FOUS30 KWBC 140830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Fri Aug 14 2026

Day 1  Valid 12Z Fri Aug 14 2026 - 12Z Sat Aug 15 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE MIDWEST...

...Iowa to the Appalachians...
A flash flood threat will continue today into tonight on the
periphery of the strong upper ridge to the south, and near and
north of a surface front. Convection will be ongoing at 12z this
morning from NE to IN and spreading into WV. While the activity
over IL/IN should generally weaken as it tracks southeast, a
continuous localized flash flood risk should persist along this
instability/moisture axis.

The primary, higher end event is expected to materialize this
afternoon into the overnight hours. A strong shortwave/MCV
advancing out of the Plains is forecast to enter IA by 00z,
accompanied by a sharp increase in 850mb moisture transport and
convergence. This uptick in forcing will interact with a very moist
airmass characterized by PWs approaching 2.4". While convection
may eventually consolidate into a forward propagating MCS,
backbuilding and cell training are still likely, especially along
the south-southwestern flank. HREF FFG exceedance probabilities
exceed 50%, with the HREF PMM QPF exceeding 200% of FFG and
depicting an FFG exceedance spatial coverage over 25%. These high
signal metrics support an organized flash flood event this
afternoon into tonight, with scattered to numerous instances of
flash flooding, some of which could be locally significant.

Some latitudinal uncertainty regarding the primary heavy rainfall
axis remains among the model suite. In fact there is a split in
recent guidance, with some trending north and others south. Based
on current observational trends and the overall model consensus,
think an axis across central/southern IA and into west central IL
is most likely, possibly extending into far northern MO. The
inherited Moderate risk area was in decent shape, but we did expand
it a bit southeast. This generally aligns with the 00z/06z
HRRR/RRFS and 00z ARW/ARW consensus, which closely matches the
typically reliable AIFS and AIGFS track (leaving a few northern
models as outliers). Additionally, the Slight risk was expanded
southward into the southern Appalachians to account for an uptick
in HREF FFG exceedance probabilities and coverage.

...High Plains...
A Slight risk is maintained from northeast CO into southwest NE.
Convective initiation this afternoon should focus along the
periphery of the upper level ridge centered to the southeast. In a
setup similar to yesterday, thunderstorms should initiate over the
higher terrain before tracking eastward into the instability pool
and low level inflow. While cell motions should be largely
progressive, some cell mergers are again expected to produce
isolated to scattered instances of flash flooding. HREF and REFS
FFG exceedance probabilities remain comparable to yesterday.

...Southwest...
A Slight risk remains across portions of northwest AZ, southeast NV
and southwest UT. While PWs are slightly lower than over the past
few days, a robust pool of 1000-2000 J/kg of CAPE should compensate
for this. Furthermore, large scale forcing will increase ahead of
an eastward moving mid level trough. Slow moving storms and cell
mergers will drive a scattered flash flood threat, supported by
HREF/REFS FFG exceedance probabilities that are forecast similar
to or slightly higher than yesterday across this corridor.

Chenard
 

Day 2  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 AND OHIO VALLEY...

...Midwest and Ohio Valley...
A Slight risk extends from IL eastward into WV. A favorable
environment for convective training will persist along the northern
periphery of the broad upper ridge and near a lingering surface
frontal zone. At 12z Sat, an MCS will likely be ongoing, with
additional afternoon and overnight redevelopment expected along
this corridor. PW values around 2.2" and CAPE exceeding 2000 J/kg
will readily support intense rainfall rates.

Latitudinal guidance spread remains, but the Slight risk was
adjusted southward with this update to better align with the
consistent and typically skillful AIFS and AIGFS solutions. This
southward shift is also supported by the 00z RRFS/REFS, as well as
recent southward trends in the GFS. This axis may overlap areas
heavily impacted by flash flooding on prior days, where saturated
soil conditions will heighten hydrologic sensitivity. Given
lingering uncertainties regarding exact axis positioning and
convective mode, we will not upgrade to a Moderate risk at this
time...but this will remain a possibility in future updates and
this is considered a higher end Slight risk.

...High Plains...
A Slight risk is maintained from eastern CO into western KS and
southwest NE. The overall synoptic setup will resemble previous
days, with shortwave forcing riding the periphery of the ridge and
interacting with a moist, unstable airmass. While model spread
remains regarding the exact location of the heaviest QPF footprint,
good spatial overlap exists among the 00z RRFS, Gem Reg and AIGFS.
The Slight risk was shifted a bit south to reflect this consensus.

Chenard
 

Day 3  Valid 12Z Sun Aug 16 2026 - 12Z Mon Aug 17 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID-ATLANTIC AND NORTHEAST NEW MEXICO INTO SOUTHEAST COLORADO...

...Mid-Atlantic...
A Slight risk is delineated from WV eastward into PA, MD, NJ, DE
and into southeast NY. A mid level shortwave/MCV will track into
the region alongside a synoptic cold front. Strong moisture
advection will increase PWs towards 2.3", which combined with
strong low level moisture transport will create an environment
primed for heavy rainfall rates.

Stronger synoptic scale forcing will broaden the overall
convective coverage, though it may also increase the forward speed
of convection. Should convection move through rapidly, the flash
flood threat could remain somewhat limited. However, given the
very high PW values and robust moisture transport, any mesoscale
training or backbuilding will quickly result in some flash
flooding, particularly over sensitive urban corridors.
Additionally, strong westerly low level moisture transport
impinging on the central Appalachians could promote terrain
enhanced training and localized flash flooding. The Slight risk
was expanded southward to capture the convective axis depicted by
the AIGFS and AIFS, which have consistently been on the southern
end of the guidance spectrum.

...Southwest and High Plains...
The Slight risk over this region has been shifted southward
relative to previous forecasts, now covering northeast NM into
southeast CO. A southward advancing cold front will provide focus,
enhancing low level convergence and easterly upslope flow. The 00z
AIGFS and RRFS focus organized convection within this corridor.
While PW values are not anomalously extreme, they remain above
average...which when combined with strong CAPE values, slow storm
motions and cell mergers...will support an isolated to scattered
flash flood threat.

Chenard
 

Day 4 and Day 5  Valid 12Z Mon Aug 17 2026 - 12Z Wed Aug 19 2026

...THERE IS A SLIGHT RISKS OF EXCESSIVE RAINFALL ON TUESDAY FOR
AREAS OF THE UPPER MIDWEST...

...Day 4/Monday...

Overall synoptic scale pattern across the CONUS will be in a
relatively unchanged state when it comes to the primary upper
features over North America by Monday. Some shifting in the
general ridge across the Southern U.S. will lead to some deviations
in the typical shortwave propagation as we've seen for several days
prior. That said, the main theme of the setup for the beginning of
next week will continue to be prevailing westerly flow aloft
leading to instances of convection initiating and impacting areas
om the northern periphery of the ridge leading to
isolated/scattered bouts of flash flooding in their wake. The most
notable areas of interest will reside over the North-Central U.S.
as a shortwave out of the Prairies of Canada is forecast to
migrate southeastward into the Northern Plains Plains and Upper
Mississippi Valley leading to some convective initiation on the
diffluent side side of the disturbance. This feature will be moving
fairly quickly offering more of a threat for heavy rainfall Monday
evening into early Tuesday before the chances dwindle. PQPF for >1"
remains fairly low across the region, but environmental conditions
do favor at least some isolated instances of heavy rainfall and
perhaps some flash flooding in those hardest hit zones.

Elsewhere over the CONUS, the Southwest will continue to lie within
the western fringes of the upper ridge centering over the Lower
Mississippi Valley, extending through the Southern and Central
Plains as we move into the beginning of next week. Prevailing
southerly flow and sufficient heating will maintain a suitable
environment for at least scattered thunderstorms across eastern AZ
into NM, extending north into the CO Front Range. Another case of
heavy rain prospects over the terrain and any burn scars in the
above areas will lend to at least a MRGL risk for excessive
rainfall, maintaining continuity from the previous forecast. Last
area(s) of interest reside over the east as a linger front along
the Tidewater/Eastern NC, and shortwave propagation into the
Central Appalachians will offer more chances for flash flooding
given the expected pattern evolution. As of now, the signal is less
prevalent for flash flood concerns compared to previous periods,
however, it stands to reason that the areas in question will be
monitored closely due to the local sensitivity factors that
typically crop up with each individual setup, especially WV given
the saturated ground conditions.

...Day 5/Tuesday...

Main threat for the CONUS on Tuesday will reside across the Upper
Midwest/Mississippi Valley thanks to a strong mid-level trough
sliding out of western Canada providing a significant increase in
large scale forcing across the North-Central U.S. with a target on
MN/WI as of the latest ensemble trends. Environmental conditioning
will occur prior to the trough's appearance as amplified flow
ahead of the mean trough will allow for a steady poleward advection
of rich warm, moist air up from the Mid and Lower Mississippi
Valley. Surface low will propagate southeast out of Canada with an
expectation for the low to deepen as it moves east-southeast over
the Upper Midwest providing sufficient low to mid-level ascent with
its presence. A trailing cold front will extend over the Dakotas
up into the northern Great Lakes by the end of the period. Along
and ahead of the front, widespread convective initiation is
forecast leading to a swath of heavy rain prospects, including the
opportunity for >3" of rain in areas hardest hit. PQPF 90th
percentile is already between 2.5-3" across southern MN where the
consensus is strongest for where the heaviest precip potential may
occur. ML guidance is also in agreement on the heavy precip axis
located between eastern SD across southern MN into southwest WI
with the max located south of I-94. A SLGT risk was added for the
new D5 across the above areas. This setup will be monitored closely
as any further south push in the QPF core would put it closer to
areas that will still be recovering from previous bouts of flash
flooding over the last week.

Kleebauer
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