with marginal shear (0-6km shear around 25-30 kts and 0-1km SRH
around 100-150 m2/s2). There is a risk of a couple strong storms
this afternoon, with a very low but non-zero threat of a damaging
wind gust/a brief tornado mainly in West Alabama. This will be
conditional on a little bit of heating and for cellular convection
to maintain itself within an overall shield of rain. This
potential appears too low to mention in the HWO at this time, with
the main focus on a marginal tornado threat holding off until
tonight (see below). Widespread rain associated with Zeta`s core
will move through the area tonight, decreasing in coverage from
southwest to northeast late in the night.

IMPACTS:

Wind:

Due to the quick movement of Zeta (the opposite of Sally), and its
expected strengthening today, its core of strong winds will move
much further inland than would normally be expected. Several
models are now explicitly indicating sustained winds near tropical
storm force well into our CWA. The compact and fast-moving nature
of the system will cause a large pressure gradient. Of bigger
concern will be the potential for high wind gusts with 925mb wind
progged around 75-80kts in a swath to the southeast of the center
of Zeta. Neutral stability during the overnight hours will be a
limiting factor in mixing down higher winds to the surface, but
showers/convective activity will be able to transport some of this
momentum downward. A somewhat conservative reduction of these
winds down to the surface still would suggest potential wind gusts
of 50-60mph. This would be true both in our southwest counties
closer to the landfall point, and further northeast where Zeta
will encounter higher terrain/ridgetops. Some 70 mph wind gusts
may not be out of the question, especially in exposed higher
elevation locations. Potential impacts would include numerous
trees down and power outages. Folks that have a lot of large trees
around their mobile home or house may want to consider moving to
their safe place when high winds approach. The period of highest
winds will only last 1-3 hours at any given location. High winds
look to move in around 9-11PM in our southwest counties, and exit
our northeast counties around 4-6AM.

An upgrade to a tropical storm warning is likely to occur later
today. Did expand the tropical storm watch a little to the
southeast based on the low-level wind fields/potential wind gusts
indicated by several models. Areas on the fringes may eventually
need a wind advisory, but will let the day shift re-assess which
counties to include.

Flooding:

Rainfall amounts have trended downward slightly due to the fast-
moving nature of the system. The rainfall last night/this morning
is priming the area somewhat, however. Additional rainfall amounts
of 2-3 inches with locally higher amounts are expected, and since
this rain will fall in a relatively short period of time, isolated
flash flooding remains possible. Made no changes to the Flash
Flood Watch.

Tornadoes:

Higher CAPE values that develop this afternoon are expected to
diminish with the loss of daytime heating before the stronger low-
level SRH arrives with Zeta this evening. Instability near the CDO
rain shield of Zeta will likely be very limited, with any tornado
threat likely to occur with convective bands on the east/southeast
side. Some CAMs have trended a little upward with instability
across our southeast counties late tonight with a band with
possible embedded cells extending southeastward from Zeta, with
impressive hodographs. However, the strength of the updrafts with
this convection is uncertain with limited forcing. Will monitor
for a possible need for a slight risk in our southeast counties,
but confidence is too low to go above marginal right now.

32/Davis

.LONG TERM...
/Updated at 0418 AM CDT Wed Oct 28 2020/
Thursday through Tuesday.

Looks like we will start out Thursday morning wet as Zeta continues
to exit stage right. We may be dry across the entire area by the
afternoon. However there is enough of a chance to continue with
slight chance of showers in the northeast through 10 pm. High
pressure slides in behind Zeta and remains over the east through the
weekend. Will need to continue to monitor the trends for Sunday, but
there is a potential for a wedge front to move in from the east as
the high pressure slides into the Mid-Atlantic and easterly flow
returns. For now will highlight this period with mainly clouds but
may need to add in some light rain as we get closer.

A dry cold front will slide through on Sunday night from the west,
associated with a strong low pressure system in the Great Lakes.
This will clear out the wedge and return us to a drier airmass.
Will need to keep an eye on temperatures Monday morning and Tuesday
morning as a potential frost is possible, and even a few local areas
reaching the freezing mark as November starts.

16


&&

.AVIATION...
12Z TAF Discussion.

Widespread IFR cigs cover cntrl Alabama early this morning due to
a moist tropical air mass over Alabama. An elevated warm front
along I-20 was producing sct showers across north Alabama, mainly
north of the northern TAF sites. Cigs will slowly rise through the
morning hours, but likely staying below 3000 feet agl across the
northern TAF sites, with VFR cigs at MGM/TOI. Sct showers will
continue thru the day, and could briefly reduce vsbys blo 3
miles. Instability is limited, and tstm potential too low for
inclusion at any TAF site. Conds will deteriorate after 02z as
tropical cyclone ZETA approaches southwest Alabama. Wind speeds
will increase rapidly between 04z and 08z with sustained speeds
arnd 30 kts and gusts to 45 kts as the center of ZETA tracks along
the I-20 corridor. Cigs will likely stay in the 1500-2500 feet
agl range, with IFR cigs forming after 09z.

58/rose

&&

.FIRE WEATHER...

Showers and isolated thunderstorms will begin to move in from the
south, becoming more numerous by the afternoon. Widespread rain,
heavy at times, will move through tonight into early Thursday
morning as tropical system Zeta moves through along with strong
winds. Wind gust up to 60 mph will be possible with the band of rain
as the system moves through. Drier and cooler conditions will return
to the area Thursday night and remain through the weekend.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     78  69  76  47  62 /  90 100  80  10   0
Anniston    78  69  77  49  63 /  80 100  80  10   0
Birmingham  78  69  73  49  62 /  80 100  60  10   0
Tuscaloosa  80  69  71  49  64 /  90 100  40  10   0
Calera      78  69  73  48  63 /  80 100  40  10   0
Auburn      78  71  78  50  65 /  60 100  60  10   0
Montgomery  82  72  79  50  68 /  60 100  40  10   0
Troy        81  72  80  50  67 /  50  90  40   0   0

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Flash Flood Watch from 4 PM CDT this afternoon through Thursday
morning for the following counties: Autauga...Bibb...Calhoun...
Cherokee...Chilton...Clay...Cleburne...Coosa...Dallas...Elmore...
Etowah...Greene...Hale...Jefferson...Marengo...Perry...
Randolph...Shelby...St. Clair...Sumter...Talladega...
Tallapoosa...Tuscaloosa.

Tropical Storm Watch for the following counties: Autauga...
Bibb...Bullock...Calhoun...Chambers...Cherokee...Chilton...
Clay...Cleburne...Coosa...Dallas...Elmore...Etowah...Greene...
Hale...Jefferson...Lee...Lowndes...Macon...Marengo...
Montgomery...Perry...Pike...Randolph...Shelby...St. Clair...
Sumter...Talladega...Tallapoosa...Tuscaloosa.

&&

$$


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