Flash Flood Watch 2, NWS Birmingham, AL

Area Forecast Discussion
National Weather Service Birmingham AL
1256 PM CDT Fri Oct 25 2019

.UPDATE...
For 18Z aviation & added a severe threat for Saturday.

&&

.SHORT TERM...
/Updated at 1251 PM CDT Fri Oct 25 2019/

A Marginal Risk for isolated damaging winds and a couple of
tornadoes will be added to our products for Saturday as Tropical
Depression #17 moves toward Louisiana and merges with a dynamic
upper-level trough located over Texas. This should result in
strong pressure falls across Mississippi and a substantial
increase in the low-level jet on Saturday morning. Short-range
models continue to show backed surface winds, from the late
morning into the afternoon, ahead of enhanced pressure troughing
along a cold front. Forecast soundings indicate large hodographs
with 0-1 km shear of 35-45 kt and SRH greater than 300 m2/s2,
particularly north of Highway 80. Though SBCAPE values will
probably only reach 500-1000 J/kg, stronger forcing along the cold
front might yield some relatively deep convection and a potential
for isolated damaging winds and a couple of tornadoes. While all
of our area will be included in the risk area, the best overlap of
instability and shear may be across the center of our forecast
area. The threat may begin around 10 AM in the west and continue
through 10 PM in the east. An upgrade to a Slight Risk cannot be
ruled out in future updates.

87/Grantham

Previous short-term discussion:
/Updated at 1017 AM CDT Fri Oct 25 2019/

Through early tonight.

Much needed rainfall has fallen across Central AL throughout the
morning hours among a moistening airmass with PWATs of 1.79
inches measured on the 12Z KBMX sounding. Nuisance flooding has
been reported across low-lying roadways and poor drainage areas
with ~2.5 inches of rain measured so far in the Birmingham area
and ~3 inches measured further west. A decrease in intensity has
been noted on radar reflectivity as of 16Z with the greatest axis
of isentropic lift slowly shifting northward in conjunction with
the northward progression of the warm front which is located
generally just south of Montgomery. Lower rain rates will help to
reduce any flood threat, but minor nuisance flooding may continue
this afternoon with widespread rain expected to continue into
Saturday ahead of Tropical Depression Seventeen.

Meanwhile, there has been a lull in activity across much of
southern Alabama. Guidance increasingly indicates the potential
for marginal destabilization (SBCAPE ~1000 to 1500 J/kg) to occur
across the southern third of the forecast area this afternoon
generally south of a line from Pickens to Randolph County. Dense
cloud cover with occasional passing light showers should make this
outcome difficult to realize, but current obs indicate
temperatures slowly rising into the 70s across the south.
Additionally, wind fields remain enhanced with veering winds
through the column and 45-50 knots of bulk shear nearly
perpendicular to the warm front. Mesoanalysis indicates a slight
increase in effective SRH of ~200 m2/s2 this afternoon in
conjunction with marginal instability across the southwest.
Additional showers and storms are expected to move into these
areas from the south this afternoon and will need to be monitored
for isolated damaging winds and/or a brief tornado. This forecast
remains highly conditional on the amount of instability the
atmosphere is able to achieve before rainfall moves in and
stabilizes surface conditions. As such, confidence remains low
with this threat, but the lull in activity across the south has
warranted enough concern to add a Marginal Risk into the HWO for
areas south of a line from Pickens to Lowndes Counties this
afternoon.

86

.LONG TERM...
/Updated at 0249 AM CDT Fri Oct 25 2019/

Sunday through Thursday.

The front should clear the area before it stalls as it becomes
parallel to the southwest flow aloft between a positively tilted
trough over the western CONUS and the subtropical ridge over the
western Atlantic, so PoPs have been reduced Sunday and could be
removed in later updates. The front will begin to retrograde back
to the northwest during the Monday/Tuesday timeframe as impulses
rotate around the trough to the west, which may bring some rain
chances back into the area as well as milder temperatures. There
remains a lot of uncertainty regarding the timing of the next cold
front. The GFS is trending slower but the ECMWF and its ensembles
and the Canadian have also trended slower, indicating the front
may not arrive until Thursday. Have continued to trend the
forecast warmer and wetter towards the end of the period.

32/Davis

&&

.AVIATION...
18Z TAF Discussion.

/MVFR-IFR-LIFR conditions forecast during the period/

At 17:50Z/12:50PM, terminal reports showed MVFR-IFR conditions due
to persistent low ceilings, with drops in visibility associated
with rainfall. This will be the theme through the rest of this
afternoon.

Part of tonight, there's potential for LIFR conditions for TCL-
BHM-EET (perhaps toward ANB-ASN as well, but isn't mentioned there
at this time). For those same terminals, the question is what
happens later in the overnight period. While I've maintained the
bump to MVFR conditions around 09Z, I have hinted for the
possibility that this could go the other way by lowering the
height of the overcast MVFR deck and included a scattered IFR
deck, compared to the prior forecast. So, future TAFs could extend
the IFR conditions deeper into the night. Surface winds will be
on the increase overnight. Overall, the 'least impacted' terminals
will be MGM-TOI.

It looks like all terminals could lie within MVFR range by
Saturday afternoon, though gusty winds continue alongside
additional spells of rainfall ahead of and with a cold frontal
passage. We'll monitor for any necessary adjustments.

89^GSatterwhite

&&

.FIRE WEATHER...

High rain chances will continue through Saturday night, followed
by drier conditions on Sunday behind a cold front.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     61  73  53  69  50 /  70  90  70  10  10
Anniston    64  75  55  71  51 /  60  80  70  10  10
Birmingham  64  75  54  69  53 /  80  90  40  10  10
Tuscaloosa  65  75  51  70  51 /  90  90  20  10   0
Calera      65  75  53  70  52 /  80  90  40  10  10
Auburn      68  75  59  74  54 /  50  70  60  20  10
Montgomery  70  79  57  74  53 /  50  80  40  10  10
Troy        70  79  59  75  53 /  50  70  50  20  10

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
None.

&&

Hazardous Weather Outlook
National Weather Service Birmingham AL
134 PM CDT Fri Oct 25 2019

ALZ011>015-017>050-262145-
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-
134 PM CDT Fri Oct 25 2019

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

.DAY ONE...Outlook through Tonight.

There is a Marginal Risk of severe storms from 1 PM to 10 PM near
and south of a line from Panola in Pickens County to Letohatchee in
Lowndes County. The primary threats include a brief tornado and
isolated damaging winds winds.

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

There is a Marginal Risk of severe storms on Saturday from 10 AM to
10 PM across all of the forecast area. The primary threats include
brief tornadoes and isolated damaging winds.

.SPOTTER INFORMATION STATEMENT...

Activation of storm spotters and emergency management may be needed
this afternoon and evening and again on Saturday.