FOUS30 KWBC 310806
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Sat Oct 31 2026

Day 1
Valid 12Z Sat Oct 31 2026 - 12Z Sun Nov 01 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL ARIZONA...

...Southwest...

The start of a brief, but nonetheless potent atmospheric river
event will impact much of the Southwest tonight. For much of the
day today, most areas will remain dry, or have nominal amounts of
rainfall. The real event will be tonight. A large upper level
longwave trough will dig south along the immediate west coast.
Meanwhile, a subtropical disturbance and attendant surface low will
be picked up by the trough and carried east into the Southwest.
The surface low will have plentiful tropical moisture with it,
which in turn will be hurled into the mountains. Southwesterly flow
off the very warm Gulf of California will advect additional
moisture into the Mogollon Rim region of central Arizona. This will
lead to a concentrated area of heavier rainfall in an area that has
seen numerous heavy rainfall events over the past few weeks,
leading to saturated soils. Given the favorable hydrology, and that
the heavy rain tonight will be the start of a potentially high
impact event in some areas, a Slight Risk upgrade was introduced
with this update, in coordination with PSR/Phoenix, AZ forecast
office.

The surrounding Marginal risk area was expanded to include much of
Nevada and the foothills of the Sierras as well. Heavy snow is
likely on the higher elevations of the Sierras, but just below
that, much of the river valleys that drain the Sierras will see 1-3
inches of precipitation. This could lead to isolated flash flooding
concerns. Other areas with isolated concerns include the Peninsular
and Transverse Ranges in southern California, as well as the slot
canyons of southern Utah.

...Mississippi and Western Tennessee...

In coordination with LIX/Slidell, LA and JAN/Jackson, MS forecast
offices, the Marginal Risk inherited for portions of the
Mississippi Valley were trimmed to a less than 5% risk with this
update. Soils across the Mississippi Valley remain very dry, with
less than 2% of the normal climatological moisture in the surface
layer according to NASA Sport imagery. Further, instability today
will be poor, as is common for very late October, due to extensive
cloud cover. Thus, nearly all of the rain expected today will be
stratiform, and well handled, in fact welcomed by the soils in this
area. A very isolated instance of flash flooding can't be ruled out
in poor drainage areas or where an elevated thunderstorm or two
develops over much of Mississippi and into western Tennessee, but
this is a very low-end and low confidence threat.

...East & Southeast Florida...

Another day of onshore flow ahead of a tropical wave pushing
towards Florida across the Gulf will lead to widely scattered
training showers with a few embedded storms possible. Since this is
the latest of multiple days of this kind of rainfall, the inherited
Marginal was retained with no changes.

Wegman


Day 2
Valid 12Z Sun Nov 01 2026 - 12Z Mon Nov 02 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE FOUR CORNERS REGION...

...Southwest...

Two separate Moderate Risk areas were retained with this update for
portions of the Four Corners. The smaller Moderate Risk along the
San Juans of southwest Colorado was left largely unchanged with
this update. An atmospheric river of extremely anomalous moisture
will make its passage across much of the Southwest during the day
Sunday. PWAT values of 3/4 inch to 1.5 inches are anywhere from 4
to 6 sigma above normal, threatening daily records. All of this
moisture will slam first into the Mogollon Rim region, then
continue into the San Juans. The San Juans have recently seen a
high-impact flash flooding event from a similar feature about a
week ago, and soils have not had a chance to dry out due to a
series of smaller rainfall events in the meantime. Now this event
will threaten to rival the first. Thus, the Moderate Risk looks in
very good shape.

More significant changes/expansions were made across Utah with this
update. In coordination with SLC/Salt Lake City, UT forecast
office, the Moderate Risk was expanded north up I-15 to include the
several very large burn scars dotting around the I-15 and I-70
junction. These burn scars have a history of producing flash
flooding due to heavy rainfall, so the outsized impacts that are
expected with the 1-2 inches of rain forecast led to the Moderate
Risk upgrade. In addition, the surrounding Slight Risk was expanded
north to include the Salt Lake City metro area, due to urban
concerns worsening any localized flash flooding from the nearby
mountains. The heaviest rainfall remains across central Arizona and
the Mogollon Rim region. A higher-end Moderate Risk may be needed
given the impacts that have occurred from the lesser rainfall
amounts that have fallen there over the past couple days. Expect
widespread 3 inch totals north and east of Phoenix, with localized
higher amounts. All of this rainfall will convert to runoff due to
already well-saturated soils.

...Central to Northern Plains...

A small Marginal Risk area that was in place across Kansas and
eastern Nebraska was shifted northward with this update with
changing guidance. Soils are around normal for moisture amounts,
which should help to absorb much of the rainfall expected. Further,
a complete lack of instability will hold any rainfall rates down.
Thus, this Marginal is a low-end one and could be dropped entirely
with future updates.

...Southeast...

In coordination with MLB/Melbourne, FL and JAX/Jacksonville, FL
forecast offices, the inherited Slight Risk along the east coast of
FL was expanded north through the Jacksonville metro. A wave of
tropical moisture with PWATs on either side of 2 inches will cross
the Florida Peninsula on Sunday. This will lead to widespread
rainfall all across the Peninsula. The east coast of the state
continues to have saturated soils from numerous consecutive days of
rainfall. Thus, the soils will be unable to absorb much of the
heavy rains expected. This will lead to widely scattered instances
of flooding. Urban and low-lying areas will be particularly
vulnerable.

Meanwhile, across the Ohio and Tennessee Valleys, a lack of
instability and generally light rainfall over parched ground should
limit any flash flooding threat, even if a stray thunderstorm were
to form. The Marginal Risk was trimmed from these areas, and may be
able to be trimmed further to close to the Gulf Coast, where any
and all instability will reside on Sunday.

Wegman


Day 3
Valid 12Z Mon Nov 02 2026 - 12Z Tue Nov 03 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE EAST COAST
OF FLORIDA...

...Florida...

In coordination with MLB/Melbourne, FL and JAX/Jacksonville, FL
forecast offices, a Slight Risk upgrade was introduced for the
entire I-95 corridor of Florida with this update. Heavy rain from
Day 2/Sunday night will continue through much of the day Monday
before tapering off Monday night. A tropical wave moving eastward
will finish crossing the Florida Peninsula Monday, then exit into
the Atlantic. Heavy rain that began on Day 2/Sunday will continue
into Monday. While technically there should be less rainfall after
12Z/Monday as compared to before then, the flooding impacts will
increase as the rain continues. Thus, the Slight roughly follows
where the Day 2 Slight was, showing continuity. Easterly flow will
continue frictional convergence over a portion of Florida that has
seen numerous days of rain over the past week or so. Thus, this
additional hit of rainfall is likely to continue causing widely
scattered instances of flash flooding, especially in urban areas
from Jacksonville to Miami. With the rain ending late Monday
afternoon, so too will end the flooding threat due to rainfall, so
this Slight Risk is really for the first half of the period.

...Midwest and Plains...

A sprawling mid-latitude cyclone will take shape across the Midwest
and Plains, resulting from the atmospheric river event that slammed
the Southwest. The upper level trough will still have a pair of
vigorous shortwaves associated with it, and highly anomalous
moisture to 5 sigma above normal. This will lead to a large area of
2-4 inches of rain in the comma head region of the storm from
eastern South Dakota through much of central Minnesota. Despite
that signal, guidance shows there will be bupkis for instability.
Thus, expect a long-duration stratiform rain event. Near normal
amounts of antecedent soil moisture should help to absorb much of
this rain as it comes down at a slow-enough pace for the soils to
keep up. Thus, for now, a Marginal Risk remains in place. The
storm's influence is likely to spread further east than previous
forecasts, so the Marginal was expanded to cover all of Minnesota,
Wisconsin, and the U.P. of Michigan. Meanwhile, lingering rain over
South Dakota will also keep a large area of 1-3 inches of rain over
that state, which could cause isolated flooding issues.

Where more attention will be needed in the coming days will be with
the cold frontal precipitation. Better instability, though nothing
to write home about, could support some thunderstorms capable of
locally heavy rainfall over the terrain of the Ozarks. There was
enough of a change in the location of the rainfall footprint, and
soils across the Ozarks become increasingly dry the further east
you go, both of which precluded there being enough confidence for a
Slight Risk issuance with this update, though one may be needed
with future updates and CAMs guidance resolution of the system.

Wegman


Day 4 and Day 5
Valid 12Z Tue Nov 03 2026 - 12Z Thu Nov 05 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE SOUTHERN PLAINS AND MID AND LOWER MISSISSIPPI VALLEY TO THE
TENNESSEE VALLEY (DAY 4) AND FROM THE TENNESSEE AND LOWER OH
VALLEYS TO THE SOUTHERN APPALACHIANS (DAY 5)...

An amplified upper trough and its associated frontal band will
provide the primary focus for heavy rain and potential flash
flooding as it moves from the Plains to the Mississippi Valley
midweek. Southerly flow ahead of the system will tap Gulf moisture,
supporting widespread showers and storms, with strong upper
forcing contributing to areas of heavy rainfall.

Model uncertainty with regards to where the heaviest rainfall
amounts are expected to occur is driven in large part by the timing
and evolution of a southern stream wave moving out into the
southern Plains early Tuesday. Some models, such as the GFS and
ECMWF, leave this feature detached from the westerlies and move it
slowly east across the mid and lower Mississippi Valley Wednesday
into Thursday; while others, such as the CMC phase it back into
the northern stream and lift it out to the north.

Regardless of how the system evolves, there remained enough model
support to maintain a Marginal Risk extending from parts of
eastern Oklahoma and Texas over to the Mid-South on Day 4.
Deepening moisture and sufficient instability supported by strong
southerly flow, as well as favorable mid-to-upper level forcing
are likely to contribute to periods of heavy rain rates. While the
possible progressive nature of the developing storms and dry soils
will likely limit the potential for widespread flooding, the
threat for at least isolated concerns could not be ruled out.

If the system does slow, as the GFS and ECMWF suggests, the threat
for flash flooding may amplify as the system continues to tap Gulf
moisture and the potential for training storms increases. While
some solutions suggest a Slight Risk may be needed at some point,
given the uncertainty, opted to introduce just a Marginal Risk on
Day 5 at this point. The highlighted area, covering much of the
Tennessee Valley and extending north into Kentucky and east into
the southern Appalachians, reflects where the ensemble guidance
indicated the greater threat for heavier amounts.

Pereira


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



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