Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Fri Oct 30 2026
Day 2
Valid 12Z Sat Oct 31 2026 - 12Z Sun Nov 01 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST...

..20z Update..

Southwest: The forecast remains for deep tropical moisture to
advect into the Southwest with ample divergence aloft from a
nearing shortwave and associated jet streak to produce half an
inch to an inch of rainfall across southern California into
Arizona, Utah, and Nevada. Burn scars, slot canyons, and sensitive
terrain will be the most vulnerable to instances of isolated flash
flooding. However, the Slight Risk was removed across the Mogollon
Rim across south central Arizona on Saturday as not all atmospheric
ingredients are aligning. Model guidance indicated rainfall to
generally range around an inch in these areas, which seemed on the
lighter side for prior level 2/4 risk for flash flooding. The
limiting factor will be instability with model guidance struggling
to show more than a 100 J/Kg. This was reflected in the 12z HREF,
which showed virtually no probabilities for rainfall rates
exceeding an inch per hour. Instability has the highest correlation
with flash flooding in the West, but it is expected that the ample
cloud cover will hinder convective thunderstorms that normally
provide increased coverage and/or higher end impacts from flash
flooding across the Southwest. Other changes included removing
northern portions of the Marginal across Nevada where an
approaching cold front will bring cooler temperatures and reduce
instability needed for higher rainfall rates and flash flooding in
the West.

Lower Mississippi Valley: The latest afternoon update included
removing northern sections of the Marginal Risk across the southern
Ohio Valley where the QPF generally remained less than an inch and
had lower instability and available moisture.The rest of the
forecast remains on track as a shortwave entering the lower
Mississippi Valley will interact with anomalous moisture in the
area to produce scattered thunderstorms capable of producing
locally heavy rainfall and nuisance flooding.

East Coast of Florida: Northeasterly flow around an area of high
pressure in the Atlantic will stream moisture off the Atlantic
Ocean and nearby Gulf Stream and converge along the I-95 corridor
from Cape Canaveral to Miami. Instability on the order of 1000-1500
J/Kg and PWATS between 1.5-2.0 inches will support modest
probabilities for 1-3 inches of rain as supported by the 12z
guidance. Saturated soils and the urbanization factor across the
area will also support the potential for water ponding and perhaps
a flash flood warning.

Wilder

...Previous Discussion...

...Southwest...

A plume of deep tropical moisture will be drawn northeastward by a
longwave positively tilted trough into the Desert Southwest. A
Pacific low at the surface will track towards southern California,
forced by the abundant divergence aloft in the right entrance
region of a 130 kt southwesterly jet. A vigorous shortwave riding
the base of this upper level trough will further enhance lift
supporting heavy rainfall. The jet will allow IVT values to surge
as high as 750 kg/ms. This pattern will advect near record amounts
of moisture for this time of year into the Southwest. PWATs between
1 and 1.5 inches are 4-6 sigma above normal for this time of year.
The result of this highly unusual amount of moisture racing
northeastward will mean the topography of the coastal ranges like
the Peninsular Ranges of southern California and the Mogollon Rim,
which are roughly oriented perpendicular to that southwesterly
flow, will squeeze out ample moisture in the form of heavy rainfall
from the atmosphere.

Expect 1-2 inches of rain along the Mogollon Rim, much of which
occurring after midnight Saturday night. While instability will be
nominal, values will remain high enough to support some elevated
convection, which will occur in addition to heavier rain in the
favored upslope regions. Antecedent soil conditions are also much
wetter than normal, which will support fast developing fast
flooding in flood-prone areas, especially burn scars, dry washes,
slot canyons, and arroyos.

...Lower Mississippi Valley...

Return flow behind an area of high pressure over the Northeast will
draw abundant Gulf moisture northward up the lower Mississippi
Valley into the Ohio Valley on Saturday. While moisture flux and
amounts will be well above normal (2-3 sigma), instability will
also be limited for areas away from the Gulf. Soils in this area
are bone dry, and well below normal for this time of year. Thus,
the Marginal Risk is a low-end one, to cover urban and particularly
flood prone areas.

...Florida...

The Marginal Risk along the east coast of Florida from Cape
Canaveral through Miami is unchanged with this update. Portions of
the Treasure Coast have soils that are water-logged from multiple
days of rainfall. However, as on previous days, robust convection
will struggle to make it too far inland before falling apart, so
the flooding threat remains confined to urban and coastal areas
where soils remain near saturation.

Wegman


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