Heavy Rain in Alabama, September 27, 2024

Area Forecast Discussion
National Weather Service Huntsville AL
252 AM CDT Fri Sep 27 2024

...New NEAR TERM, SHORT TERM, LONG TERM...

.NEAR TERM...
(Today and Tonight)
Issued at 249 AM CDT Fri Sep 27 2024

The outer rain bands from Hurricane Helene have begun to move into
the region early this morning as the remnants of the system push
into east central Georgia. As Helene moves further inland, it is
forecast to interact with a deepening trough of low pressure over
the Mid South region. This interaction will pivot Helene back to
the NNW and into the Southern Appalachians of NC/TN by Friday
morning -- and eventually further to the west into Middle
Tennessee by the afternoon as the two systems merge. It should be
noted that the current track of Helene is further to the east than
originally thought and this may slightly impact projection on
rainfall/QPF.

Deep tropical moisture will continue to advect into the region
early this morning as evidenced by PWATs creeping into the 1.5" to
2.0" range over much of the area by 09-12z and potentially over
2" across portions of northeast Alabama. As Helene pivots to the
north and eventually NNW, greater forcing along a convergence
boundary along the AL/GA border may focus tropical rain bands
across portions of northeast/north central Alabama into southern
middle Tennessee from roughly a 4 AM to 1 PM window. This could
produce rainfall amounts in the 2 to 4" range (with locally
higher amounts) over this 6-9 hour period. Should this
materialize, a localized flash flooding threat would evolve where
the heaviest bands setup, especially in poor drainage areas. Rises
on rivers, creeks, and streams would also occur late in the day
and into the overnight and evening hours.

Our highest confidence in this evolving would be in areas east of
I-65, especially in locations along the AL/GA border. HREF Grand
Ensemble show probabilities of 70-100% of total rainfall
exceeding 3" (thru 1 AM Saturday) northeast of a line from Ft
Payne to Scottsboro to Winchester, with much lower probs south and
west of that line. Thus, a lower confidence in a flash flood
threat exists in our northwest Alabama counties, where QPF is
forecasted to be close to 1". Still, these areas received very
heavy rainfall from TC Francine a couple weeks ago and we'll have
to watch closely the location of these rainbands to see if they do
setup a little further to the west. A Flood Watch is in effect
for the entire area through 1 AM Saturday morning.

Due to the strong pressure gradient from Helene and its eventual
merger with the aforementioned trough later today, strong
gradient winds will also be a concern (especially across our
eastern areas along the Cumberland Plateau). Wet soils in these
areas (combined with gusty winds up to 30-40 MPH) could knock down
trees and a few power outages are a distinct possibility. A Wind
Advisory is in effect to highlight this threat through 1 AM
Saturday morning. One more note about today: the dense cloud
cover and rain chances will keep high temperatures much lower and
in the upper 60s to mid 70s.

&&

.SHORT TERM...
(Saturday through Monday)
Issued at 249 AM CDT Fri Sep 27 2024

The remnant center of Helene will stall over TN/KY later tonight,
and although QPF amounts will be much lighter, medium to high rain
chances (30-80%) remain in the forecast Friday night into
Saturday due to light shower activity. Rises on rivers, creeks,
and streams may still be ongoing and persist into the weekend,
mainly in locations that receive the heaviest rainfall. Ridging to
the west will amplify and eventually help to nudge the remnant low
to the east into the central Appalachians by Monday. Rain chances
will continue to drop to 20-50% on Sunday and 10-20% by Monday as
the system pulls away. A gradual warming trend will take place as
high temperatures finally return to the upper 70s to lower 80s by
Monday.

&&

.LONG TERM...
(Monday night through Thursday)
Issued at 249 AM CDT Fri Sep 27 2024

High pressure will build into the Deep South and Tennessee Valley
during the middle to later part of the work week as we finally
begin to dry out. Highs will return to the low to mid 80s with
ample sunshine each day. In this drier, more comfortable air mass,
we'll finally get a decent taste of autumn heading into the first
week of October with Min RH values drop to around 40-50% percent
during the afternoon hours.

&&

.AVIATION...
(06Z TAFS)
Issued at 1238 AM CDT Fri Sep 27 2024

Ceilings will lower quickly overnight to MVFR/IFR conditions as
rainbands associated from the remnants of Helene push into the
region. Visibilities will drop into the 3-5 SM range from the
early morning hours through much of the afternoon due to SHRA
and/or mist as the rainbands move through the TN Valley. Strong,
gusty winds will increase throughout the morning hours into the
afternoon and evening into the 25-30 kt range due to the strong
pressure gradient from Helene passing to the east. Amendments may
be needed during this timeframe due to the potential for strong
gusty winds and reduced visibilities from SHRA.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Flood Watch through late tonight for ALZ001>010-016.

     Wind Advisory until 1 AM CDT Saturday for ALZ001>010-016.

TN...Flood Watch through late tonight for TNZ076-096-097.

     Wind Advisory until 1 AM CDT Saturday for TNZ076-096-097.

&&

NEAR TERM...AMP.24
SHORT TERM....AMP.24
LONG TERM....AMP.24
AVIATION...HC