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=20
00z HREF guidance has a decent handle on this activity and=20
indicates it should gradually take on more of a southeastward=20
motion and be impacting portions of southeast KS at 12z this=20
morning, moving into southwest MO through the morning hours. At=20
least some flash flood risk will probably continue into the=20
morning hours with some training potential persisting. Although=20
the overall expected progressive nature of convection by this time
should limit the magnitude of the threat, and convection should=20
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=20
training convective axis. Robust instability will be advecting in=20
from the southwest, likely resulting in a tight instability=20
gradient somewhere in the vicinity of eastern KS into western to=20
central MO. Meanwhile the low level jet will increase, resulting in
strong and persistent moisture transport and convergence over the=20
region. Corfidi vectors end up becoming quite weak, and what=20
movement they do indicate is generally parallel to the expected=20
moisture convergence axis and instability gradient. Thus we very=20
well may end up with backbuilding upscale convective growth over=20
this area by Tuesday night into Wednesday morning.=20
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=20
the setup this seems reasonable. This is likely a higher end=20
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=20
18z today the model consensus is for an area of low pressure to be=20 positioned
near the GA/SC border. HREF guidance indicates a=20
relatively narrow axis of higher instability within the easterly=20
flow just north the stationary front extending from the low. Weak=20
mid level shortwave energy and stronger northwesterly flow aloft=20
moving over top this axis of convergence should aid in convective=20 development
today. Quicker deep layer flow should mean faster cell=20
motions off to the southeast...however the stationary front could=20
act as a focus for some training. And easterly low level flow=20
countering the deeper layer westerly flow may also support some=20
cell merger activity. Absent wet antecedent conditions this is=20
probably more of a Marginal risk level threat. However with soil=20
saturation and streamflows continuing to run well above average,=20
and HREF 3" neighborhood probabilities of 40-70% and 5"=20
probabilities of 15-30%, think that scattered flash flooding could=20
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=20
for convective development. Upslope easterly flow north of this=20
front should aid in locally enhancing lift/convergence, and in=20
many ways this setup is similar to Monday. With several flash=20
flood warnings Monday, and some of these same areas expected to=20
see this round of storms, think isolated to scattered flash=20
flooding is again probable. Easterly flow may be a bit weaker=20
today, and guidance does suggest quicker forward propagation off to
the east than Monday...so it is possible that rainfall magnitudes=20
stay a bit lower. However the setup appears favorable enough for=20
some slower cell motions and mergers near the front before the=20
faster propagation takes hold...that think Slight risk level=20
impacts are possible.
..Western U.S...
Scattered convection is again likely from portions of AZ/NM=20
northward into ID/WY/MT. There will be a bit more in the way of=20
westerly flow compared to earlier days, which may allow cells to=20
move along at a faster pace. This should keep the flash flood risk=20
isolated in nature...however heavy rainfall rates will again be=20
probable in the more intense cells...which could result in a=20
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=20
wherever the axis sets up. By late morning into the afternoon the=20
convection should become more progressive and/or weaken as it=20
shifts southeastward.
Thereafter the risk of excessive rainfall shifts northward into
portions of IA and southern MN. Convection here will likely=20
develop by afternoon and continue into the overnight hours, with=20
the focus expected to be near and just north of a warm front=20
slowly lifting north across the region. Stronger mid/upper forcing=20
will move into the region, which combined with strengthening low=20
level moisture transport, should allow for organized convective=20
development. With the front not moving much and the moisture=20
transport axis rather persistent, expect we will see some=20
backbuilding/training convection near the front. Where exactly this
sets up remains a bit of a question. The 00z GFS/NAM/UKMET have=20
trended northward into more of MN, however the GEM reg, AIFS and=20
GFS Graphcast all still support IA into far southern MN as the=20
preferred location. With models often too far north with convective
QPF, hesitant to expand the Slight risk too far north. Thus still=20
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=20
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=20
questionable, and global model solutions show a bit more spread on=20
the convective evolution over this area. Given this uncertainty=20
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.=20
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=20
this time, but the overall ingredients remain in place for areas=20
of excessive rainfall. Expect one round of convection to be moving
across the region Thursday morning, but with broad troughing=20
moving through, this will likely not be the end of the convective=20
risk. Instead we should be able to reload with instability and get
additional round(s) of convection into Thursday night. Exactly=20
where the best risk of multiple heavy convective rounds exists is=20
still uncertain, but the most likely axis appears to be across=20
portions of IL/IN. This is a bit south and east of the inherited=20
Slight risk, and so the risk was adjusted accordingly. There is=20
still some excessive rainfall threat further north over portions of
WI and MI, but at the moment think the greater training risk is=20
further south and east where the favorable ingredients should=20
linger longer.
Chenard
Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!7XqyqeyUISZKXoFmRVAnutNI0dPC62ZeBccNnfSKkIgK=
YcLqT9eBJl9n4yJZQ6uVFAAA1gR_9sQ_RVve_6tUOL1bOCk$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!7XqyqeyUISZKXoFmRVAnutNI0dPC62ZeBccNnfSKkIgK=
YcLqT9eBJl9n4yJZQ6uVFAAA1gR_9sQ_RVve_6tU35qQxdQ$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!7XqyqeyUISZKXoFmRVAnutNI0dPC62ZeBccNnfSKkIgK=
YcLqT9eBJl9n4yJZQ6uVFAAA1gR_9sQ_RVve_6tUM8RGjYk$=20
=3D =3D =3D
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