Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
728 PM EDT Sat Oct 10 2026

Day 1
Valid 01Z Sun Oct 11 2026 - 12Z Sun Oct 11 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHERN APPALACHIANS & PORTIONS OF SOUTHERN CALIFORNIA...

Southeast/Mid-Atlantic/Southern Appalachians...
Widespread heavy rains are likely to continue with Post-Tropical
Cyclone Isaias through tonight/early Sunday morning. Heaviest
rains likely into the upslope regions of the Southern Appalachians
from northeast Georgia into the Upstate of South Carolina and
western North Carolina. Strong upslope flow will support heavy
precip totals in the 3-5"+ range across these areas, with maximum
totals in the 6-10 inch range across far western NC into the
Upstate of SC. Changes to the risk areas were confined to their
western sides to account for areas where the heavy rain threat is
waning.

To the south of the maximum upslope precip area...there continues
to be potential for organized convection in the developing cold
frontal band southeast of the surface center across northwestern
FL as enough moisture, effective bulk shear, and instability
remains to fuel organized convection. Hourly rain amounts to 3" are
possible, though the thunderstorm area should be somewhat
progressive. Additional 5" totals are possible.

Frictional convergence along the coast favors the potential for
very heavy totals along the immediate northeast SC and NC coastal
regions. Both the HREF and RRFS neighborhood probabilities are high
for 2 and 3"+ totals, with isolated maximum values in excess of 5"
possible.


...Southwest...
PW values will be increasing towards 2" as moisture associated
with Rachel is drawn northeastward.  More concentrated precip
areas likely after 0300 UTC as Rachel approaches the northern
Baja/Southern CA coast and a more favorable right entrance jet
region develops northeastward into the Great Basin. Probabilities
of 2"+ through early Sunday morning are highest in far southern CA,
where a Moderate Risk of flash flooding/excessive rainfall remains.
Burn scars, dry washes/arroyos, and urban areas would be most at
risk from the heavy rainfall expected.

Roth


Day 2
Valid 12Z Sun Oct 11 2026 - 12Z Mon Oct 12 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN THE
SOUTHWEST...

2030Z Update...
Widespread coverage of heavy rainfall still on track from
California into the Great Basin and Southwest US as anomalous deep
moisture streams across the region. With comparable QPF amounts
with previous amounts...only minor nudges were needed to the
boundaries of the outlook areas. Elsewhere there may still be
flooding concerns with the remnants of Isaias before it heads off-
shore.

Bann

Previous Excessive Rainfall Discussion.,.

A widespread heavy rainfall threat will continue across a large
region from Southern California, into the Great Basin and portions
of the Southwest on day 2. The combination of the remnants of
Rachel and the continued very favorable right entrance region jet
dynamics over the Great Basin will keep PW anomalies very high day
2, 2 to 4+ standard deviations above the mean. Model consensus is
for widespread heavy precip totals across these areas, with a
moderate risk maintained from Southern CA into southern NV,
including the Las Vegas region, far northwest AZ and far southwest
UT. A broad slight risk area continues northeastward into the
Great Basin, with a northeastward extension of this slight risk
area made from the previous issuance into far northeast NV,
northern UT and far southeast ID to match the model consensus heavy
qpf axis.

The remnants of Isaias will produce widespread moderate to heavy
precipitation from the Upper OH Valley, Central Appalachians, Mid-
Atlantic, Northeast and coastal NC day 2. Aside from isolated
urban runoff issues, believe there will not be any excessive
rainfall issues across these areas. There may be some convection
along the immediate NC coast Sunday, with isolated runoff issues
possible.

Oravec


Day 3
Valid 12Z Mon Oct 12 2026 - 12Z Tue Oct 13 2026

...THERE IS A Slight RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE GREAT BASIN AND THE SOUTHWEST...

2030Z Update...
Few changes were needed to the previously-issued Excessive Rainfall
Outlook.

Bann

Previously Issued Excessive Rainfall Discussion...

While the remnants of Rachel will have accelerated rapidly
northeast day 2 away from the Southwest and Great Basin, a strong
amplified mid to upper level trof will continue across the Great
Basin into central to southern California. This will maintain
above average PW values to the east of this upper trof from
Southern California into the Southwest and Great Basin, although
values will be lower than the day 1 and 2 time period. Model
consensus is for additional moderate to heavy precip totals from
northwest Arizona into eastern Nevada and much of Utah. While
precip totals are much lower than the day 2 period, a slight risk
was maintained from northwest Arizona into much of Utah along the
Wasatch as there will likely be overlap of heavy rain areas day 2
with the potential day 3 moderate to heavy totals.

Oravec


Day 4 and Day 5

Valid 12Z Tue Oct 13 2026 - 12Z Thu Oct 15 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
ARIZONA INTO UTAH MONDAY INTO EARLY WEDNESDAY...

...Western U.S...

The previous discussion remains in good shape. Only adjustments
were to expand the Marginal Risk into southern Arizona where
anomalous moisture coupled with saturated soils could support a
very localized flash flood threat. The rest of the western U.S. Day
4 threat areas remain on track.

Mullinax

---Previous Discussion---

Remnants from Rachel plus the influence of a mature upper level low
centered over the Great Basin will maintain another active period
with convection centered over the Mogollon Rim in AZ up through UT
where the model consensus continues to depict scattered heavy
convective prospects in-of the terrain and the valley areas to the
north. The good news compared to previous periods is the remaining
energy from Rachel will be diffuse by this juncture of the
forecast, however the upper level progression just to the east will
be plentiful in providing a continued ascent regime capable of
maintaining a broad convective threat with locally heavy rainfall
thanks to PWATs lingering between 1-2 standard deviations above
normal. Considering the environment prior, this is progress to
waining the worst of the flash flood threat that will ensue over
the weekend. That said, antecedent soils may be pretty well-
saturated by this point leading to perhaps a bit of an easier
chance for flash flooding despite rates and expected rainfall
accumulations being lower than previous periods. A broad MRGL risk
remains over the interior west with an embedded SLGT still
lingering over northwest AZ up through much of UT where the best
mid and upper level forcing is progged for much of Tuesday before
the threat finally ends heading into mid-week.

Kleebauer

...Southern High Plains into the Central Plains...

Day 4...

The rationale for the Day 4 ERO remains on track, so other than a
minor expansion of the Marginal Risk, no additional risk upgrades
were made this cycle. Some heavier QPF could occur towards the end
of the period, but confidence in the timing as to where the
heaviest QPF would occur in late Day 3 or early Day 4 was low to
consider an upgrade as of this forecast cycle.

Mullinax

---Previous Discussion---

An increasing threat of heavy rainfall will be brought into the
Central and Southern Plains by D4 thanks to the introduction of
remnant energy and moisture advection from now Hurricane Simon in
the Eastern Pacific off Mexico. The current upper pattern depicts
Simon taking a turn and moving due north through Mexico with its
primary energy becoming sheared as it migrates poleward across
northern Mexico and eventually makes headway into the CONUS.
Current ensembles are in agreement for the energy of Simon to
maneuver through Coahuila and up through southwest TX before
pivoting north-northeast around a broad ridge centered over the
Southeast U.S. This will bring the main energy, albeit sheared and
weaker than the prior 72 hrs, across the Central and Southern
Plains leading to a focused area of ascent to couple with elevated
PWATs bring brought into the area. A surface ridge to the north
will only act to increase low-level convergence in tandem with the
primary energy ejection leading to an area of convection spawning
after 00z with the introduction of a budding nocturnal LLJ. Models
are consistent with a signal for 1-3" of rainfall in areas where
convection develops with a few pockets with higher totals. Ensemble
mean QPF is much lower overall, so the maxima is more in line with
the deterministic output from the global suite. In any case, this
signal is reputable for a broad MRGL risk with the alignment of the
risk basically right within the expected path of the mid-level
energy as it moves north-northeast into the CONUS. The best
potential will likely be over OK/KS into MO thanks to the added
benefit of the low-level convergence regime being introduced with
the combination LLJ and surface ridge to the north.

Kleebauer

Day 5...

Latest 06Z and 12Z ensemble guidance have trended wetter in parts
of the Southern Plains, spear-headed by the EC-AIFS guidance. EC-
AIFS members show hints of Simon maintaining a surface circulation
by 12Z Wednesday just off the Mexico coast and its moisture being
directed into the South Central U.S.. This is thanks to the
interaction between the anomalous trough in the West and the
impressive upper-level ridge over the Southeast. The newest 12Z
EC-AIFS ensembles show a stronger 500mb low than subsequent cycles
over the Intermountain West, with pockets of 30-50% for >1" of
rainfall from the Permian Basin to as far north as eastern KS and
western MO for Day 5. Individual deterministic model members show
some of the higher-end potential from this setup with 2-4" totals
showing up across various global models. Where some of these higher
amounts come to fruition remains unclear, but the ECMWF does shows
PWs approaching monthly record levels in the Ozarks and eastern
OK. This implies that with sufficient instability and excellent
upper-level forcing via right-entrance region jet streak dynamics
aloft, that excessive rainfall rates are a growing concern from
the Southern Plains to the Middle MS Valley. With increased QPF
becoming focused farther south, the Slight Risk was extended into
West Texas, and the Marginal Risk in the Northern Plains was
dropped given drier trends.

Mullinax


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