Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
854 PM EDT Tue Aug 11 2026
Day 1
Valid 01Z Wed Aug 12 2026 - 12Z Wed Aug 12 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY...
0045Z Update:
Only changes made were applied to the area from the southern Great
Lakes eastward toward the Mid-Atlantic states. The changes were
primarily to remove territory from the outlook areas across the
northern and eastern areas where the airmass tended to be more
stable. Satellite imagery early this evening still showed training
convection across parts of the Ohio and Tennessee Valleys gradually
easing southward. The latest runs of the HREF and HRRR both suggest
that additional development will occur later tonight over Iowa or
Illinois that tracks over areas which received rainfall over the
past 12 to 89 hours...so largely left the western portion of the
Slight Risk area untouched despite only modest confidence. Farther
west...maintained the Marginal/Slight risk over the Southwest US
without change given the late day instability and meager forcing
can still result in isolated instances of flash flooding for a few
more hours.
Bann
Previous discussion below..
...Midwest/Ohio Valley/Mid-Atlantic States...
The general flow pattern remains fairly static since yesterday.
The equatorward edge of the Westerlies is forecast to lie from the
Northern Plains into portions of the Ohio Valley and Mid- Atlantic
States where there is also low- level confluence, difluent 1000-500
hPa thickness, and an instability gradient to focus organized
convection. Precipitable water values are forecast to be 2"+ in
spots. Hourly rain amounts to 3" and local totals to 6" remain
possible given the available ingredients. Previous days' rains over
these areas have saturated the soils further across portions of
northern IL/northern IN/OH/WV/southwest PA. When combined with the
general very flashy nature of the rivers/terrain of West Virginia,
the Moderate Risk level continues to make sense. Recent guidance
led to some southward shift/expansion in the Slight/Moderate Risk
areas. An eastward expansion of the Slight Risk area into Richmond/
Baltimore/Washington DC was made as several members of the global
guidance had a heavy rain signal over or very near the region which
received heavy rainfall on Monday.
Roth
...Cen-Srn Rockies...4 Corners...Cen Great Basin...Southwest...
1600 UTC Update: Southeast AZ and southwest NM remains most at risk
for more persistent heavy rainfall, and as such the Slight Risk
from yesterday's Day 2 ERO remained intact based off last night's
00Z and today's 12Z guidance. Across the Great Basin, the Marginal
Risk was broadened as moisture subtly increases as an upper level
trough settles in to the CA Coast and ridging near the Four Corners
weakens, which should lead to less mid-level capping and broader
convective coverage.
Hurley/Roth
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Wed Aug 12 2026
Day 1
Valid 12Z Wed Aug 12 2026 - 12Z Thu Aug 13 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL PLAINS THROUGH THE TENNESSEE VALLEY, AND FOR THE DESERT
SOUTHWEST..
...Central Plains southeast to the Tennessee Valley...
Extremely active convective weather continues Wednesday as a "ring
of fire" type setup continues with mid-level waves rotating atop a
ridge to bring convective clusters to the region.
The ridge responsible for this setup amplifies across the Southern
Plains but does retreat subtly in the east resulting in pinched
flow around its northern periphery. Within this flow, multiple
shortwaves will lift out of the Four Corners and then race E/SE
from the Central Plains through the Ohio Valley in a pattern very
repetitive from previous days. While the exact placement of any of
these shortwaves is difficult to determine even on D1, each ripple
interacting with a stationary front draped across the region will
result in clusters of thunderstorms moving repeatedly across the
region, with ascent additionally enhanced by a jet streak
positioned near the Canada/U.S. border leaving its diffluent RRQ
overtop this area. While storms will generally track NW to SE
within the region of greatest thickness gradient, small variations
in mesoscale features can play a significant role in where the
heaviest rainfall will occur.
This activity will develop within robust thermodynamics as a plume
of elevated PWs reaching +1.5 to +2 sigma arcs around the ridge and
pools along the front, collocated with impressive MUCAPE driven by
an EML noted in regional forecast soundings lifting out of Mexico
and around the ridge. Although lapse rates weaken farther east into
the Ohio Valley than the Central Plains, this results in deeper
column saturation to support intense rainfall rates, for which the
HREF neighborhood probabilities suggest will exceed 2"/hr at times.
The most intense rain rates will occur within any MCS development
as 0-6km bulk shear of 40-60 kts drives strong convective
organization, which with mean 0-6km winds aligned to both the front
and the Corfidi vectors, indicates a strong likelihood for periods
of training. While there continues to be uncertainty as to the
exact footprint of the heaviest rainfall, anywhere prolonged
training of these intense rain rates can occur, more than 5 inches
of rain is possible.
As noted above, confidence in the exact placement of the heaviest
rainfall remains uncertain due to model variations in the placement
of these, already notoriously difficult to forecast, mid-level
impulses. However, both the REFS and HREF depict a region of higher
probabilities from Iowa through Kentucky, with a subtle SW trend
noted in the accompanying deterministic guidance. Some of this rain
will fall atop recently saturated soils from prior heavy rain, but
in general the axis of heaviest rain may just miss those areas. For
this reason changes to the inherited ERO were modest and cosmetic.
...Southwest/Great Basin/Four Corners...
The subtle eastward retreat of the expansive mid-level ridge over
the southern tier of the CONUS in response to a trough diving along
the Pacific Coast will allow for more intense moisture and
thermodynamic return into the region today. This evolution will not
only allow for elevated PWs to stream northward (widespread +2 to
+2.5 sigma and above the 97th percentile according to NAEFS), but
will also weaken the cap as mid-level temps fall to enhance
available instability. Additionally, forcing for ascent will be
maximized as multiple weak impulses lift northward from Mexico
around the ridge periphery, acting upon the favorable
thermodynamics to drive widespread convective development.
Coverage of thunderstorms today should be quite widespread,
potentially one of the more active days of the entire monsoon
season so far. Simulated reflectivity from the high-res CAMs
suggest storms will develop as far west as the Sierra, while
covering as well most of the Four Corners, Great Basin, and into
the High Plains. Mean 0-6km winds of 10-15 kts will allow for at
least some more progressive movement than previous days, but bulk
shear of 20-30 kts and persistent southerly moist inflow will help
to organize convection resulting in periods of possible
training/backbuilding. With rain rates likely reaching 1"/hr at
times, this could produce locally 3" of rain, but even where total
rainfall is less, the intensity of the rain could still cause flash
flood related impacts. The greatest threat will be across
vulnerable features like burn scars, dry washes, canyons, and urban
areas, but activity is expected to be widespread today, especially
where any morning cloud cover does not limit instability later
today. After coordination with the affected WFOs, a SLGT risk was
expanded to include most of Arizona, the slot canyon region of
Utah, and far southern Nevada.
Weiss
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Wed Aug 12 2026
Day 2
Valid 12Z Thu Aug 13 2026 - 12Z Fri Aug 14 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDWEST THROUGH THE OHIO VALLEY, THE CENTRAL AND NORTHERN HIGH
PLAINS, PORTIONS OF THE SOUTHWEST, AND THE EASTERN SIERRA/WESTERN
GREAT BASIN...
...Midwest/Ohio Valley...
The active pattern from Wednesday continues almost without respite
into Thursday. A wavering stationary front positioned north of an
impressive mid-level ridge (500mb heights climbing above the 90th
percentile over the Gulf Coast/Southeast) will again be the focus
for waves of thunderstorms. As is the case on D1, and even moreso
on D2, ripples of energy moving within the pinched westerlies and
atop this boundary will help focus ascent for thunderstorm
development, and the upstream connection to the Mexican EML and
Pacific moisture will provide favorable thermodynamics for intense
rainfall. In fact, PWs on Thursday may be higher than any other day
this period, with a plume of 2.25" PWs progged to align near the
front, reflected in the NAEFS with an area exceeding the 99th
percentile of the CFSR climatology. This is focused in a stripe
from central IA through southwest OH, which is also where the ECMWF
EFI reaching 0.7/SoT of 1-2.
This is a more cohesive signature among the models for focused
heavy rainfall, although the greatest risk area may occur on the
upstream portion of this region as reflected by REFS indicating a
high risk (>60% chance) for at least 1"/hr rates, with the
environment supporting the training of 2"/hr rates at times also.
This is also where the AI (both GFS and ECMWF) focus the heaviest
rainfall and is also where the greatest REFS/GEFS/ECENS
probabilities for 3+" of rainfall exist. Conceptually this could be
the region of greatest training as convection develops over NE/IA
and backbuilds along the boundary before organizing into
clusters/MCS and moving more progressively eastward. While anywhere
across this region could experience flash flooding, especially
since the vulnerability will be increased due to antecedent
rainfall, the inherited SLGT (and enhanced SLGT) was expanded to
the west with this update.
...Four Corners through the Northern High Plains...
Widespread convection is again expected in a broad area from the
Four Corners into the Northern High Plains as moisture streams
around a ridge centered over the Gulf Coast. This re-centering of
the mid-level ridge will continue the evolution which begins on
Wednesday of allowing more open 700-500mb flow out of Mexico and
the Pacific to support elevated PW anomalies across a broad region
of the Western CONUS. This is reflected by NAEFS PW anomalies
above the 90th percentile covering most of the this region, with a
focused area of anomalies above the 99th percentile spreading into
the Northern High Plains.
Within this impressive thermodynamic plume, forcing for ascent
will intensify thanks to broad height falls as a trough digs in
from the Pacific, with several mid-level impulses rotating up
through the flow to locally enhance deep layer lift. At the same
time, a stationary front will be draped from Montana through the
Central High Plains, with additional ascent provided via upslope
mechanisms in its wake, especially within the High Plains. As bulk
shear increases to 35-45 kts, the setup will become increasingly
supportive for clusters of thunderstorms containing rain rates
above 1"/hr traversing generally SW to NE (W to E farther north),
and repeating through multiple rounds. The REFS/SREF/GEFS all
indicate regions of 3+" of rain occurring, with the general focus
matching the locations of highest EFI, which also concurs with the
inherited ERO and first guess fields. For these reasons, changes to
the inherited risk areas were modest and cosmetic due to overall
consistency for these areas.
...Southwest/Great Basin...
Broadening upper level trough moving along the Pacific Coast will
bring expanding height falls to the Southwest, with pinched flow
between this trough and a ridge over the Gulf Coast/Southeast
driving pronounced thermodynamic advection northward. PWs will
again exceed 1 inch all the way into Utah (+1 to +2 sigma), but
this will be actually slightly less anomalous than what is
anticipated to exist on Wednesday /D1/. Still, with these elevated
PWs overlapping MLCAPE of 500-1000 J/kg, another active day of
convection is anticipated, with locally elevated bulk shear of
20-25 kts helping to organize development, especially near the
AZ/NM border. Mean winds will be generally light at 5-10 kts across
AZ/NM and from the south, but any organization could locally result
in some training/merging to enhance rainfall. This would be most
problematic over any urban areas, dry washes, canyons, and burn
scars, and the inherited SLGT risk was only modified cosmetically
for new guidance.
However, a second SLGT risk area was added for portions of the
Great Basin/western Nevada where the overlap of high PWs and
significant CAPE will support multiple rounds of thunderstorms.
Both the HREF and REFS guidance indicate an elevated risk of flash
flood guidance exceedance over this area, so after coordination a
targeted SLGT risk was added here as well.
Weiss
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Wed Aug 12 2026
Day 3
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, AS WELL AS PARTS OF THE UPPER
MIDWEST...
...Colorado and Adjacent Areas...
An easterly low level jet branching off of broad southerly flow up
the Plains and around an expansive ridge to the east will support
a combination of upslope and convective rainfall across much of
northern Colorado on Friday. PWATs near the Kansas border will
approach 1.75 inches, lowering to around 1 inch into the Rockies
behind the Front Range. This ample moisture will funnel north and
west ahead of a large upper level trough expanding over the West
Coast to provide increasing synoptic lift through height falls.
Upper level shortwaves will also support more widespread convective
development across much of Colorado. The inherited SLGT area was
cosmetically adjusted over primarily NE Colorado where terrain-tied
storms may move slowly, or repeatedly, over areas that will also
receive heavy rain on Thursday to precondition the soils.
...Upper Midwest...
The inherited small SLGT risk was cosmetically adjusted to account
for new guidance, but in general changes were modest. There
continues to be quite a bit of uncertainty D3 as to the convective
evolution across the Upper Midwest. The region will remain firmly
entrenched within pinched mid-level westerlies north of an
amplified ridge, with low-level flow becoming easterly as a
seemingly semi-permanent stationary front wavers to the south. As
waves of energy ripple along this boundary and the periphery of the
ridge, another day of active convection with clusters of
thunderstorms is expected. The exact placement of this activity
remains uncertain as there are considerable latitudinal and
longitudinal discrepancies among the various models, but it is
likely that some areas will receive heavy rainfall once again on
Friday. Should this overlap heavy rain axes from D1 and D2,
upgrades to this risk area may be needed. However, at this time,
the threat appears slightly lower by Friday than
Wednesday/Thursday, and the SLGT was adjusted to match best the AI
QPF and the ECENS probabilities for 3"/24 hrs.
Weiss
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Wed Aug 12 2026
Day 4 and Day 5
Valid 12Z Sat Aug 15 2026 - 12Z Mon Aug 17 2026
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR NORTHEASTERN
COLORADO AND SOUTHERN WISCONSIN ON SATURDAY...
...Northeastern Colorado...
A third consecutive day of heavy rain into portions of northeastern
Colorado is expected on Saturday. A Slight Risk upgrade was
introduced with this update for this reason. A plume of moisture
that will have been in place across the region will still be in
place on Saturday. Meanwhile in the upper levels, the ridge over
the Southeast continues to gradually expand east along the Mexican
border. This will continue shifting the upper level flow from
southerly to southwesterly. Shortwave disturbances riding the top
of the ridge will continue tracking across the region, which will
provide ample forcing given the abundant moisture for widespread
shower and thunderstorm activity, including the potential for
backbuilding and training storms. While soils across much of the
upper Plains are very dry based on NASA Sport imagery, that
situation is likely to change, especially on Day 2/Thursday. With
repeated rounds of storms into Friday as well, by Saturday expect
much of this area to have saturated soils. Some of the guidance
suggests that Saturday will be the wettest of the upcoming days.
Thus, it stands to reason that additional upgrades may be coming
with future updates.
...Southern Wisconsin...
A Slight Risk was also introduced for southern Wisconsin with this
update. The combination of storms that originate in Colorado on
Friday, a plume of highly anomalous moisture riding the ridge, a
large trough over the northern Plains, and a strong cold front
moving in from the north will all play roles in setting up this
heavy rainfall event. The anomalous moisture will probably be the
single most important ingredient supporting heavy rainfall, as
PWATs exceed 2.25 inches in many areas. A morning MCS crossing the
area will start the soaking process for the soils, then new
convection will form overnight with the increase of the nocturnal
LLJ. The strong cold front will begin to clear the convection from
the northwest late Saturday night, eventually ending the excessive
rainfall and flooding threat. Urban and small stream flooding will
be the greatest threat, especially in areas such as Milwaukee.
However, given the impressive amount of moisture the storms will
have to work with, and the terrain in the area, flash flooding
could occur anywhere there is persistent heavy rain from training
storms.
...Day 5/Sunday...
The heavy rain from the Wisconsin/Midwest region continues off to
the east on Sunday. Thus, the Northeast will have the greatest
threat for locally heavy rainfall. Unlike over the Midwest where
there will be an extended period where anomalously high PWATs over
2.25 inches will be in place, allowing storms to have more than
abundant fuels for flooding, by Sunday the entire complex of storms
will be much more progressive. However, that anomalous moisture
will still be there fueling the storms, and there still remains
some potential for training, especially where any storms form
ahead of the main line of storms. Thus, there is some potential
that some areas of the interior Northeast, generally along the I-90
corridor, could eventually need an upgrade. For now, primarily
given the average soil moisture and progressive nature of the
storms, a large Marginal has been introduced for all of the
Northeast outside of northern Maine.
Lingering storms into New Mexico and Colorado, then extending into
Kansas and Nebraska are expected Sunday afternoon. While the storms
may be numerous once again along the mountains, the coverage will
be much more widely scattered over the Plains.
Wegman
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Wed Aug 12 2026
Day 4 and Day 5
Valid 12Z Sat Aug 15 2026 - 12Z Mon Aug 17 2026
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR NORTHEASTERN
COLORADO AND SOUTHERN WISCONSIN ON SATURDAY...
...Northeastern Colorado...
A third consecutive day of heavy rain into portions of northeastern
Colorado is expected on Saturday. A Slight Risk upgrade was
introduced with this update for this reason. A plume of moisture
that will have been in place across the region will still be in
place on Saturday. Meanwhile in the upper levels, the ridge over
the Southeast continues to gradually expand east along the Mexican
border. This will continue shifting the upper level flow from
southerly to southwesterly. Shortwave disturbances riding the top
of the ridge will continue tracking across the region, which will
provide ample forcing given the abundant moisture for widespread
shower and thunderstorm activity, including the potential for
backbuilding and training storms. While soils across much of the
upper Plains are very dry based on NASA Sport imagery, that
situation is likely to change, especially on Day 2/Thursday. With
repeated rounds of storms into Friday as well, by Saturday expect
much of this area to have saturated soils. Some of the guidance
suggests that Saturday will be the wettest of the upcoming days.
Thus, it stands to reason that additional upgrades may be coming
with future updates.
...Southern Wisconsin...
A Slight Risk was also introduced for southern Wisconsin with this
update. The combination of storms that originate in Colorado on
Friday, a plume of highly anomalous moisture riding the ridge, a
large trough over the northern Plains, and a strong cold front
moving in from the north will all play roles in setting up this
heavy rainfall event. The anomalous moisture will probably be the
single most important ingredient supporting heavy rainfall, as
PWATs exceed 2.25 inches in many areas. A morning MCS crossing the
area will start the soaking process for the soils, then new
convection will form overnight with the increase of the nocturnal
LLJ. The strong cold front will begin to clear the convection from
the northwest late Saturday night, eventually ending the excessive
rainfall and flooding threat. Urban and small stream flooding will
be the greatest threat, especially in areas such as Milwaukee.
However, given the impressive amount of moisture the storms will
have to work with, and the terrain in the area, flash flooding
could occur anywhere there is persistent heavy rain from training
storms.
...Day 5/Sunday...
The heavy rain from the Wisconsin/Midwest region continues off to
the east on Sunday. Thus, the Northeast will have the greatest
threat for locally heavy rainfall. Unlike over the Midwest where
there will be an extended period where anomalously high PWATs over
2.25 inches will be in place, allowing storms to have more than
abundant fuels for flooding, by Sunday the entire complex of storms
will be much more progressive. However, that anomalous moisture
will still be there fueling the storms, and there still remains
some potential for training, especially where any storms form
ahead of the main line of storms. Thus, there is some potential
that some areas of the interior Northeast, generally along the I-90
corridor, could eventually need an upgrade. For now, primarily
given the average soil moisture and progressive nature of the
storms, a large Marginal has been introduced for all of the
Northeast outside of northern Maine.
Lingering storms into New Mexico and Colorado, then extending into
Kansas and Nebraska are expected Sunday afternoon. While the storms
may be numerous once again along the mountains, the coverage will
be much more widely scattered over the Plains.
Wegman
