FOUS30 KWBC 120822
QPFERD

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 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
$$
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