FOUS30 KWBC 130825
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Tue Aug 13 2024

Day 1
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, NORTHEAST COLORADO, AND SOUTH CAROLINA...

...Portions of KS/MO/AR...
As of 07z organized convection is moving eastward across KS. The 
00z HREF guidance has a decent handle on this activity and 
indicates it should gradually take on more of a southeastward 
motion and be impacting portions of southeast KS at 12z this 
morning, moving into southwest MO through the morning hours. At 
least some flash flood risk will probably continue into the 
morning hours with some training potential persisting. Although 
the overall expected progressive nature of convection by this time
should limit the magnitude of the threat, and convection should 
see a weakening trend by afternoon as it moves into northern AR.

The greater flash flood risk is likely to evolve tonight into
Wednesday morning across portions of eastern KS into MO. The
ingredients remain favorable for a potentially significant 
training convective axis. 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 and convergence over 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 backbuilding upscale convective growth over 
this area by Tuesday night into Wednesday morning. 

Confidence in there being flash flood impacts continues to grow,
although the axis of heaviest rainfall will likely be quite
narrow...thus there remains uncertainty on where exactly this ends
up. HREF data supports a narrow axis of 3-5"+ rainfall, and given 
the setup this seems reasonable. This is likely a higher end 
Slight risk, with a narrow corridor of locally significant impacts
possible. If confidence in the location increases later today, can
not rule out the need for a focused MDT risk upgrade.

...Southeast...
Convection is again likely today across portions of GA/SC/NC. At 
18z today the model consensus is for an area of low pressure to be 
positioned near the GA/SC border. HREF guidance indicates a 
relatively narrow axis of higher instability within the easterly 
flow just north the stationary front extending from the low. Weak 
mid level shortwave energy and stronger northwesterly flow aloft 
moving over top this axis of convergence should aid in convective 
development today. Quicker deep layer flow should mean faster cell 
motions off to the southeast...however the stationary front could 
act as a focus for some training. And easterly low level flow 
countering the deeper layer westerly flow may also support some 
cell merger activity. Absent wet antecedent conditions this is 
probably more of a Marginal risk level threat. However with soil 
saturation and streamflows continuing to run well above average, 
and HREF 3" neighborhood probabilities of 40-70% and 5" 
probabilities of 15-30%, think that scattered flash flooding could 
evolve today. Thus will go ahead and upgrade to a Slight risk.

...Northeast CO..
Opted to introduce a small Slight risk area across portions of
northeast CO and adjacent areas of far southeast WY and southwest
NE. Frontal forecasts this afternoon/evening are for a stationary
front to be draped across this area potentially acting as a focus 
for convective development. Upslope easterly flow north of this 
front should aid in locally enhancing lift/convergence, and in 
many ways this setup is similar to Monday. With several flash 
flood warnings Monday, and some of these same areas expected to 
see this round of storms, think isolated to scattered flash 
flooding is again probable. Easterly flow may be a bit weaker 
today, and guidance does suggest quicker forward propagation off to
the east than Monday...so it is possible that rainfall magnitudes 
stay a bit lower. However the setup appears favorable enough for 
some slower cell motions and mergers near the front before the 
faster propagation takes hold...that think Slight risk level 
impacts are possible.

..Western U.S...
Scattered convection is again likely from portions of AZ/NM 
northward into ID/WY/MT. 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.


Day 2
Valid 12Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024

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

Training convection will likely be ongoing at 12z Wednesday across
portions of MO. See the day 1 discussion for more details on this
expected evolution...but in summary the ingredients will be there
for a significant training threat to continue into Wednesday
morning. Just like on day 1, there remains some uncertainty on the
exact axis of maximum training, but this is likely a higher end
Slight risk at the moment, with some flash flooding likely 
wherever the axis sets up. By late morning into the afternoon the 
convection should become more progressive and/or weaken as it 
shifts southeastward.

Thereafter the risk of excessive rainfall shifts northward into
portions of IA and southern MN. Convection here will likely 
develop by afternoon and continue into the overnight hours, with 
the focus expected to be near and just north of a warm front 
slowly lifting north across the region. Stronger mid/upper forcing 
will move into the region, which combined with strengthening low 
level moisture transport, should allow for organized convective 
development. With the front not moving much and the moisture 
transport axis rather persistent, expect we will see some
backbuilding/training convection near the front. Where exactly this
sets up remains a bit of a question. The 00z GFS/NAM/UKMET have 
trended northward into more of MN, however the GEM reg, AIFS and 
GFS Graphcast all still support IA into far southern MN as the 
preferred location. With models often too far north with convective
QPF, hesitant to expand the Slight risk too far north. Thus still 
think keeping it centered over IA and only into far southern MN is
the way to go for now...generally favoring the the further south 
model solutions mentioned above.

A secondary rainfall maximum may occur over portions of ND into far
northern SD along another slow moving convergence axis. The 00z
HREF data is supportive of Slight risk level impacts across this
region. However HREF skill at these longer lead times is sometimes
questionable, and global model solutions show a bit more spread on 
the convective evolution over this area. Given this uncertainty 
and the fact that this area is typically not as flash flood prone,
decided to keep the risk level at Marginal for now and continue to
monitor. 


Day 3
Valid 12Z Thu Aug 15 2024 - 12Z Fri Aug 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
ILLINOIS, INDIANA AND SOUTHERN WISCONSIN...

The heavy rainfall threat shifts east into the Great Lakes and OH
Valley by Thursday. A bit more uncertainty with the details by 
this time, but the overall ingredients remain in place for areas 
of excessive rainfall. Expect one round of convection to be moving
across the region Thursday morning, but with broad troughing 
moving through, this will likely not be the end of the convective 
risk. Instead we should be able to reload with instability and get
additional round(s) of convection into Thursday night. Exactly 
where the best risk of multiple heavy convective rounds exists is 
still uncertain, but the most likely axis appears to be across 
portions of IL/IN. This is a bit south and east of the inherited 
Slight risk, and so the risk was adjusted accordingly. There is 
still some excessive rainfall threat further north over portions of
WI and MI, but at the moment think the greater training risk is 
further south and east where the favorable ingredients should 
linger longer.

Chenard
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