Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Fri Aug 14 2026

Day 1
Valid 01Z Sat Aug 15 2026 - 12Z Sat Aug 15 2026

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

2030Z Update...
Few changes needed to the on-going excessive rainfall outlook over
the western and central portions of the United States. The
corridor from Iowa into northern/central Illinois continued to have
the greatest concern for excessive rainfall during the overnight
hours...with the 18Z CAMs and associated neighborhood probabilities
continuing to be focused in southeast Iowa. Rainfall amounts tail
off over parts of the Ohio Valley but are likely to be problematic
given the amount of area which has received heavy rainfall this
week. Late afternoon/early evening satellite and radar imagery
showed convection being slow to diminish across the western United
States. Was able to trim some area from the eastern states given
the loss of daytime heating and subsequent stabilization.

Bann

Previous Discussion...

...Iowa to the Appalachians and Southeast states...

16Z Update: After coordination with the local field offices, the
existing Moderate Risk area was expanded a couple rows of counties
to the east to the Indiana state line to account for highly
impactful rainfall overnight and early this morning, and the
expectation for additional heavy rainfall later today. In addition,
the Slight Risk area was expanded about a row of counties to the
south of the existing area to include more of southern Illinois and
southwestern Indiana where deeper instability feeding into the MCS
could allow for heavier storms to build farther south that what the
CAM guidance currently suggests.

There was also an extension of the Marginal Risk area southward to
include most of Georgia and into the northern half of interior
Florida given recent heavy rainfall yesterday and the potential for
scattered but potentially very high QPF maxima with slow moving
storms that develop later this afternoon, with rainfall rates up to
three inches per hour in isolated instances. The latest 12Z HREF
exceedance probabilities are now up to 25% across portions of
northern Florida into southern Georgia. The previous forecast
discussion for this region is included below.
--------------

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.

The updated ERO features a reduction of the Marginal Risk area
over north-central Nebraska in the wake of the departing MCV, and
rainfall rates over the sandhills region will likely remain under
flash flood guidance.

...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.

For the midday update, the existing Marginal Risk area was reduced
in areal coverage, with generally lower probabilities of heavy
rainfall from central Wyoming southward to extreme northwestern New
Mexico. Intense convection is already developing across
northwestern Arizona this morning, and this merits keeping the
Slight Risk area there intact.

Chenard/Hamrick


Day 2
Valid 12Z Sat Aug 15 2026 - 12Z Sun Aug 16 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL OHIO AND INTO EASTERN INDIANA...

...Midwest and Ohio Valley...

A Moderate Risk has been introduced in the update Day 2 outlook
across portions of east-central Indiana into central Ohio, between
Fort Wayne and Zanesville. The latest 12Z CAM guidance suite has
impressive QPF totals on the order of 2 to 4+ inches, with the
RRFS, ARW, and ARW2 more widespread with these higher totals, and
the HRRR generally has a narrower axis. In addition, the HREF 3
hour flash flood guidance exceedance probabilities are impressive
at nearly 60% for this region during the 00Z-09Z time period
Sunday, and low end probs for 100-year ARI. There is still some
latitudinal differences in the main QPF axis, but given the
severely saturated ground conditions across this region from
antecedent rainfall over the past few days, there is now enough
confidence for the Moderate Risk area.

A Slight risk extends from northern Illinois eastward into West
Virginia. 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. This axis may overlap areas heavily impacted by flash
flooding on prior days, where saturated soil conditions will
heighten hydrologic sensitivity.

...Central Plains to the Northern Rockies...

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.

For the 20Z Update, the Slight Risk was extended a little eastward
across northern Kansas given 12Z CAM guidance trends. The Marginal
Risk was also extended back across northern Wyoming into northeast
Idaho and western Montana where some isolated but potentially
heavy showers and storms could develop during the afternoon and
evening hours.

Chenard/Hamrick


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.

For the 20Z update, the Slight risk was expanded a little to the
north to include the southern tier of counties in Upstate New York,
and the southern portion of the DelMarVa trimmed back some given
the latest 12Z guidance trends.

...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. No changes were needed for this Slight Risk
area with the 20Z update, although the Marginal Risk area was
connected into a continuous corridor across Kansas.

Chenard/Hamrick


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...

2030Z Update...
Maintained the Slight Risk of excessive rainfall on Tuesday across
parts of the Upper Midwest with only some minor change. The
Marginal risk areas on each Day were expanded to cover a growing
signal...albeit not a resounding signal...for convection to develop
in a moisture-laden atmosphere over parts of the middle third of
the county. Also expanded the Marginal risk area surrounding the
Slight risk given the uncertainty about where an upper low north of
the border is located and any associated mid-level height falls
brushing the northern states.

Bann

...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


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