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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