Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Fri Oct 9 2026
Day 2
Valid 12Z Sat Oct 10 2026 - 12Z Sun Oct 11 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
SOUTHEAST...

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST
CALIFORNIA AND FAR SOUTHERN NEVADA...

...Southeast (Isaias)...

The threat of heavy rainfall and flash flooding from Isaias will
continue over a large portion of the Southeast on Saturday and
Saturday Night. The latest outlook is very consistent with the
previous forecast, with mostly small changes to account for the
latest guidance and updated forecasts from NHC. See
www.hurricanes.gov for the latest forecast information on all
hazards associated with Isaias.

Although the Slight Risk area is large, there are several areas of
note where the threat is somewhat higher (on the high end of the 15
to 40 percent probabilistic range associated with that risk
category), and could warrant an upgrade to a Moderate Risk in the
future if guidance becomes more consistent and/or the potential for
locally significant rainfall and flash flooding increases.

1. Convective rain band east of the center of Isaias (Florida Big
Bend into much of Georgia). There is high confidence in a
convective rain band setting up about 200 miles east of the track
of the center of Isaias. This will ramp up Friday Night and likely
be ongoing on Saturday morning. Given the access to moderate
instability and precipitable water values approaching 2.5 inches,
the rain rates could be intense at times (2-3 inches per hour). The
key uncertainties are how far north the strongest convection will
be able to reach, and how stationary the band will be overall. The
00Z HRRR suggested some northern limit to the extent of the
instability due to a wedge of cooler air associated with a surface
ridge and northeasterly low-level flow from the Piedmont of the
Carolinas down into northern Georgia. However, other models suggest
the effective warm front will lift further north and bring
locations near Atlanta under risk of greater rain rates. This will
be monitored for the need for a risk upgrade.

2. Southern Appalachians (far northeast Georgia, far upstate South
Carolina, western North Carolina). Model QPF has been increasing in
general in this region; for example, the NBM QPF in the upslope
areas of western North Carolina has increased from a general 1-3
inches to 3-5 inches in the past 12 hours of updated runs. A
contributing factor is the event coming into view of the hi-res
model runs, of which many are showing more terrain enhancement and
a wetter forecast, but there are some overall adjustments to the
ensemble envelope with some trends in the overall forecast of
Isaias. Not all hi-res models are suggestive of the highest rain
rates reaching this region -- the 00Z HRRR kept instability
suppressed further south with a wedge of cooler air in place closer
to the mountains. However, other hi-res models are beginning to
suggest double digit totals are not out of the question depending
on the mesoscale details (particularly strength and orientation of
the low-mid level flow and available instability). It is noted here
that a Moderate Risk upgrade was strongly considered, but after
collaboration with WFO GSP and the uncertainties described here,
it was held at a high-end Slight Risk for now. Continued model
trends in a wetter direction and reduction in rainfall uncertainty
would likely drive an upgrade later today.

3. Coastal Carolinas. Significant coastal convergence will favor
occasional clusters or bands of low-topped convection capable of
producing efficient rainfall along the coastline of the Carolinas.
Although this area has been relatively dry of late, sustained heavy
rain rates could overcome unfavorable antecedent conditions to
produce a more concentrated area of heavy rainfall and flash
flooding. Additionally, hi-res ensembles (HREF and REFS) do place
the axis of heaviest rainfall offshore, although it would only take
a small displacement error to put this over the coastline.
Therefore, this area will also be monitored for a Moderate Risk,
especially with potential complications from higher than average
tides expected this weekend.

...Southwest (Moisture from Rachel)...

Scattered convection may occur during the daytime on Saturday over
the Southwest as precipitable water values increase in the tropical
moisture plume downstream of Rachel over the eastern Pacific. By
Saturday afternoon PWs over much of the region should be over the
99th percentile for early-mid October. However, the scattered
nature of the convection and lack of a prominent focusing mechanism
may keep the individual convective clusters or cells moving enough
and isolated enough to prevent more widespread instances of flash
flooding despite the arrival of the deep moisture.

Later in the forecast period -- on Saturday night -- the threat may
begin to ramp up in a more concentrated fashion from southeast
California into southern Nevada and northwest Arizona as Rachel
approaches the Pacific coast of Baja California. A Slight Risk was
added in these areas, as the RRFS and REFS members show potential
for a greater concentration of stronger convective bands and
clusters in an area of increasing convergence. This will likely
continue into the Day 3 period.

Lamers


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