FOUS30 KWBC 091704
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
104 PM EDT Fri Oct 9 2026

Day 1  Valid 16Z Fri Oct 09 2026 - 12Z Sat Oct 10 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN SOUTHERN
ALABAMA, THE FLORIDA PANHANDLE, AND SOUTHWEST GEORGIA...

16Z update... The latest hi-res guidance is suggesting an increase
in QPF amounts inland across Florida, Alabama and eastern
Mississippi as Isaias advances onshore, and into Georgia with the
convective feeder band that is expected to the east of the low
center. As of now that feeder band looks to be in the vicinity of
the Big Bend region. Rain rates with the immediate landfall may be
as high as 3-4 inches/hour before reducing to 1-3 inches/hour
during the overnight period as it tracks further north across the
Gulf states/Coastal Plains. The Moderate Risk was expanded into
central Alabama and across southwest Georgia. The high-end Slight
Risk was expanded into northwest Alabama and west-central Georgia.

Additional convection is expected along the coastline from
northeast Florida to South Carolina as tropical moisture advecting
northward interacts with the frontal boundary draped to the north.
Cells will be capable of yielding 1-2+ inches/hour rain rates
along this urban corridor. A Slight Risk was added to cover this
elevated threat for localized excessive rainfall and flash flooding.

Campbell

Previous discussion...

Hurricane Isaias is expected to make landfall on the Central Gulf
Coast later today, bringing a threat of heavy rainfall and flash
flooding to the region. Numerous instances of flash flooding are
likely, some of which will be significant. Overall, the ERO for
this forecast period is very consistent with the previous forecast.
See http://www.hurricanes.gov
for the latest forecast information on all hazards associated with Isaias.

The northern fringes of the rain shield associated with Isaias are
already beginning to approach the Gulf Coast, but the threat of
heavier rain rates will ramp up closer to the time of landfall and
thereafter, as the region of strongest instability in the southeast
quadrant of the system will also begin to reach the coast. Thus,
the threat of flash flooding should begin to increase markedly in
the latter half of the forecast period, or from late Friday
afternoon into Friday night.

The primary areas of concern are related to (1) higher rain rates
in the inner core and any eyewall convection that can be sustained
as Isaias pushes onshore, and (2) a convective rain band to the
east along the instability gradient.

As Isaias advances inland, the inner core convection should
diminish and rain rates near the center should decrease with a lack
of available instability. Therefore, the threat of flash flooding
near the track of the center will be highest closest to the coast,
particularly in the Moderate Risk area of southern Alabama and the
western Florida Panhandle. Heavy rain and flash flooding may extend
along the track of the center further north and northwest into
eastern Mississippi and central Alabama, but this will be highly
dependent on the track and overall organization of the system as it
reaches the coast. Shear will be increasing on Isaias; if it can
resist that shear and maintain a well-organized inner core, the
flash flood threat associated with the center may extend further
inland. For now the risk level in these areas further inland has
been held at a Slight Risk given the uncertainties.

Further east, a training band of convection is expected. This band
will have much greater access to instability, and the combination
of that with extremely deep tropical moisture will promote very
efficient rainfall and very high rain rates. If the rain band
trains in a specific location for several hours, that could easily
produce double digit rainfall totals (10+ inches), and potentially
the event maximum storm total rainfall, as well as significant
flash flooding. This would be about 200 miles east of the landfall
location, so people in the threat area are encouraged not to
concentrate too much on the landfall location to understand the
rainfall threat from Isaias. The main changes to the ERO in these
areas were to nudge the risk contours slightly to the east to
account for guidance that suggests the rain band could set up even
east of Tallahassee at times, and increase the area of the Moderate
Risk into southwest Georgia as it is likely the rain band will
affect areas at least 100 miles inland (possibly more).

Lamers
 

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

...Southeast (Isaias)...

...1700Z Update...
A Moderate Risk of Excessive Rainfall was introduced for portions
of the western Carolinas southward into northern Georgia given
latest QPF trends. Additional details will follow shortly.

Bann

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 http://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
$$
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