FOUS30 KWBC 080823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Thu Oct 8 2026

Day 1
Valid 12Z Thu Oct 08 2026 - 12Z Fri Oct 09 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
FLORIDA PENINSULA...

A stalled out front demarcating the boundary between a tropical air
mass over the Gulf and a much cooler and drier one to the north
over much of the Eastern U.S. will remain the primary driver for
continued rainfall across the Florida Peninsula through tonight.
Localized convection embedded within the broader rainfall shield
will be the cause of any localized flash flooding. Of course, soils
are fully saturated over north Florida after multiple consecutive
days of rain over the area. Thus, continued light to moderate rain
in most areas, with pockets of heavier rain will largely convert to
runoff. For most areas, the sandy soils and flat terrain should be
able to handle any rainfall, especially since most of it will be
light to moderate in intensity. Where any convection develops is
where there is the best chances for isolated flash flooding,
especially if that happens to occur over a larger city such as
Tampa or Orlando. Thus, just as on previous days, any flash
flooding will be isolated, though broader flooding issues are
possible in poor-drainage and urban areas.

Wegman
 

Day 2
Valid 12Z Fri Oct 09 2026 - 12Z Sat Oct 10 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR SOUTHERN
ALABAMA AND THE WESTERN FLORIDA PANHANDLE...

Hurricane Isaias will make its way out of the central Gulf, make
landfall near the Alabama/Florida border, then continue north
across Alabama on Friday. See http://www.hurricanes.gov
for the latest track forecast.

As regards rainfall, increasing southwesterly shear over Isaias as
it makes landfall will push most of the associated rainfall east
and northeast of the center. Thus, much of the higher risk area are
along and east of the track. Frictional convergence on the
southerly flow east of the center will locally enhance rainfall
rates from Mobile east along the coast and into the Florida
Panhandle. 3-7 inches of rain, with localized totals up to 10
inches are forecast. Considering the soils are already wetter than
normal over much of this area from recent rainfall, this amount of
heavy rain... much of it occurring in a 12-18 hour period will
cause numerous flash floods, some of which will be significant.
Urban areas such as Mobile and Pensacola are at greatest risk. Also
at risk are areas around any concave shaped bays (which most are),
that can funnel any storm surge up the rivers and block any
drainage of rainfall.

As the track of Isaias is little changed, the rainfall totals are
also little changed. The risk areas over this region are also
little changed. Isaias will be moving at a steady clip as it moves
into the Southeast, so for most areas, any heavy rains will be
relatively short in duration. This fast movement will be the saving
grace preventing much more widespread and significant flooding
issues. As the center moves inland Friday night, the heaviest rains
will pivot northwest of center with the greatest forcing into the
drier air mass to the north, as well as in lingering feeder bands
that will promote localized training storms. The Slight Risk area
continues to highlight much of the rest of Alabama, eastern
Mississippi, and western Georgia. Frictional convergence will
increase in intensity along the Atlantic coast of Georgia and South
Carolina in the predawn hours Saturday morning. This phenomenon
will be just beginning, so the Marginal Risk that extends to the
Atlantic coast looks good.

Wegman
 

Day 3
Valid 12Z Sat Oct 10 2026 - 12Z Sun Oct 11 2026

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

...Southeast...

The remnant circulation of Isais over the Southeast will rapidly
complete extratropical transition on Saturday. With the storm
exhibiting much more of a classic comma or T shape, the heaviest
rains will elongate more east to west. To the west, the bulk of the
circulation will track across central Tennessee, with heavier rains
expanding northwest to the Mississippi River near Memphis. The
feeder bands from the Gulf will be nearly stationary from the
Florida Panhandle near Apalachicola north over much of Georgia.
Finally, the bulk of Isaias' moisture will track northeast up into
the Carolinas.

Within the large Slight Risk area, there are several higher risk
areas worth noting. In the Florida Panhandle, feeder bands
impacting many of the same areas that the core or at least the
eastern portion of the core hit on Friday will continue through
much of Saturday and possibly into Saturday night. While rainfall
amounts will be significantly lower on Saturday as compared to
Friday, especially on a larger scale basis, any storms will be
capable of localized rates to 2.5 inches/hour. Rates that high will most
likely be in any convection that is closer to the Gulf, where much
of the instability will be.

In the southern Appalachians, there will be a 12-18 hour period
when the center of Isaias over Tennessee will support screaming
winds at several thousand feet up, or for the Appalachians, around
mountain top level. Winds will be in the 40-50 kt range. This will
favor significant upslope along the mountains. Thus, much higher
rainfall amounts are expected into the western Carolinas. The short
time these conditions will be favorable, despite a complete absence
of instability, will save this area from much more significant
flash flooding. For this reason, no Moderate Risk upgrade is
included with this update, though that area is in a higher-end Slight.

Finally, along the Carolina coast, significant frictional
convergence will favor much heavier rainfall amounts from about
Hilton Head, SC north to around Morehead City, NC. Alone, this may
favor a lower-end Slight, with widespread 2-4 inches of rain
expected, and localized higher amounts to 6 inches. However, as
Saturday is the New Moon, the king tides, especially Saturday
morning, will be particularly high. While the high tide cycle will
be slightly muted Saturday evening, the heavy convection will be in
full force at that time. Thus, expect a combination of tidal and
rainfall/fresh-water flooding to impact coastal communities. For
this reason, a higher-end Slight has been included with this update.

...Southwest...

On the other side of the country from Isaias, a deep positively
tilted trough over the West Coast will begin to pick up deep
tropical moisture from the Pacific and advect it across much of the
Southwest on Saturday. Later on, the remnant circulation and
associated moisture of Rachel will move in, though that's far more
of a concern on Day 4/Sunday. A predecessor rainfall event of sorts
will begin on Saturday. Strong southwesterly flow will uplift the
moisture being transported into the lower Colorado River Valley, as
well as further north across much of Utah. Any east-west oriented
ranges will be particularly exposed to the incoming moisture, where
upslope will locally increase rainfall amounts. Instability will be
a limiting factor, due mostly to extensive cloud cover and the
weakening October sun. Nonetheless, since many of the areas of the
Southwest are simply poorly equipped to handle a surge of tropical
moisture, a large Marginal was expanded across Utah to as far north
as southeastern Idaho. Further north into the Yellowstone region,
enough cold air will be in place to favor snow, which cuts the
flooding risk significantly.

Wegman
 

Day 4 and Day 5
Valid 12Z Sun Oct 11 2026 - 12Z Tue Oct 13 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEAST
CALIFORNIA INTO SOUTHERN NEVADA, WESTERN ARIZONA AND SOUTHERN
UTAH...

Day 4 (12Z Sun - 12Z Mon)...

The closed low off the southern CA coast will help push 
Rachel or its extra-tropical remnants northeastward into southern 
CA/western AZ and the southern Great Basin. While confidence is not
high on the models handling of whatever surface center still 
remains, there is higher confidence that anomalous PW values will 
push northeastward day 4 across Southern CA into the Southwest and 
Greate Basin. A fairly broad slight risk for day 4 was maintained 
across southeast CA/western AZ into southern NV and southern UT 
where model consensus is for widespread heavy rainfall potential. 
The previous slight risk was expanded farther into southern UT to 
include the sensitive slot canyon regions.

Across the coastal Mid-Atlantic...the extra-tropical remnants of
Isaias will continue to weaken day 4. While heavy rain is possible
day 4, believe it will be over several hour period, precluding any
significant flash flood threat. A marginal risk was maintained
primarily for the urban corridor from NYC to DC....decreasing the
western extent of it. The marginal risk was extended farther
south into eastern NC where there is potential for convection along
and ahead of the associated eastward moving cold front. 

Day 5 (12Z Mon - 12Z Tue)

The mid to upper level trof along the West coast expected to
amplify further Monday into Tuesday. This will help to draw
Simon northward into western Mexico with anomalous PW values 3 to 
5+ standard deviations above the mean moving into the Southwest 
and Southern High Plains. A marginal risk is depicted day 5 where 
there will be modest instability in the anomalous PW axis. 
Additional moderate precip totals also likely day 5 from Southern 
CA, northeastward into the Great Basin, but with more stable
conditions expected, rainfall rates wil likley be lower and no risk
area depicted at this time.

Oravec
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