As we receive the remnants of post-tropical Storm BETA, we are expecting rainfall anywhere from 2-4″ on Thursday and Friday. A Flash Flood Watch is currently in effect for northern Alabama/southern-middle Tennessee to account for this.

URGENT - IMMEDIATE BROADCAST REQUESTED
Flood Watch
National Weather Service Huntsville AL
239 PM CDT Wed Sep 23 2020

ALZ001>009-016-TNZ076-096-097-240345-
/O.NEW.KHUN.FF.A.0011.200924T0900Z-200925T0000Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Lauderdale-Colbert-Franklin AL-Lawrence-Limestone-Madison-Morgan-
Marshall-Jackson-Cullman-Moore-Lincoln-Franklin TN-
Including the cities of Russellville, Winchester, Red Bay, Cullman,
Town Creek, Cowan, Estill Springs, Huntsville, Sheffield, Arab,
Sewanee, Decatur, Lynchburg, Athens, Muscle Shoals, Decherd,
Fayetteville, Albertville, Guntersville, Boaz, Tuscumbia, Moulton,
Florence, and Scottsboro
239 PM CDT Wed Sep 23 2020

...FLASH FLOOD WATCH IN EFFECT FROM LATE TONIGHT THROUGH THURSDAY
EVENING...

The National Weather Service in Huntsville has issued a

* Flash Flood Watch for portions of Alabama and southern middle
  Tennessee, including the following areas, in Alabama, Colbert,
  Cullman, Franklin AL, Jackson, Lauderdale, Lawrence, Limestone,
  Madison, Marshall and Morgan. In southern middle Tennessee,
  Franklin TN, Lincoln and Moore.

* From late tonight through Thursday evening

* Periods of moderate to heavy rainfall associated with the remnants
  of Tropical Storm Beta will impact the Tennessee Valley on
  Thursday. Efficient tropical rainfall rates will likely overpower
  absorption rates into the ground, leading to flash flooding across
  the area.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A Flash Flood Watch means that conditions may develop that lead to
Flash Flooding. Flash Flooding is a very dangerous situation. You
should monitor later forecasts and be prepared to take action should
Flash Flood Warnings be issued.

WPC Day 2 Outlook
WPC Day 1 Outlook

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
903 PM EDT Wed Sep 23 2020
 
Day 1
Valid 01Z Thu Sep 24 2020 - 12Z Thu Sep 24 2020 

...A MODERATE RISK OF EXCESSIVE RAINFALL IS IN PLACE FOR PORTIONS 
OF THE LOWER MISSISSIPPI VALLEY...

...Lower MS Valley and portions of the Mid-South...
Post-T.C. Beta this evening is moving steadily off to the 
northeast across central LA and will soon be advancing into 
southwest MS as thee system gradually continues to accelerate 
ahead of a mid to upper-level trough over the mid-MS Valley. A 
fairly large shield of moderate to heavy rain continues across the 
lower MS Valley, and is seen streaking northeast toward the 
mid-South. Generally a large portion of this rainfall is 
stratiform in nature owing to the strong isentropic ascent over 
top the relatively cool/stable boundary layer that is in place to 
the north and northeast of Beta's circulation. However, closer 
into the low center from areas of southwest to central MS, there 
is a substantial area of very heavy rainfall associated with 
strong convection that is being aided by enhanced mid-level 
ascent/dynamics and which is working in tandem with focused deep 
layer moisture convergence, and a nose of moderate instability 
that is lifting up over a warm front extending east from Beta's 
low center. MLCAPE values ahead of Beta are as high as 1000 to 
1500 j/kg aiming in across much of southeast LA and toward far 
southwest MS, and with a south-southeast low-level jet of 30 to 40 
kts. Areas of the lower MS Valley involving especially southwest 
to central MS and perhaps to a lesser degree downstream across 
areas of northeast MS and northwest AL are expected to see the 
focus of the heaviest rainfall overnight as Beta continues to 
advance northeast and drives the ongoing heavy rainfall swath off 
to the northeast with it. Generally, the thermodynamic environment 
will tend to wane just a tad given the loss of daytime heating, 
but the presence of very favorable jet-aided dynamics aloft and 
focused low-level moisture transport/convergence near and ahead of 
the low center (generally along and just north of the 
aforementioned warm front) is expected to maintain a focused area 
of heavy showers and thunderstorms. PWs across the region are on 
the order of 2 to 2.25 inches, and with strong deep layer forcing, 
the rainfall rates within the stronger convective cores are likely 
to exceed 2 inches/hr. The latest HRRR/HRRRexp guidance and 18Z 
HREF suite of guidance still supports locally an additional 3 to 5 
inches with isolated storm total amounts of 5 to 7 inches going 
through 12Z/Thursday. However, they still collectively appear to 
be a tad too far north with their axis of heaviest rainfall, and 
so some modest southward adjustments were made to the Slight and 
Moderate Risk areas were made to account for the latest radar 
trends. Numerous instances of flash flooding are expected, and 
potentially some locally significant flash flooding within the 
Moderate Risk area. Adjacent areas of the mid-South involving 
western and central TN should see notably lower rainfall totals 
with a relatively diminished threat of flash flooding since the 
rainfall here should tend to be more of a longer duration moderate 
to locally heavy warm-advection rainfall evolution with minimal 
(if any) convection expected. Farther back to the west over the 
remainder of the lower MS Valley, the excessive rainfall/flash 
flooding concerns will gradually be diminishing from southwest to 
northeast, as the back edge of the rainfall shield moves to the 
northeast, and the latest ERO update reflects this with 
adjustments made based on the latest radar trends. 

...Upper Mississippi Valley...
Ahead of a shortwave trough moving currently through the Dakotas, 
showers and thunderstorms are expected to develop shortly after 
00Z across north-central Minnesota just north of a surface front 
oriented from WSW to ENE. Precipitable water values of 1.0 to 1.3 
inches in the pre-convective environment and MUCAPE values in the 
1000 to 2000 J/kg range are forecast across south-central to 
north-central Minnesota. As southwesterly 850 mb winds of 30+ kt, 
ahead of a surface trough extending into the Central Plains, 
ascend over the front, mean storm motions are expected to parallel 
the frontal boundary supporting training. There is a relatively 
brief window between 03Z and 09Z when flash flood potential will 
be greatest, before instability weakens to the north and forward 
propagation increases toward the south, ahead of the surface low 
and in the wake of the surface trough axis. Rainfall rates locally 
as high as 1 to 1.5 in/hr are expected which may cause some 
isolated flash flooding or runoff concerns. A Marginal Risk area 
remains highlighted across portions of north-central MN and 
extending east across far northern WI and the far western portion 
of the U.P. of MI. Some modest adjustments were made to the area 
based on the 18Z HREF suite of guidance.

...Pacific Northwest...
A rather strong atmospheric river for this time of the year is 
impacting the Pacific Northwest with IVT values locally in excess 
of 750 kg/m/s just ahead of a cold front which is currently 
analyzed near the coastal ranges of western WA. Southwesterly 850 
mb winds of 40 to 50 kts continue to focus strong moisture 
transport, and there is also some weak instability just ahead of 
the front and a few convective elements moving onshore across 
western WA. Expect locally an additional 2 to 3 inches of rain 
over especially the higher terrain of the Olympics and the WA 
Cascades overnight, with some hourly rainfall rates in the 0.5 to 
0.75 inch/hr range possible. Lesser amounts of as much as 1 to 2 
inches with isolated heavier amounts are expected farther south 
over areas of western OR as the cold front moves inland. There 
will be some concern for some of the burn scar areas of especially 
northwest OR and southern WA to see some enhanced runoff problems 
from these locally heavier rainfall rates that get up into the 
0.50+ inch/hr range, and as such the Marginal Risk area is 
maintained at this time.

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
356 PM EDT Wed Sep 23 2020
 
Day 2
Valid 12Z Thu Sep 24 2020 - 12Z Fri Sep 25 2020 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE 
TENNESSEE VALLEY INTO THE SOUTHEAST...

...Lower Mississippi Valley/Tennessee Valley/Southeast...
...20Z Update...
Modified the Marginal and Slight Risk areas based on the latest 
model trends/WPC QPF and after coordination with the local 
offices.  Models are coming into better agreement with respect to 
the axis of heaviest rain as the ingredients look to overlap just 
along and north of the surface low track/instability gradient.  
And when comparing the previous forecast amounts and the overall 
footprint, did increase values and nudged the higher rainfall 
totals northward at this update.  The swath of heaviest rain 
stretches from northeast MS into northern AL and much of southeast 
TN with on average 2-4+ inches, with locally higher amounts 
possible. In addition to these changes, the guidance is spreading 
the heavy rain quickly to the northeast across portions of the 
southern/central Appalachians with some trough interaction.  Given 
that much of the Deep South/TN Valley has received little to no 
precipitation over the past week, decided it was best to retain 
the Slight Risk category for flash flooding.  However, if QPF 
values come up substantially, then a more focused Moderate Risk 
area may be necessary.  

...Previous Discussion...
As remnants of Beta get picked up by a positively tilted long wave 
trough extending from the OH Valley into the Lower MS Valley 
during Day 2, deep moisture and instability are transported across 
the TN Valley and Southeast states. There is a fair amount of 
spread concerning just how far north the remnants are drawn 
northward, and how much instability follows, particularly into the 
TN Valley. The 00z NAM/ECMWF look too far north with the weakening 
surface system, apparently allowing too much instability to reach 
northern MS/AL. The 00z GFS looks too fast with the system, as it 
lifts out the surface low into the eastern TN Valley. The 00z 
UKMET looks like a fair proxy for the expected rainfall 
distribution, so it was used as a template for the WPC QPF and 
Excessive Rainfall Outlook. 

With the mid level trough lifting to the northeast from the OH 
Valley into the Lower MS Valley, the remnants of Beta gets lifted 
northeast. However, as mentioned earlier, there is quite a bit of 
spread with respect to how quickly this occurs, and how far north 
the surface system gets. Since the model guidance has been too 
quick to lift Beta with the trough over the past couple of days, 
it seems as though the track of the system will be further south 
than much of the guidance. With a track further south with the 
surface system, the northern edge of the instability gradient is 
expected to lie from central MS into central and possibly northern 
AL during the afternoon and evening hours (when instability 
peaks). During this time, a 50 knot jet streak passing just to the 
north across the OH Valley provides synoptic scale ascent for 
banded rainfall to extend from central and northern MS across 
northern AL into far south central TN. 

Much of the high resolution guidance showed a band of 4.00/6.00 
inches of rainfall along the northern edge of the instability 
gradient. While placement looks reasonable, the amounts seem too 
high, as these models appears to be bringing too much instability 
into northern MS/northern AL. The moisture plume with Beta is 
transported along the isentrope toward an elevated boundary over 
this area, so heavy rainfall is expected, but lacking deeper 
instability, the high resolution values look too high. Hourly 
rainfall rates could top out near 1.50 inches, with total rainfall 
amounts of 3.00/4.00 inches possible in spots. This area was 
spared the highest rainfall associated with Sally, and three hour 
flash flood guidance values are generally above 2.50 inches. 

Further south, instability is better across southern MS/southern 
AL into the western FL Panhandle, with MUCAPE values between 
1000/2000 J/KG. Inflow of 2.00/2.25 inch precipitable water air 
within the instability axis could support bands of convection 
tracking north into these areas from the northern Gulf of Mexico. 
Banding should be fairly progressive as the mid level flow becomes 
more westerly at the base of the mid level trough to the north. 
Still, hourly rainfall rates here could peaking near 2.00 inches, 
with local 3.00+ inch rainfall amounts possible, despite 
relatively modest model QPF amounts.