Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 PM EDT Thu Oct 8 2026
Day 1
Valid 16Z Thu Oct 08 2026 - 12Z Fri Oct 09 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS CENTRAL
FLORIDA...
Given the increasing signal in both the HREF and REFS for localized
very heavy amounts, parts of central Florida were upgraded to a
Slight Risk with the 16Z Update.
Thunderstorms developing along a stalled boundary in central
Florida -- extending from the Tampa Bay area to the Space Coast --
will be capable of producing rainfall rates in excess of 3 in/hr.
A very moist airmass (~2.25-2.50 inches), along with favorable
upper- level jet dynamics and cloud layer shear will combine to
support organized, highly-efficient, rain-producing storms. Given
the antecedent moist soils and the potential for training storms,
localized instances of flash flooding appear more likely,
especially in urbanized and poor drainage areas.
The Slight Risk reflects an axis where both the HREF and REFS show
high probabilities for 3 inches or more, with both indicating that
heavier amounts over 5 inches around the Tampa Bay area are likely.
Refer to WPC MPD #1460 for additional information regarding the
near-term threat for heavy rain and flash flooding in this area.
Pereira
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 PM EDT Thu Oct 8 2026
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...
20Z Update...
As with the expected track for Isaias, the ERO remains largely
unchanged with this update. The Moderate Risk was expanded a little
farther north and east, extending from the anticipated landfall
point near the western Florida-Alabama border over to the Florida
Big Bend and far southwestern Georgia where guidance continues to
indicate that a lingering inflow band may produce a prolonged
period of training storms and very heavy amounts. REFS neighborhood
probabilities for amounts exceeding 5 inches are greater than 70
percent within much of the Moderate Risk area. The only other
adjustment of note was to continue to trim some of the western
extend of the outlook as guidance continues to show drier air on
the western extent supporting a tight accumulation gradient.
Pereira
Previous discussion...
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 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 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 PM EDT Thu Oct 8 2026
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...
20Z Update...
Similar to Day 2, the overall messaging remains much the same, with
a few areas of higher concerns within a broad Slight Risk area
that extends from the Florida Big Bend region to the Tennessee
Valley, southern Appalachians, and the Southeast Coast.
Following the 12Z model trends, adjustments included shifting the
axis of the Slight Risk a little farther east across the Florida
Panhandle and the northern Peninsula into the interior Southeast.
Training storms along a persistent inflow band may continue to
elevate the threat for flash flooding from the eastern Panhandle-
Big Bend region northward across Georgia, into an area of
orographically-enhanced amounts covering portions of the southern
Appalachians. Currently considering this corridor a 'higher-end
Slight,' with potential upgrades forthcoming if forecast confidence
in the positioning of the inflow band increases.
Also, remaining a 'higher-end Slight' is much of the coastal
Carolinas, where strong inflow and onshore convergence, potentially
enhanced by a weak wave is expected to raise the threat for heavy
amounts and flash flooding. With the potential for the heaviest
amounts to remain confined to the immediate coast or just offshore,
opted not to upgrade to a Moderate with this update but will
continue to monitor.
Pereira
Previous discussion...
...Southeast...
The remnant circulation of Isaias 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 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
