FOUS30 KWBC 130904
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
504 AM EDT Thu Aug 13 2026

Day 1
Valid 12Z Thu Aug 13 2026 - 12Z Fri Aug 14 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL CENTERED OVER
CENTRAL TO SOUTHERN INDIANA...

...Iowa to Kentucky...
A broad higher end Slight risk (25%) area extends from portions of
IA into KY on the periphery of the strong upper ridge centered
over the South. An MCS will likely be ongoing across portions of
IA at 12z this morning, moving off to the east into IL by
afternoon. Most indications are that this system will be forward
propagating by this time, which should limit the coverage and
magnitude of the flash flood threat to some extent. With that
said, high rainfall rates are expected (2-3" an hour possible),
and scattered flash flooding is likely. Another round of convection
is likely later tonight as low level moisture transport increases
into the stalled front. This setup has training/backbuilding
potential, and a more significant flash flood threat could evolve.
It is this second round of convection that could push this to a MDT
risk across portions of IA and IL...however there is a bit more
uncertainty with the exact magnitude and axis of this overnight
activity. The MCS rolling though this morning should play some role
in the boundary position and strength by tonight. Thus we will
hold just shy of a MDT risk and let day shift evaluate
observational trends and the 12z HREF.

The corridor across IN and KY that got hit hard by flash flooding
yesterday will need to be closely monitored again today and
tonight. The MCS moving across IA/IL should be approaching eastern
IN and northern KY by late this afternoon or evening. Most
indications are that the MCS will be in a weakened state by that
time as it outruns the better forcing and low level flow. However
with an axis of instability likely present, would expect some
convection to persist, and certainly a possibility the MCS
maintains intensity more than currently forecast. Hopefully any
activity that is there will be moving at a decent clip...but at
least localized flash flooding is likely given the saturated
conditions. Then attention turns to tonight into early Friday...as
the training convective axis mentioned earlier over IA/IL should
also extend into portions of IN/OH/KY. Again some uncertainty with
the exact axis and magnitude, but areas of heavy rainfall are
likely.

There is high confidence that areas of at least locally heavy
rainfall will be in the vicinity of the corridor hit with major
flash flooding yesterday. Lower confidence that high end rainfall
amounts intersect the corridor of most saturated conditions...but
there is some chance that this does indeed occur. So overall, there
is high confidence in additional lower end impacts, and low to
medium confidence in more significant considerable to locally
catastrophic impacts. Thus we went ahead and upgraded this corridor
to a Moderate risk with this update.

...High Plains...
A favorable pattern for scattered to widespread showers and
thunderstorms from CO into MT today into tonight. PWs will be
around mid August max values across this corridor, so heavy
rainfall rates are likely. One area of increased convective
coverage is expected across portions of northeast CO into
northwest KS. This is highlighted by HREF FFG exceedance
probabilities around 25% and the HREF PMM QPF getting upwards of
150-200% of FFG. Convection developing over the terrain should move
eastward into the low level inflow, likely helping produce some
cell training and mergers. Given the aforementioned PWs and over
1000 j/kg of CAPE, some flash flooding appears likely.

A second relative max in QPF is forecast over northeast MT where
850mb moisture transport and convergence is maximized. Less CAPE
this far north, and so rainfall rates are not forecast as high.
Thus not surprising that we see less in the way of FFG exceedance
in the HREF. Nonetheless, the pattern is persistent, which does
support some training of activity, and should be at least some
instability to drive higher rainfall rates. So still would expect
some flash flooding issues to develop...just maybe more localized
in nature than further south where the instability is greater.

...Southwest...
A broad Slight risk stretches from AZ into NV today. The core of
strongest forcing and highest moisture shifts a bit northwest
compared to Wednesday. Thus HREF/REFS FFG exceedance probabilities
are a bit less over AZ, but higher over portions of NV and UT. So
while isolated to scattered flash flooding is likely over the
entire Slight risk area, the greater coverage of convection should
be over NV and UT.

Chenard


Day 2
Valid 12Z Fri Aug 14 2026 - 12Z Sat Aug 15 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE FRONT RANGE
AND HIGH PLAINS OF COLORADO AND WESTERN NEBRASKA, PORTIONS OF THE
UPPER MIDWEST AND OHIO VALLEY, AND PORTIONS OF THE SOUTHWEST...

...Iowa to West Virginia...
A corridor of convection will continue to be favored near a
stalled front on the periphery of the strong upper ridge centered
over the southern U.S.. There are some larger latitudinal
differences by this time time in the models, however, the AIFS and
AIGFS have been pretty consistent with a farther south axis, and
this aligns well with the 00z 3km NAM and RRFS. Thus we leaned in
this direction with the Slight risk corridor given the good track
record of the AI guidance, and the agreement with the available
CAMs.

The strongest forcing and moisture convergence should end up
along the western end of this Slight risk area...and PWs over this
region should be even higher than what we have on day 1. While some
models have this axis farther north into WI...our current
preference keeps it across IA into IL. This may very well overlap
areas that see heavy rainfall and flash flooding on day 1. Thus we
will consider this area a higher end Slight risk, and there is the
potential for a MDT risk upgrade on future updates pending model
trends and how convection on day 1 evolves. Significant flash
flooding is certainly a possibility in this setup over portions of
IA into IL, especially if/where there is overlap from day 1
rainfall. The QPF signal is a bit less pronounced farther
southeast into IN/OH/WV...but the environment will be conducive
for heavy rainfall rates and periodic training here as well. Thus
given wet antecedent conditions stretching across this corridor the
Slight risk was expanded to the southeast.

...High Plains...
A Slight risk remains for portions of northeast CO into southwest
NE. A similar setup as Thursday, with high PWs and CAPE supporting
heavy rainfall rates. Also some model spread here...but a solid
model clustering between the AIFS/AIGFS/RRFS/Gem reg supports the
Slight risk area as having the greatest convective coverage Friday.

...Southwest...
A Slight risk was added to portions of northwest AZ, southeast NV
and southwest UT. PWs will probably be a bit lower by this time,
but still expecting sufficient CAPE, and stronger forcing given
the approaching mid level trough. The HREF members and RRFS depict
good convective coverage, and FFG exceedance probabilities similar
to day 1 (Thursday)

Chenard


Day 3
Valid 12Z Sat Aug 15 2026 - 12Z Sun Aug 16 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE FRONT RANGE
AND HIGH PLAINS OF COLORADO AND WESTERN KANSAS AND NEBRASKA, AS
WELL AS PARTS OF THE UPPER MIDWEST...

...Wisconsin to Ohio...
The favorable corridor for potential training convection shifts a
bit northeastward on Saturday into IL/WI and northern IN/OH as the
upper ridge to the south slowly shifts eastward and expands north.
Much of the same ingredients are in place from prior days...with
shortwave energy riding along the periphery of the ridge, an upper
jet to the north and strong low level moisture transport into the
northward lifting boundary. We do see a bit stronger forcing
develop by this time and an area of low pressure to the
north...which should help kick things eastward a bit more than
previous days. Nonetheless, the strong low level moisture transport
is supportive of some backbuilding near the warm front. Again we
see some latitudinal differences in the guidance...with our
preference continuing to stay on the southern edge of the model
spread. This aligns with typical biases, along with the better
performing AIFS and AIGFS, and the 00z RRFS (the only CAM that
goes this far out). So while some adjustments in the risk area are
likely, the corridor from northern IL into northern OH seems most
favored at this time.

...High Plains...
The wet pattern should persist across eastern CO into western
KS/NE. Scattered convection within a high PW and unstable airmass
will result in some cell mergers and heavy rainfall rates. Some of
these areas will likely be more hydrologically sensitive by this
time as well, with previous days rainfall increasing soil
saturation.

Chenard


Day 4 and Day 5
Valid 12Z Sun Aug 16 2026 - 12Z Tue Aug 18 2026

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL ON SUNDAY FOR
PORTIONS OF THE COLORADO FRONT RANGES AS WELL AS FOR THE MID-
ATLANTIC...

...Colorado Front Ranges and High Plains...

The pattern of the previous days will continue into Sunday across
the High Plains. A large ridge covering much of the southern tier
of states finds its northwestern periphery over this region.
Another shortwave driving northeastward along the ridge interface
will spark yet another round of storms Sunday afternoon and
evening. This will be supported by an incredibly persistent pool of
anomalous moisture stuck over the area, likely due to the daily
rounds of storms that reinforce the atmospheric moisture in place.
Further, easterly flow into the mountains from off the Plains is
continually reinforced, including into Sunday. At the moment, the
guidance is not as aggressive on the rainfall amounts across
Colorado on Sunday as compared to previous days. This means Slight
level impacts will be more reliant on favorable antecedent
conditions across this area from previous days. There remains some
uncertainty specifically with how far south the storms will form.
Any offset of storm formation on Sunday compared to prior days will
greatly mitigate the flooding threat. Thus, there remains some
uncertainty with the location of the Slight. However, given the
persistent and unmoving pattern, there is enough confidence to
continue the Slight for yet another consecutive day.

...Mid-Atlantic...

A pair of fronts emanating from a low up over Quebec on Sunday will
sweep across the Mid-Atlantic. The warm front will come first,
advecting a plume of deep tropical moisture into the Mid-Atlantic.
PWATs will increase to over 2 inches in most areas with the warm
frontal passage. The front will move across this region from the
morning through the early afternoon. Widespread shower and
thunderstorm activity will be close on its heels. A cold front
tries to push across the area, but is likely to get hung up over
northern Pennsylvania Sunday night. This will keep much of the area
in the soupy warm and muggy air mass. A secondary low forms along
the fronts Sunday night, which will invigorate another round of
showers and storms likely persisting well into Sunday night.
Southwesterly flow aloft will support the storms' backbuilding and
persisting in this pattern as more moisture is advected into the
region ahead of the cold front. Urban and small stream flooding
will be the primary threats with these storms.

The Slight Risk area covers the mountainous regions of western
Pennsylvania and West Virginia which were both hit hard by recent
heavy rains, are prone to flash flooding due to the terrain, as
well as the urban area of Pittsburgh. The eastern part of the
Slight Risk area covers the I-95 corridor region from Wilmington,
DE through New York City. Of course, urban concerns will be a large
factor in any flooding concerns, as well as the increased danger
from the likely nighttime convection affecting the area.

...Day 5/Monday...

A series of Marginal Risks were drawn from New Mexico and Colorado
for continued afternoon convection (albeit on the down trend from
prior days), a MCS affecting portions of the upper Midwest and
northern Plains, continued WNW flow of deep moisture into the
mountains of Kentucky, West Virginia, and southwest Virginia, and
finally the approach of the same cold front that brings heavy rains
on Sunday to areas of the Mid-Atlantic further north brings rounds
of storms Monday to the Tidewater of Virginia and eastern North
Carolina. The Plains and coastal features are both quite transient,
and are likely to adjust in location and intensity over the coming
days. Meanwhile the Appalachians and High Plains areas have more
uncertainty on rainfall amounts over areas that have both been
hard-hit with heavy rains.

Wegman


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt



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