FOUS30 KWBC 150836
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
436 AM EDT Sat Aug 15 2026

Day 3  Valid 12Z Mon Aug 17 2026 - 12Z Tue Aug 18 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE OHIO AND TENNESSEE VALLEY AND APPALACHIANS, THE SOUTHWEST
AND UPPER MIDWEST ...

...OH/TN valleys into the Appalachians...
A southward dropping cold front will serve as a focus for scattered
convection on Monday. While overall storm coverage will be reduced
compared to previous days, lingering moisture and instability just
behind the boundary will maintain a heavy rainfall threat from KY
and TN into the central Appalachians, enhanced by local terrain
upslope. A Marginal risk is maintained for isolated flash flooding.

...Southwest...
A Marginal risk encompasses much of NM and adjacent portions of
southeast AZ and southern CO. Beneath an upper level ridge strong
instability and slightly above average PWs will interact with
embedded shortwave energy trapped in the weak flow. Terrain based
initiation is expected during the afternoon, with scattered
coverage and cell mergers likely as convection drifts off the
higher elevations.

...NE to MN...
A Marginal risk continues from central NE into southern MN ahead
of an advancing cold front. Frontal forcing will support convective
development with localized training or backbuilding potential
along the south to southwest flank of convection. While the threat
is expected to remain localized...and storms may prove progressive
enough to limit impacts...a Marginal risk reflects the low end risk.

Chenard
 

Day 4 and Day 5  Valid 12Z Tue Aug 18 2026 - 12Z Thu Aug 20 2026

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

...Day 4/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
the Upper Mississippi Valley into the Corn Belt as of the latest
ensemble and global deterministic trends. One of the main changes
from the previous forecast is a further south focus of the heavier
rainfall as the advancement of a cold front and elevated theta_E
advection will likely cause a development of widespread convection
further south than the previous forecast. The latest National
Blend came in further south as well, actually matching more of
the ML algorithms in the EC-AIFS and AI-GEFS ensembles meaning the
blend is shifting away from the previous northern output that put
the QPF maxima over MN. A deepening surface low materializing
within the diffluent mid-level evolution ahead of the trough is
slated to propagate southeast out of Canada with an expectation for
the low to strengthen quickly as it moves east- southeast over the
Upper Midwest. A trailing cold front will extend over the Dakotas.
through Iowa/Southern MN, arcing 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 remains steady in its
presentation from previous forecasts depicting between 2.5-3"
across Iowa into southwest WI 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 in the same areas
across from IA to WI with the max located across Central IA as the
00z ML suite.

Considering the latest trends in guidance for a further south
impact in the heavy rainfall axis, the SLGT risk inherited was
shifted further south to encompass much of IA into WI. The shift
into IA is noteworthy due to the antecedent soil moisture that will
likely still remain elevated across much of the state due to the
previous periods of rainfall that generated widespread flash
flooding in the southern and eastern portion of the state. General
trends will be monitored closely as any overlap of the some of the
more sensitive areas in IA and WI could introduce some targeted
upgrades, especially with the signal likely to bear locally higher
totals compared to what is being forecast at lead. For now,
maintained the SLGT risk, but adjusted based on the latest shift in
the QPF mean further south.

...Day 5/Wednesday...

Broad ridging across the Southern CONUS and a deep, amplified upper
trough across Canada into the Northern U.S. will generate a
progressive flow that will entice periods of convection around the
western and northern periphery of the ridge with scattered heavy
rainfall signatures within those aforementioned domains. The most
notable area of precip will likely lie within the Great Lakes down
into the Ohio Valley where a cold front advancement will continue
to be a focal point for convection along and ahead of the front as
it migrates east of the Mississippi Valley towards the east coast.
Current signals are not as prolific as the previous periods of
excessive rainfall, however the antecedent soils will still likely
be in recovery over the areas of interest for the D5 leading to at
least isolated instances for flash flooding in the anticipated
evolution. Models remain mixed in their magnitude and location of
heaviest rainfall, but ensembles maintain some areal QPF maxima in
the vicinity of both the Great Lakes down through OH/KY into the
western half of WV. These areas are the most susceptible to
appreciable impacts given the nature of the soils allowing for at
least a MRGL risk to encompass the area to note the potential.

Across the Southwest into the Front Range, the western periphery of
the ridge will allow for a continued southerly push of warm, moist
air poleward with lee troughing likely to be situated from CO down
through NM as it materializes within a modest theta_E ridge.
Scattered thunderstorms across the terrain will transpire leading
to at least the threat for isolated flash flood prospects,
especially within any remnant burn scars and complex topography
within the Sangre de Cristos down into the Sacramento Mountains,
extending southwest towards the terrain of southeastern AZ into the
Bootheel of NM. A MRGL risk was added over the area to account for
the threat.

Kleebauer
$$
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