FOUS30 KWBC 120833
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Mon Aug 12 2024

Day 1
Valid 12Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN COLORADO AND WESTERN KANSAS, EASTERN OKLAHOMA AND FAR
WESTERN ARKANSAS, AND THE COASTAL CAROLINAS...

...Northeast OK and Western AR...
Training convection is expected to be ongoing across this corridor
at 12z Monday. As of 07z the coverage and intensity of convection 
over northeast OK is generally greater than most of the 00z high 
res guidance. With the low level jet only intensifying, expect this
trend to continue, and thus would expect the convective training 
threat into Monday morning to be more significant than the 00z high
res guidance would suggest. At least scattered flash flooding is 
anticipated across portions of eastern OK, possibly pushing into 
portions of far western AR as well. This convection should be on a
downward trend by late morning into the early afternoon hours.

...Eastern CO into KS...
Convection is expected to develop over portions of eastern CO by
later this afternoon. Models indicate that a weak shortwave will
eject eastward originating from the ongoing convective activity 
over UT...with an upper jet to the north supporting at least weak 
divergence aloft in its right entrance region. Meanwhile, easterly 
flow in the low levels will advect in higher moisture, with PWs 
forecast to increase towards 1.5", above the climatological 90th 
percentile for mid August. Given the forcing and moisture in 
place, the forecast ~2000 j/kg will allow for robust convective 
development. With deep layer mean flow from the west, and low 
level flow from the east, cells may initially end up slow moving, 
with cell mergers a possibility. Given the environment in place, 
tend to think some of more aggressive high res models may end up 
closer to reality for this event. This would support localized 
rainfall upwards of 3-5" over portions of eastern CO and far 
western KS. HREF QPF, environmental ingredients in place, and the 
CSU machine learning ERO are all supportive of an isolated to 
scattered flash flood threat. Thus we will go ahead and upgrade 
this region to a Slight risk.

This activity is likely to grow upscale overnight and then move 
eastward across KS. Convection will likely be quicker moving by 
this time, although can not rule out a west to east training axis 
setting up. If this were to occur then Slight risk level impacts 
may continue overnight across KS. However given some lingering 
uncertainty on these convective details, and the fact that soil 
saturation and streamflow conditions across KS are running below
average...will keep the risk level at Marginal for now.

...Carolinas...
Upgraded portions of the eastern Carolinas to a Slight risk for
today. We should see another round of convection near the stalled
front this afternoon, with HREF 3" neighborhood probabilities in 
the 40-70% range. Totals of this magnitude will likely stay 
isolated to scattered in nature...however given the increased 
sensitivity over the area after Debby and post Debby 
rainfall...expect another day of isolated to scattered flash 
flooding. Today is a bit trickier of a forecast with a weak low 
along the front. While the higher probabilities of excessive 
rainfall are along the coast this afternoon, we could see at least
a localized flash flood risk a bit further inland this morning 
into the early afternoon as well.

...Southwest...
A broad Marginal risk remains across much of the Southwest, with
isolated flash flooding possible over this large geographic region.
The coverage/organization of intense rainfall may be a bit less
than what we saw on Sunday, but nonetheless an isolated risk 
likely continues. Do see some more elevated probabilities over 
southern AZ, so may end up with a bit better concentration of 
convection there, but still seems to fall shy of Slight risk 
levels. Portions of central/northern UT also stand out as 
potentially seeing greater convective coverage...but this signal is
generally north of the more sensitive basins in UT...and thus not 
thinking a Slight risk is needed at this time.

Chenard


Day 2
Valid 12Z Tue Aug 13 2024 - 12Z Wed Aug 14 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN KANSAS INTO MISSOURI AND NORTHERN ARKANSAS...

...Portions of KS/MO/AR...
Likely to have some ongoing convection at 12z Tuesday across
portions of southeast KS and southwest MO. This will be what
remains of the Monday night organized convection that is expected 
to move across KS. Most indications are that this activity should 
be on a weakening trend after 12z, so while an isolated flash flood
threat may persist, not currently anticipating a widespread 
threat. The greater threat is the likelihood of training 
convection Tuesday night across this corridor.

This appears to be evolving into a potentially significant training
convective setup. Robust instability will be advecting in from the
southwest, likely resulting in a tight instability gradient 
somewhere in the vicinity of eastern KS into western to central MO.
Meanwhile the low level jet will increase, resulting in strong and
persistent moisture transport into the region. Corfidi vectors end
up becoming quite weak, and what movement they do indicate is 
generally parallel to the expected moisture convergence axis and 
instability gradient. Thus we very well may end up with repetitive
backbuilding upscale convective growth over this area Tuesday 
night. There remains some uncertainty on the exact details of 
convective evolution and placement of highest totals...but 
confidence is increasing in an excessive rainfall threat, with 
potentially significant localized impacts. The coverage of excessive
rainfall will likely be quite a bit narrower than the Slight risk
area...however it remains broader to account for uncertainty in the
maximum rainfall axis.

...Southwest...
Scattered convection is again likely from portions of AZ/NM
northward into ID/WY. There will be a bit more in the way of
westerly flow compared to earlier days, which may allow cells to 
move along at a faster pace. This should keep the flash flood risk
isolated in nature...however heavy rainfall rates will again be 
probable in the more intense cells...which could result in a 
localized flash flood within the broad Marginal risk area.

...Southeast...
Convection is again likely Tuesday across portions of GA/SC/NC.
Overall it is an interesting setup, with model guidance suggesting
a more well defined low over SC by this time, with stronger 
forcing and flow aloft moving in as well. This should seemingly 
allow for some strong convective development...with the stronger 
flow aloft supporting quicker cell motions off to the southeast. 
These cell motions would typically suggest a lowered flash flood
risk...however we will need to keep an eye on how the convergence 
axis east of the low plays out. This boundary could act as a focus
for some training convection, which would offset the quicker cell 
motions and still produce a flash flood risk. Overall think enough 
uncertainty on these details remains to keep the risk level at 
Marginal for now. However will continue to monitor trends and 
would not rule out an eventual Slight risk upgrade over portions of
the area.

Chenard


Day 3
Valid 12Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDDLE MISSISSIPPI VALLEY...

The threat of excessive rainfall shifts into Plains and MS Valley
on Wednesday. Likely to have some ongoing risk to start the day
across portions of central MO...with the threat likely shifting to
the north by late in the day into the overnight hours. It again 
looks like a favorable environment for training/backbuilding 
convection with an increasing low level jet into a warm front. 
Strong and persistent moisture transport into this boundary is 
typically supportive of backbuilding convection and a flash flood 
risk. 

There is some model QPF spread, with convection and locally heavy
rainfall likely all the way from the Dakotas and MN into MO.
However it looks like the best ingredients for excessive rainfall
are over MO to start the day, and then across portions of IA and 
western IL later in the day/overnight. This is where the better 
instability and most persistent moisture transport is currently 
forecast. And while the location of greatest QPF differs from model
to model and run to run...this axis is favored by the majority of 
solutions. Thus will place the Slight risk here for now, with the
understanding that some adjustments are likely as the event nears.
So while confidence on the exact location is only average...do 
think we are getting towards above average confidence in the 
impacts from this event...with at least isolated to scattered flash
flooding expected, some of which could be locally significant.


Chenard
--- SBBSecho 3.20-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)