Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Fri Oct 9 2026
Day 1
Valid 01Z Sat Oct 10 2026 - 12Z Sat Oct 10 2026
...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FROM THE FLORIDA BIG
BEND INTO SOUTHERN GEORGIA...
Weakening Hurricane Isaias is expected to make landfall on the
Central Gulf Coast shortly not far from Eglin Air Force Base,
bringing a threat of heavy rainfall and flash flooding to the
region. Numerous instances of flash flooding are likely with this
cyclone, some of which will be significant. Core convection should
continue to diminish and rain rates north of the center should
decrease with a lack of available instability. The threat of flash
flooding near the track of the center for some distance inland lies
from the FL Panhandle into southern Alabama where a Moderate Risk
continues to exist. Gradients on the western edge across southwest
AL and MS were tightened, based on recent mesoscale guidance and
convective trends thus far.
Further east across the FL Big Bend, a training Band Of Deluge is
poised to be efficient from roughly 06z onward, whose incipient
form appears to have two foci -- one east of Apalachicola which is
moving eastward and another broken shower/thunderstorm band that
lies from offshore northwest FL into the Big Bend. This band has
much greater access to instability (2000+ J/kg of ML CAPE) and
more than sufficient effective bulk shear (25-50 kts) to organize.
Such bands have produced hourly amounts in the 3-6" range in other,
previous tropical cyclones in and near the Gulf Coast, and there's
little reason to believe this band will be any different. If the
rain band trains in a specific location for several hours -- as
hinted at by both the 18z HREF and 12z REFS -- double digit
rainfall totals will be possible (10+ inches), potentially the
event maximum storm total rainfall, as well as significant flash
flooding. While there has been recent dryness in the region and the
geology of the area favors quick percolation into the local
aquifer, 3-6" in an hour would be extremely hazardous for any
towns/transportation routes across the region. The signal in the
12z REFS/18z HREF along with conceptual models of such events in
the past led to coordination with the Tallahassee/TAE Forecast
Office to raise to a High Risk within this potentially dangerous
rain band.
Roth
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Fri Oct 9 2026
Day 2
Valid 12Z Sat Oct 10 2026 - 12Z Sun Oct 11 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST UNITED STATES...
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN
CALIFORNIA, NORTHWEST ARIZONA, SOUTHERN NEVADA AND SOUTHWEST UTAH...
21Z update...
...Southwest (Rachel)... A Moderate Risk area was raised for the
Peninsular Range and adjacent area as tropically enhanced rainfall
ahead of Rachel makes its way across southern California. Rainfall
rates of 1-3 inches will be possible, particularly for the
mountains of southern California.
...Southeast (Isaias)... A Moderate Risk of Excessive Rainfall
was introduced for portions of the western Carolinas southward into
northern Georgia given latest QPF trends. Guidance shows
persistent south-southeasterly flow up against the higher terrain
to help bolster the already tropically enhanced rainfall from
Isaias as it. Neighborhood probabilities for reaching/exceeding 5
inches is over 70% for much of this part of the region. There is
also a 60-70% likelihood that somewhere within that same area could
reach/exceed localized 8 inches, so confidence is fairly high that
there will be numerous areas with flash flooding from north-
central Georgia to western North Carolina. The exact location will
be dependent on the track of Isaias and its transition to being an
extratropical feature.
Campbell
Previous Forecast Discussion...
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
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Fri Oct 9 2026
Day 3
Valid 12Z Sun Oct 11 2026 - 12Z Mon Oct 12 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN THE
SOUTHWEST...
21Z update... Remnant energy of Rachel and tropically enhanced
rainfall will be lifting northeast during this period spreading
from southern California into the Great Basin and toward Central
and Northern Rockies. The greatest amounts will likely be in
proximity to the low track from southern California to northeast
Utah/southwest Wyoming where a Moderate Risk was raised for this
update.
Campbell
Previous discussion...
Regardless of the exact timing of the post-tropical transition of
Rachel as it approaches the Pacific coast and then moves into the
southwest U.S., the ingredients will be there for heavy rainfall
regardless. A broad Slight Risk was maintained over the region with
deep tropical moisture in place and models showing increasing
instability during the day on Sunday, particularly over the low
desert regions of southeast California, southern and western
Arizona, and southern Nevada. Additionally, increasing low-mid
level convergence just ahead of the track of Rachel (or
eventually the remnant circulation aloft), coinciding with the
combination of instability and deep moisture, should lead to a
focused area of convection. Some training of heavier rain rates may
occur and lead to narrow corridors of more significant rainfall and
flash flood potential. The area of greatest concern is from
Riverside County in southeast California into northwest Arizona,
southern Nevada (including the Las Vegas area), and then into the
slot canyon regions of southwest Utah. There is still some
uncertainty as to the exact track of Rachel, so that will play a
key role in the placement of the heaviest rainfall. However, as
uncertainty is reduced, it's possible a Moderate Risk upgrade could
be required in the highest confidence areas.
Lingering rainfall associated with Isaias will be will persist
along the Mid-Atlantic region and may lead to isolated areas of
flash flooding to sensitive areas. A Marginal Risk area was
maintained.
Lamers
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
