Flash Flooding and Flood in Tennessee, September 20, 2021

Area Forecast Discussion
National Weather Service Nashville TN
605 AM CDT Mon Sep 20 2021

.UPDATE...
FOR 12Z AVIATION DISCUSSION.

&&

.DISCUSSION...

Radar this morning shows numerous showers developing once again
across the area, especially over our southern and eastern
counties. This activity is occurring in deep southerly flow
throughout the troposphere out ahead of an upper trough axis
along the Mississippi River. As the trough moves across the area
today, showers and possibly a few thunderstorms should become
fairly widespread this morning then gradually diminish from west
to east during the afternoon. The inverted surface trough that has
provided a focus for the heavy rain over the past several days
has thankfully lifted out of the area and dissipated. However, our
area remains in convergent flow between the upper trough to the
west and high pressure off to our east, and the atmosphere is
still very moist with PWATs around the 2 inch mark. Forecast
soundings also show a bit more MLCAPE today versus the past few
days, which could allow for higher rainfall rates in the stronger
showers and storms. Thus some additional heavy rainfall appears
possible today and will keep the Flash Flood Watch going until
expiration at 21/00Z. Latest WPC guidance and model QPF has
trended downward for today, but still shows most of Middle
Tennessee receiving another 1/2 inch to 2 inches of rain, which
could cause flooding of roads and low lying areas, as well as
rises on some area rivers.

Our forecast area will get a welcome break in the rainfall
tonight into Tuesday morning as the upper trough axis lifts off
to the east. NBM pops appear way too high with the lack of precip
expected and have undercut them significantly during this
timeframe. Strong cold front will arrive in our northwest
counties Tuesday afternoon, but will not make it fully through
our cwa until Wednesday morning due to the parent upper trough
axis taking on a negative tilt and cutting off over the Ohio
Valley - which is unusually early in the year for such a setup to
occur. With ample moisture remaining ahead of the front, another
round of numerous showers and storms are expected both along and
behind the front. WPC and 00Z models show potential for another 1
to 2 inches of rain from Tuesday afternoon through Wednesday
morning, so some additional flooding issues are not out of the
question especially since we are so saturated after the recent
rains. In addition, forecast soundings show MLCAPE increasing up
to 1000 J/Kg ahead of the front with significant dry air aloft, so
a few strong storms with gusty winds are also possible.

Precip should come to an end across our western half on Wednesday
morning and the east by late Wednesday afternoon or early
evening, with strong CAA bringing a lengthy period of dry and
much cooler weather for the rest of the week into early next
week. Once again, models have trended cooler with the post
frontal airmass, and high temperatures may not get out of the 60s
for most locations on Wednesday, which will likely be a chilly
shock to the system after our warm and humid month so far. In
fact, lows on Thursday and Friday mornings are expected to drop
into the 40s for nearly everyone outside of the Nashville urban
heat island, with highs only reaching the mid 60s to mid 70s
those two days. These temperatures are around 5 to 10 degrees
below normal for this time of year. Temps will slowly rebound
over the weekend closer to normal levels, but dewpoints will
remain low in the 40s and 50s with skies staying mostly sunny, so
it will definitely feel like Fall has arrived outside.

&&

.AVIATION...
12Z TAF DISCUSSION.

Southerly flow will bring abundant Gulf moisture.  Wet weather
continues with more showers and thunderstorms again today.
Conditions become MVFR later this morning and stay that way
through just about 00Z when there will be a break in the rain and
conditions become VFR. As the inverted troff and low approach
winds will strengthen and at times gusts 10-15 kts.

&&

.OHX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this evening for Bedford-Cannon-
Cheatham-Clay-Coffee-Cumberland-Davidson-De Kalb-Dickson-
Fentress-Giles-Grundy-Hickman-Houston-Humphreys-Jackson-Lawrence-
Lewis-Macon-Marshall-Maury-Montgomery-Overton-Perry-Pickett-
Putnam-Robertson-Rutherford-Smith-Stewart-Sumner-Trousdale-Van
Buren-Warren-Wayne-White-Williamson-Wilson.

&&

DISCUSSION......Shamburger
AVIATION........12

Hazardous Weather Outlook
National Weather Service Nashville TN
238 AM CDT Mon Sep 20 2021

TNZ005>011-023>034-056>066-075-077>080-093>095-211000-
Stewart-Montgomery-Robertson-Sumner-Macon-Clay-Pickett-Houston-
Humphreys-Dickson-Cheatham-Davidson-Wilson-Trousdale-Smith-Jackson-
Putnam-Overton-Fentress-Perry-Hickman-Lewis-Williamson-Maury-
Marshall-Rutherford-Cannon-De Kalb-White-Cumberland-Bedford-Coffee-
Warren-Grundy-Van Buren-Wayne-Lawrence-Giles-
238 AM CDT Mon Sep 20 2021

This Hazardous Weather Outlook is for Middle Tennessee.

.DAY ONE...Today and tonight.

A Flash Flood Watch continues for all of Middle Tennessee through
this evening. Numerous additional showers and a few thunderstorms
with moderate to locally heavy rainfall are expected again today.
Additional rainfall up to 1 to 2 inches is possible today, which may
lead to flash flooding of roads and low lying areas. Rain will
briefly end across the area tonight.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

A strong cold front will move slowly across the area Tuesday
afternoon through Wednesday morning, bringing another round of
showers and thunderstorms. Additional rainfall amounts around 1 to
2 inches are possible, which could cause some localized flooding of
roads and low lying areas. In addition, a couple of strong storms
are possible with gusty winds.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed today due to the risk of flash
flooding. Please relay any information about observed severe weather
to the NWS while following all local, state, and CDC guidelines.

Shamburger