Areal Flood Warning 45, NWS Huntsville, AL

Area Forecast Discussion
National Weather Service Huntsville AL
643 AM CDT Sun Sep 19 2021

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 316 AM CDT Sun Sep 19 2021

A significant threat for flash flooding will exist across the
Tennessee Valley as multiple rounds of heavy rainfall are expected to
develop again today. This activity will be tied to a slow moving
trough/area of low pressure across the lower-Mississippi Valley.
Multiple shortwaves will rotate northward along the eastern edge of
this feature, with a convergence axis of sorts likely setting up
along the Interstate 65 corridor. Deep southerly flow will continue
to reinforce a tropical air mass across the Tennessee Valley, with
PWATs progged to peak around 2", well within the 95th percentile per
the BMX/OHX sounding climatologies. Thus, we expect widespread
showers and thunderstorms to redevelop rather quickly by early to mid
morning and persist throughout the day. Current radar depicts
numerous showers already developing across the area, with radar
estimated rainfall rates up to 3-4"/hr.

The environmental setup checks many of the boxes you'd want for a
higher-end flash flooding threat. Namely, deep tropical moisture in
place, with a weak convergence boundary to focus this moisture and a
low level jet nosing into central/northern Alabama to provide some
added support. The mean southerly flow will be parallel to this
boundary, with storm motion vectors around 10 kts or less. The
environment also features somewhat modest (but sufficient)
instability to sustain updrafts, with a deep warm cloud layer
favoring highly efficient rain-producing storms. Compounding the
flash flood threat will be extremely saturated soils from the recent
heavy rainfall events over the last couple of days. This has
significantly increased our sensitivity to additional heavy rainfall
-- most notably along the I-65 corridor and portions of northwest
Alabama and southern middle Tennessee. In short, this is quickly
becoming a dangerous setup for flash flooding due to the combination
of the current environmental parameters and antecedent soil
conditions which will make flash flooding much more likely.

Finally, we need to address how much rain is possible in a setup like
this. Hi-res guidance has had a good handle on the heavy rainfall
potential for the last couple of days and are in decent agreement
regarding the timing of convective development today (09z to 21z) and
where the greatest swath of rainfall will occur (along and near the
convergence boundary). The environment will easily support hourly
rainfall totals of 1-2", with 6 hours totals exceeding 3" in some
areas. Our official QPF forecast for the entire area today is 1-3" of
rain, but it needs to be stressed that in locations where training
occurs, rainfall totals in the 3-5" range will be more common, with
localized higher amounts. Our biggest areas of concern will be areas
along the I-65 corridor that saw some significant rainfall yesterday,
but in this setup, a significant flash flooding threat can
materialize very quickly anywhere in our area. Please make sure you
have multiple ways to get warnings today and use extra caution if
you're out on the roads!

.SHORT TERM...(Tonight through Wednesday)
Issued at 316 AM CDT Sun Sep 19 2021

The main change to the forecast during this window was an extension
of the Flash Flood Watch through 7 PM Monday. Otherwise, the forecast
remains on track for widespread showers and thunderstorms and a
significant threat for flash flooding persisting into Monday and
potentially Tuesday. For tonight, we expect a gradual diminishing of
convection by the late evening hours as the initial shortwave trough
lifts to the north. However, by early Monday morning and into the
day, we'll see convection redevelop quickly as the center of the low
begins to pivot to the northeast. Expect yet another shortwave to
traverse the area and subsequently spark widespread showers and
storms. Once again, the combination of the storm environment and the
antecedent saturated soils will create a potentially dangerous setup
for additional flash flooding as another 1 to 3 inches of rainfall
will be possible.

Our one saving grace may be another lull in the action Monday night
into Tuesday morning as the initial area of low pressure shifts
northeast into the Ohio Valley. However, our attention will quickly
turn to an amplifying longwave trough over the Mid South that will
push a cold front into the area Tuesday afternoon/evening into
Tuesday night. This feature will generate one final round of showers
and thunderstorms that will likely linger into the morning hours on
Wednesday. Finally, a cooler and drier air mass will work its way
into the region by Wednesday afternoon, bringing an end to our
precipitation chances and the heavy rainfall/flooding threat.
However, it should be noted that the multiple days of heavy rainfall
will also create rises on area rivers, creeks, and streams -- and it
seems plausible that river flooding issues may persist well into
next week even after all the rain has tapered off.

.LONG TERM...(Wednesday night through Saturday)
Issued at 316 AM CDT Sun Sep 19 2021

Behind the cold front on Wednesday night, Autumn will be in full
swing. A much-welcomed cold and dry air mass will filter in from the
northwest and will drop our temperatures to lows in the upper 40s to
low 50s. Thursday, persistent cold air advection will limit our
warming potential, despite abundant sunshine. High temperatures will
be in the low 70s, but some areas may struggle to break out of the
60s. The shortwave trough will remain in place over the Midwest
through late this week and finally lift into the Northeast on Friday
and Saturday. Gradually increasing thicknesses aloft will allow high
temperatures to warm into the mid 70s by the weekend. All around a
pleasant end to this forecast period as the TN Valley dries out after
our super soaking earlier in the week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 643 AM CDT Sun Sep 19 2021

A very pessimistic forecast as MVFR to IFR conditions will prevail
through the TAF period as widespread showers and storms will impact
both terminals. SHRA will increase in coverage throughout the
morning, with TSRA becoming more predominant by the afternoon. Expect
heavy rainfall that may lead to ponding of water on runways. AWWs
may be needed for lightning and gusty winds in any thunderstorms.
Coverage of showers will decrease late tonight, but think low
stratus and light fog/mist will continue through the remainder of the
period.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Flash Flood Watch through Monday evening for ALZ001>010-016.

TN...Flash Flood Watch through Monday evening for TNZ076-096-097.

&&

NEAR TERM...AMP.24
SHORT TERM...AMP.24
LONG TERM...30
AVIATION...AMP.24

For more information please visit our website
at weather.gov/huntsville.

Hazardous Weather Outlook
National Weather Service Huntsville AL
408 AM CDT Sun Sep 19 2021

ALZ001>010-016-TNZ076-096-097-200015-
Lauderdale-Colbert-Franklin AL-Lawrence-Limestone-Madison-Morgan-
Marshall-Jackson-DeKalb-Cullman-Moore-Lincoln-Franklin TN-
408 AM CDT Sun Sep 19 2021

This Hazardous Weather Outlook is for north Alabama and portions of
southern middle Tennessee.

.DAY ONE...Today and Tonight...

A significant threat for flash flooding is expected to develop across
the Tennessee Valley today as widespread showers and thunderstorms
will produce multiple rounds of heavy rainfall across the region. A
Flash Flood Watch is in effect until 7 PM Monday.

.DAYS TWO THROUGH SEVEN...Monday through Saturday...

Periods of heavy showers and storms are expected to develop across
the region Monday and Tuesday and a threat for flash flooding will
persist into early this week. Additionally, rises on rivers, creeks,
and streams are expected and minor flooding will be possible. This
activity will become more scattered in nature on Wednesday and taper
off by Wednesday evening as a cold front sweeps through the area.

.SPOTTER CALL TO ACTION STATEMENT...

Activation of storm spotters and emergency management personnel is
not anticipated at this time.

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