Flash Flood Warning 2, NWS Huntsville, AL

Area Forecast Discussion
National Weather Service Huntsville AL
537 PM CST Wed Feb 12 2025

...New AVIATION...

.NEAR TERM...
(Remainder of today and tonight)
Issued at 303 PM CST Wed Feb 12 2025

There have been no significant changes to the near term portion
of the forecast (remainder of today and tonight) since the update
earlier this morning. Current thinking is that widespread light-
moderate rain will persist across the region for several more
hours, with the combination of low stratus clouds and persistent
rainfall yielding a statically stable boundary layer across our
CWFA.

Very late this afternoon/early this evening (23-1Z timeframe),
the remnants of scattered warm sector thunderstorms (currently in
progress across southern MS) will spread northeastward into the
southeastern portion of our forecast area, but should become
elevated in doing so as they cross the effective warm front (which
may remain situated across central AL for much of the event).
Although this activity will not pose a risk for strong-severe
thunderstorms, convectively-enhanced rainfall rates may contribute
to an increasing threat for both flash and areal flooding.

Meanwhile, to the west of the forecast area, there are some
indications that a broken band of moderate-heavy showers and a few
thunderstorms may evolve along a cold front that will begin to
sweep southeastward this evening as the primary synoptic surface
cyclone deepens and accelerates north-northeastward away from the
region. This activity should enter northwest AL between 0-2Z and
may be limited in both coverage and intensity by extensive
preceding precipitation to its east, but if a contaminated warm
sector airmass (featuring dewpoints in the u50s-l60s) was to
arrive in our region, it would likely occur immediately in advance
of the cold front (particularly across the southern half of the
CWFA). Due to the magnitude of both deep-layer and low- level
shear in place across the region this evening (as well as the
potential for up to 200-400 J/kg CAPE), we will be monitoring the
stronger updrafts in this regime for both damaging winds and
perhaps a brief tornado. Present indications are that the risk for
strong-severe convection will end in the southeastern portion of
the forecast area by 6-8Z (if not sooner).

Low-level flow will quickly veer to NW and strengthen in the wake
of the cold front, with a colder/drier airmass expected to spread
southeastward to encompass the entire region by sunrise on
Thursday. Due to the combination of elevated winds and extensive
postfrontal stratus, lows will be in the m30s-l40s.

&&

.SHORT TERM...
(Thursday through Friday night)
Issued at 303 PM CST Wed Feb 12 2025

Mid-level winds will temporarily veer to WNW for a brief period
from Thursday-Thursday night as a longwave trough ejects
northeastward across the Great Lakes and into eastern Canada.
This, along with modest northerly flow at the surface, will
provide a dry/stable airmass for the TN Valley, with highs in the
m-u 40s tomorrow. As the center of a surface high shifts eastward
into the central Appalachians on Thursday night, conditions may
become quite favorable for radiational cooling as the boundary
layer decouples after Midnight, and we have indicated a hard
freeze on Friday morning with lows in the l-m 20s.

Light SE return flow will begin Friday as the surface high
migrates further eastward into the Mid-Atlantic region and a new
surface low develops across eastern CO in response to an
amplifying mid-level shortwave trough digging across the
southwestern US/northwestern Mexico. Although an increase in
mid/high-level clouds can be expected as winds aloft back to WSW
once again, highs should reach the l-m 50s. Latest model guidance
suggests that the lee cyclone will develop southeastward into
southwestern OK/northwest TX on Friday night, along the leading
edge of a surging arctic cold front. Rapid return of low-level
moisture will begin across our forecast area as a sharpening warm
front (arcing northeastward from the low) begins to lift northward
and away from the region, with an increasing chance for showers
and perhaps a few elevated thunderstorms between 6-12Z Saturday.

&&

.LONG TERM...
(Saturday through Tuesday)
Issued at 343 AM CST Wed Feb 12 2025

The main focus in the long term will be on Saturday, as a potent
storm system brings another round of heavy rain and the potential
for severe weather to the Tennessee Valley. A strong upper trough
will shift east through the Central Plains on Saturday, taking on
a more neutral to slightly negative tilt as it progresses east.
This will strengthen both upper and lower level wind fields and
will aid in advecting a moisture rich airmass north from the Gulf.
Recent trends over the past 24 hours have brought the sfc low
more north, putting us deeper into the warm sector and farther
displacing us from the axis of heaviest rain, now expected to be
closer to the TN/KY state line. A QLCS is expected along a cold
front and will make its way through the area overnight Saturday.

Digging into the severe threat first, strong upper level
divergence will strengthen synoptic lift Saturday afternoon and
evening at a time where temperatures are forecast to be in the
upper 60s with dewpoints in the upper 50s/lower 60s. Current model
soundings seem to be underdoing temps/dewpoints at the sfc, so
there is still a considerable amount of uncertainty regarding the
thermodynamic environment and whether or not there will be enough
support for discrete convection ahead of the main QLCS overnight.
If this were to occur, a late afternoon/evening round of
supercells could precede the QLCS and pose a higher risk for
severe weather. Even if that does not occur, wind fields are
strong enough to support bowing segments and embedded rotation
within the main line/QLCS. Damaging winds will be the primary
concern at this point, with the tornado and hail threat a bit more
contingent in how much thermodynamic support is available. Will
need to monitor how models trend regarding temps/lapse rates, as
this looks to be the main limiting factor as of now. The main
window for severe weather will come between 6pm Saturday and 6am
Sunday.

With the continued trend to the north, the threat for widespread
flooding has gone down some. We do still want to stress that
multiple rounds of heavy rain earlier on the week have helped to
saturate the ground, and it will take less rainfall for runoff to
begin than it did earlier in the week. PWATs still look to be at
the top end of sounding climatology and will support very heavy
rain rates within the QLCS as it moves through Saturday night.
High rain chances are expected through most of the day on Saturday
especially in northwest AL and southern TN. The current forecast
totals range from 3 inches over far NW AL to near 2 inches over NE
AL. Will point out that due to very low confidence and relatively
no signal from models, this does not account for any discrete
convection ahead of the main QLCS, only light to moderate rain
earlier in the day as the warm front surges north. The heaviest
rain will fall between 6pm Saturday and 6am Sunday with rain
chances tapering off Sunday morning as the front shifts to our
east. We do expect additional rises on area rivers and streams as
well as ponding water in ditches and low lying areas as the QLCS
moves through. Will keep an eye on rainfall trends and will
communicate any increase in flooding risk as necessary.

Dry weather is forecast by Sunday afternoon, and temperatures
will quickly plummet overnight. With lows dropping to the lower
20s, any water remaining on area roadways will freeze and result
in hazardous travel conditions due to black ice. Cooler
temperatures will remain through the beginning of the week, with
another shot of potentially heavy rain setting up by the mid-week
period.

&&

.AVIATION...
(00Z TAFS)
Issued at 536 PM CST Wed Feb 12 2025

Periods of -SHRA (with a few VCTS) will persist this evening and
overnight across the Tennessee Valley, resulting in IFR conditions
to localized LIFR conditions. AWWs and amendments may be needed
during this timeframe. Low ceilings will persist through late
Thursday morning/early Thursday afternoon, with ceilings lifting
to VFR levels as cloud cover gradually diminishes late in the day.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Flood Watch until 4 AM CST Thursday for ALZ008>010-016.

TN...None.
&&

NEAR TERM...70
SHORT TERM....70
LONG TERM....25
AVIATION...AMP.24