Severe Thunderstorm Warning 204, NWS Birmingham, AL

Area Forecast Discussion
National Weather Service Birmingham AL
1234 PM CDT Fri Aug 28 2020

.UPDATE...
For 18Z Aviation.

&&

.SHORT TERM...
/Updated at 1012 AM CDT Fri Aug 28 2020/

High res models are coming into better agreement in keeping the
zone of showers and thunderstorms roughly along I59 through the
afternoon. With PW values at a max for this time of year and
rainfall totals between 2 to 2.5 inches an hour...have decided to
issue a Flash Flood Watch to account for this prolonged period of
heavy rainfall. Localized flooding is possible where these
thunderstorms continue to move through the same areas of central
Alabama. Wind speeds have been adjusted slightly higher to the
forecast for the afternoon due to a mid level jet moving through
the northern most counties. Wind gusts up to 20 or 25 are possible
for a couple of hours across mainly areas north of I59.

The northwest fourth of the area could see severe weather this
afternoon. Several bands of thunderstorms could move through the
northwest through the afternoon with the best timing between noon
and 10 pm. With instability between 2300 and 2600 J/kg...and
decent shear...tornado formation is possible.

By the late evening...the band of convection across the central
part of the state is expected to weaken in intensity and coverage
...moving east shortly after midnight. Areas south of I59 should
remain mostly dry through the early morning with only a stray
shower possible. North of I59...showers and thunderstorms are
expected to continue through the remainder of the
night...weakening by the morning. Winds could remain gusty in
these storms with a 25-35 kts remaining through the
midlevels...though not expecting additional severe weather
overnight past 10 pm.

Saturday...Several more waves of showers and thunderstorms will
move across the state through the day...with the greatest coverage
in the far north and east. Mid and upper level dry air will advect
into the state through the morning and early afternoon keeping
rainfall totals much lower than today. Instabilities will remain
high around 3000 J/kg...though winds will be weak and PW values
much lower around 1.7. Will keep an eye on severe weather
potential...though not expecting severe on Saturday at this time.

24

.LONG TERM...
/Updated at 0404 AM CDT Fri Aug 28 2020/

Sunday through Thursday.

On Sunday, Laura will be long gone, but an area of 500 mb
vorticity will be left behind across southern Alabama. On the
back side of this disturbance, subsidence could lead to a deficit
in the moisture content across much of our forecast area. The main
jet stream will remain to our north across Tennessee, where
enhanced moisture is expected. A dead zone between these features
is evident in the 850 mb wind fields, and the signal for
widespread coverage of showers and storms has weakened in recent
model runs. Therefore, POPs have been reduced from 60-80 percent
down to 50-60 percent, which could also be too high.

The moisture deficit zone may fill in with 2+ inch PWAT values on
Monday as west-southwest flow strengthens across the region. This
suggests rainfall coverage will increase area-wide for Monday.
Height rises are expected to begin on Tuesday in response to a
trough digging toward the Texas Panhandle. Moisture still appears
sufficient for scattered to perhaps numerous showers and storms,
but the highest chances have been trimmed back to our northwestern
counties where less influence from the ridge is expected.

Due to persistent deep-layer anticyclonic flow around the ridge,
significant subsidence is forecast to develop across the western
Gulf of Mexico through Alabama on Wednesday and Thursday. This
should greatly reduce rain chances.

87/Grantham

&&

.AVIATION...
18Z TAF Discussion.

An axis of showers and thunderstorms will continue across central
Alabama through the afternoon and the evening. MVFR conditions are
possible at each TAF site that a thunderstorm moves over...but
only included a TEMPO at each site that is in the path of this
axis of storms. By the time this line slowly moves east
tonight...the storms are expected to weaken in coverage and
intensity. Saturday morning...enough low level moisture could
cause IFR ceilings at all but KMGM and KTOI. Saturday...showers
and thunderstorms are expected at sites from KEET and north as
several waves will move through the northern half of the state.

NOTE: KTOI has had a data outage and will not be reporting until
further notice...therefore included AMD NOT SKED.

24

&&

.FIRE WEATHER...

A tropical air mass will remain over Central Alabama today with
several bands of showers and storms expected. Rainfall coverage may
decrease a little bit over the weekend, but the humid airmass
will remain in place. South to southwest winds are expected over
the forecast period.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     73  89  72  89  72 /  70  50  40  50  30
Anniston    73  89  72  90  72 /  60  50  40  50  30
Birmingham  73  89  73  89  74 /  60  50  40  50  30
Tuscaloosa  74  90  74  89  74 /  50  50  30  50  20
Calera      73  87  72  89  72 /  50  50  30  50  20
Auburn      73  87  72  89  72 /  40  60  30  50  20
Montgomery  75  89  74  91  74 /  40  60  20  50  20
Troy        74  88  73  89  73 /  30  70  20  60  20

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Flash Flood Watch through late tonight for the following
counties: Bibb...Blount...Calhoun...Cherokee...Chilton...Clay...
Cleburne...Coosa...Etowah...Greene...Hale...Jefferson...
Marengo...Perry...Shelby...St. Clair...Sumter...Talladega...
Tuscaloosa.

&&

Hazardous Weather Outlook
National Weather Service Birmingham AL
456 AM CDT Fri Aug 28 2020

ALZ011>015-017>050-291300-
Marion-Lamar-Fayette-Winston-Walker-Blount-Etowah-Calhoun-Cherokee-
Cleburne-Pickens-Tuscaloosa-Jefferson-Shelby-St. Clair-Talladega-
Clay-Randolph-Sumter-Greene-Hale-Perry-Bibb-Chilton-Coosa-Tallapoosa-
Chambers-Marengo-Dallas-Autauga-Lowndes-Elmore-Montgomery-Macon-
Bullock-Lee-Russell-Pike-Barbour-
456 AM CDT Fri Aug 28 2020

This Hazardous Weather Outlook is for the counties served by the
National Weather Service office in Birmingham.

.DAY ONE...Outlook through Tonight.

The remnants of Laura may bring a threat for damaging winds and a
brief tornado between Noon and 10 PM. Areas with greatest potential
will be along and northwest of an Aliceville to Warrior line, with a
marginal risk along and north of a Livingston to Anniston line.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Activation of storm spotters and emergency management may be needed
on through tonight. Please follow all local, state, and CDC
guidelines in regards to travel.

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