Excessive Heat and Heat in Alabama, August 16, 2024

Area Forecast Discussion
National Weather Service Huntsville AL
908 AM CDT Fri Aug 16 2024

...New NEAR TERM...

.NEAR TERM...
(Rest of Today)
Issued at 908 AM CDT Fri Aug 16 2024

An active day of weather across the Tennessee Valley is on tap,
with both excessive heat impacts and strong/severe storms in the
forecast. Ahead of a surface cold front, warm moist air is pooling
over the Tennessee Valley with the combination of high
temperatures in the mid to upper 90s and dewpoints in the mid 70s
creating heat index values of between 105-109 degrees. A Heat
Advisory is in effect from 11 AM to 5 PM today for areas along and
west of I-65. This slow- moving cold front will approach the
region later this afternoon, serving as a forcing mechanism for
clusters of showers and thunderstorms. Sufficient heating the
morning hours should create a moderately unstable environment in
the afternoon with SBCAPE values around 2000-3000 J/kg in an
uncapped environment. Additionally, soundings show quite a bit of
dry air at the mid- levels with a pronounced inverted-v profile.
Thus, think there will be a threat for microbursts with the
strongest cells as well as damaging straight- line wind bursts in
any line segments that can develop. This will be very mesoscale
driven, but timing looks to be mainly this afternoon, with
convection waning by early this evening.

&&

.SHORT TERM...
(Tonight through Saturday night)
Issued at 357 AM CDT Fri Aug 16 2024

This activity will likely push east of the area after 9 or 10 PM
and dissipate. The highest and better chance of showers and
strongest thunderstorms looks to be near and east of the I-65
corridor into southern middle Tennessee. Temperatures will still
be rather warm when you wake up on Saturday morning (68-73
degrees) given expected cloud cover. Highs look like they will be
hard pressed to climb above the 90 to 94 degree range. This
should keep heat index values at bay. They could briefly hit 105
degrees for an hour, but not long enough to entertain another
Heat Advisory yet.

The actual cold front associated with this storm system doesn't
really move much on Saturday. Additional disturbances move either
ESE or SE along the front Saturday into Saturday night. Just how
they move will have a big impact on the forecast on Saturday. For
now, going with the clustering of most guidance, which moves
another disturbance east from southwestern Missouri into the area
on Saturday. This appears like it could be more widespread
coverage of showers and thunderstorms on Saturday afternoon into
Saturday night in northwestern Alabama. Shear will be strong with
more widespread shear of 35 to 40 knots shown in most models over
the area. However, due to more prevalent cloud cover with
showers/convection likely starting a bit sooner by the late
morning/early afternoon, instability and lapse rates don't look as
good as on Friday. Despite this, there will still be enough
instability (1000-2000 J/KG) for at least marginally severe storms
to occur. Main threat again will be damaging winds, frequent
lighting, and heavy rainfall.

&&

.LONG TERM...
(Sunday through Thursday)
Issued at 357 AM CDT Fri Aug 16 2024

Consensus guidance from the 16/00Z suite of global models suggests
that a mid-level longwave trough will become established across
eastern North America early in the period, with moderately strong NNW
flow aloft predicted to remain in place across the TN Valley through
the middle of next week. At the surface, an area of low pressure
will shift east-northeastward across southern Quebec and into western
New England on Sunday/Sunday night, with the trailing cold front
expected to cross the CWFA at some point Sunday afternoon. Thus, a
low-medium (20-40%) chance for showers and a few thunderstorms has
been maintained in the grids through late Sunday morning. Although
some wrap-around stratus clouds may impact the northeastern portion
of the region on Sunday night/Monday (in the wake of the departing
cyclone), probabilities for any light rain showers or sprinkles
remain very low (5-10%). Light-moderate N-NE flow will advect a
cooler/drier continental airmass southward early next week, with
highs falling into the m-u 80s for most of the region beginning
Monday, and lows dropping into the u50s-l60s Tuesday morning (and
perhaps m-u 50s Wednesday morning). Present indications are that low-
level ESE flow will return Wednesday night as a subtle surface trough
retrogrades westward across the region, and this will allow for a
minor increase in both low-level moisture and chances for showers on
Thursday.

&&

.AVIATION...
(12Z TAFS)
Issued at 653 AM CDT Fri Aug 16 2024

VCTS was included at KMSL from 14Z for isolated to scattered
showers or thunderstorms that may come close to the terminal. A
better chance of -TSRA is expected at KHSV later this afternoon.
Thus, a tempo group was left in for that terminal for MVFR
CIGS/VSBYS due to -TSRA activity. Winds should be light and
variable by the evening hours. Patchy fog was included at KHSV,
since a better chance of rain is expected there. Otherwise, VFR
conditions are expected.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Heat Advisory from 11 AM this morning to 5 PM CDT this afternoon
     for ALZ001>007-016.

TN...Heat Advisory from 11 AM this morning to 5 PM CDT this afternoon
     for TNZ096.

&&

NEAR TERM...AMP.24
SHORT TERM....KTW
LONG TERM....70
AVIATION...KTW