Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Wed Oct 7 2026
Day 1
Valid 16Z Wed Oct 07 2026 - 12Z Thu Oct 08 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE LOWER
TEXAS COAST AND MUCH OF THE FLORIDA PENINSULA...
16Z Update...
...Lower Texas Coast...
A small Marginal Risk was added along the southern portion of the
Lower Texas Coast where persistent northeasterly flow is expected
to support streaming showers and thunderstorms that may produce
isolated heavy accumulations and runoff concerns. The Marginal Risk
highlights the area where the HREF shows high probabilities for
amounts exceeding 3 inches through the remainder of the period.
Also within this area, the HREF shows some 20-40 percent
probabilities for amounts exceeding 3-hr FFGs later this morning
into the early afternoon.
Pereira
Previous discussion...
...Florida...
A very slow moving, but strong front will remain draped across the
Florida Peninsula today, specifically over the northern Florida
Peninsula. A tropical air mass to the south of the front, over the
Peninsula, characterized by PWATs above 2.25 inches will be
uplifted against a much cooler and drier air mass to the north.
This clash of air masses will promote a continued rinse- and-
repeat pattern, with widespread light to moderate rainfall
continuing along I-4, and generally south of I-10. As on previous
days, localized convection will remain possible. Where convection
forms, the abundance of moisture will support locally heavy
rainfall. The sluggish movement of any convection will prolong any
associated heavy rainfall. Thus, a Marginal Risk remains in place
for the possibility that any convection that develops will impact
an urban or specifically flood-prone area.
...Four Corners Region...
Widely scattered showers and a few thunderstorms are possible
across the Four Corners Region this afternoon. Instability values
are expected to peak on either side of 500 J/kg, which could allow
for any storms that form to produce locally heavy rainfall.
Convective coverage appears to remain isolated, so the subsequent
flash flooding potential appears to be sub-Marginal, but the area
will need to be monitored.
Wegman
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Wed Oct 7 2026
Day 2
Valid 12Z Thu Oct 08 2026 - 12Z Fri Oct 09 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF
FLORIDA...
20Z Update...
No significant changes were made to the previous forecast.
Pereira
Previous discussion...
A stationary front draped across north Florida to start the day on
Thursday will begin to retreat northwestward into the Southeast
through Thursday night. Increasing southeasterly flow ahead of the
circulation of T.S. Isaias will overcome any reinforcement of the
drier air to the north, allowing the front to begin moving.
Tropical air to the south of the front running into it will allow
for the continuation of light to moderate rain across much of the
Florida Peninsula. As on previous days, and with increasing
moisture advection, localized convection and still generally slow
storm movements will make for an isolated flash flooding threat.
Late Thursday night, the outermost rain bands associated with T.S.
Isaias will begin to sweep northwestward across the Apalachicola
area, which could lead to an isolated flash flooding threat.
Wegman
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Wed Oct 7 2026
Day 3
Valid 12Z Fri Oct 09 2026 - 12Z Sat Oct 10 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR FAR SOUTHERN
ALABAMA AND THE WESTERN FLORIDA PANHANDLE...
20Z Update...
The daytime update continued some of the overnight adjustments --
most notably trimming away some of the western extent of previous
outlook areas and creating a tighter gradient. This is a reflection
of a slight eastern shift in the preferred track guidance and the
model indication of drier air wrapping around the west side of the
system as it moves inland. The Moderate Risk was maintained along
the Gulf Coast and expanded a little farther north and east to
cover a larger portion of the Florida Panhandle and southern
Alabama. Following landfall, this area is expected to remain a
focus of heavy rainfall that will be supported by a long axis of
onshore flow remain centered over the area into early Saturday.
Pereira
Previous discussion...
The circulation of T.S. Isaias, likely as a hurricane, will
approach the Gulf Coast, make landfall, and move inland across the
Southeast Friday and Friday night. See the latest forecast from the
National Hurricane Center at www.hurricanes.gov on the track
forecast.
Rainfall-wise, increasing southwesterly shear over the storm and an
accelerated forward motion will make the rainfall shield along and
east of the track become increasingly lopsided, leaving a sharp
gradient of rainfall on the west side. With continued eastward
nudges to the track, implying a faster forward motion, the
inherited Day 3/Friday forecast was significantly changed with this
update. The Slight Risk was expanded well to the north, now
including Huntsville, AL and Atlanta. Meanwhile, New Orleans, and
all of Louisiana for that matter, look to dodge this cannon ball.
Thus, the Louisiana Gulf Coast was downgraded to a Marginal, and
will likely to continue to be trimmed with future updates.
A Moderate Risk was introduced with this update for far southern
Alabama, including Mobile, as well as the western Florida
Panhandle. It remains probable with the aforementioned eastward
shifts in the track, that the Moderate may be too far west, and the
Mobile area may be able to be dropped from the Moderate with future
updates. Conversely, the Moderate will likely be expanded north
with future updates to include much of central Alabama and possibly
western Georgia. The Moderate was drawn to focus on the Gulf Coast
for now as this area has been included in all of the guidance for
getting hit with very heavy rainfall associated with the core
eyewall of the storm.
There remains a contingent of guidance that keeps the storm further
west and with a slower forward motion. While this appears to be
increasingly unlikely, to account for this possibility, the
Moderate was limited in its northward extent in case the storm
takes its time making it to the coast.
A faster forward speed may necessitate a much higher flooding
threat with future updates for portions of the highly flood-prone
southern Appalachians. As the center of circulation tracks due
north into the Tennessee and Ohio Valleys, an upslope southeasterly
flow into the western Carolinas and north Georgia could pose a
significant flash flooding threat for that region, though for now
that threat is greatest in the Day 4/Saturday time frame.
Wegman
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Wed Oct 7 2026
Day 4 and Day 5
Valid 12Z Sat Oct 10 2026 - 12Z Mon Oct 12 2026
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE TENNESSEE VALLEY AND SOUTHEAST SATURDAY, AND THEN ACROSS PARTS
OF THE SOUTHWEST ON SUNDAY...
20z Update: Only minor changes needed to the risk areas associated
with Isaias. The Slight risk was combined across portions of MS/AL/TN
to account for some eastward trend in the storm track and
uncertainty on the track with this inland extent. A higher end
Slight risk remains for portions of the southern Appalachians. It
does look like some instability gets into these areas, which
combined with upslope enhancement, should lead to an area of 3-5"
and scattered flash flooding. Some of this could be locally
significant, but the forward speed of the system will hopefully
keep more widespread higher end impacts at bay. At the moment NBM
probabilities of exceeding 5" are pretty low, supporting holding at
higher end Slight. Can not rule out an eventual MDT risk upgrade,
especially if rainfall amounts trend more towards ~6-8", which
while a low chance at the moment, is still a possibility given the
degree of moisture transport into the terrain...so we will continue
to monitor.
A slow moving band of convection may extend south across GA and
into the FL Panhandle on saturday. This could lead to locally
significant flood impacts, but too much uncertainty on location and
magnitude to justify anything higher than a Slight risk at this
time.
Out West, we added a Marginal risk to AZ on day 4 (Saturday).
Isolated to scattered convection appears likely as higher moisture
begins to advect into the area. What remains of Rachel and the
abundant tropical moisture associated with it, will impact more of
the Southwest on Sunday. New models remain in line with the
overnight forecast, so little to no change was needed to the
inherited Slight risk area on day 5 (Sunday).
Chenard
...Previous Discussion...
Day 4 (12Z Sat-12Z Sun) -- There was a significant departure from
yesterday's Day 5 ERO, largely due to the trend in the guidance
(led by the AI ECMWF and GFS Guidance), which align with the
official NHC forecast track of T.S. Isaias. There are three main
areas of concern (thus 3 separate Slight Risk areas) -- one across
northern MS-western TN and far NW AL, associated along and just
west of track of Isaias's circulation which by that time will be
going through extratropical transition with sharpening mid-level
deformation on the western side. The other areas include the south
and east facing slopes of the southern Appalachians with the strong
upslope south-southeast flow (850 mb wind anomalies some 5+
standard deviations above normal) along with the high-PW
environment east of Isaias's remnant low (PWs ~2.5 standard
deviations above normal per the GEFS). Over this area, the ERO risk
a bit more concerning with much of the guidance indicating amounts
between 2.5-5.0+ inches on day 4, which is why we included a 25%
area or "high-end Slight" in the ERO. This area will need to be
monitored for a potential upgrade to Moderate Risk in later
outlooks.
Lastly for Day 4, included another separate Slight Risk along the
Carolina coast, given the high PW environment (PWs up to 2.25-2.5")
along with an uptick in deep-layer instability with strengthening
trajectories advecting off the Gulf Stream.
Day 5 (12Z Sun-12Z Mon) -- The Eastern U.S. will not the only part
of the country that will be affected by a remnant tropical
cyclone. Much of the global guidance, led by both the GFS, ECMWF
and their AI versions, show the remnants of Rachel having an impact
across parts of the Southwest into the Lower Great Basin. The
latest GEFS/ECENS/CMC ensemble output show the highest QPF
exceedance probabilities across southeast CA-southern NV into
southwest UT and western AZ, where PW anomalies peak between 4-5
standard deviations above normal while 850 mb wind and 850-700 mb
moisture flux standardized anomalies reach +5. Most certainly
sufficient multi-model support for at least a Slight Risk (which is
supported by the GEFS first-guess ERO), however for now have
refrained from including anything higher given (a) the event still
over 4 days out and (b) the degree of instability (and thus
rainfall rates) is too much of a unknown at this point.
Hurley
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Wed Oct 7 2026
Day 4 and Day 5
Valid 12Z Sat Oct 10 2026 - 12Z Mon Oct 12 2026
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE TENNESSEE VALLEY AND SOUTHEAST SATURDAY, AND THEN ACROSS PARTS
OF THE SOUTHWEST ON SUNDAY...
20z Update: Only minor changes needed to the risk areas associated
with Isaias. The Slight risk was combined across portions of MS/AL/TN
to account for some eastward trend in the storm track and
uncertainty on the track with this inland extent. A higher end
Slight risk remains for portions of the southern Appalachians. It
does look like some instability gets into these areas, which
combined with upslope enhancement, should lead to an area of 3-5"
and scattered flash flooding. Some of this could be locally
significant, but the forward speed of the system will hopefully
keep more widespread higher end impacts at bay. At the moment NBM
probabilities of exceeding 5" are pretty low, supporting holding at
higher end Slight. Can not rule out an eventual MDT risk upgrade,
especially if rainfall amounts trend more towards ~6-8", which
while a low chance at the moment, is still a possibility given the
degree of moisture transport into the terrain...so we will continue
to monitor.
A slow moving band of convection may extend south across GA and
into the FL Panhandle on saturday. This could lead to locally
significant flood impacts, but too much uncertainty on location and
magnitude to justify anything higher than a Slight risk at this
time.
Out West, we added a Marginal risk to AZ on day 4 (Saturday).
Isolated to scattered convection appears likely as higher moisture
begins to advect into the area. What remains of Rachel and the
abundant tropical moisture associated with it, will impact more of
the Southwest on Sunday. New models remain in line with the
overnight forecast, so little to no change was needed to the
inherited Slight risk area on day 5 (Sunday).
Chenard
...Previous Discussion...
Day 4 (12Z Sat-12Z Sun) -- There was a significant departure from
yesterday's Day 5 ERO, largely due to the trend in the guidance
(led by the AI ECMWF and GFS Guidance), which align with the
official NHC forecast track of T.S. Isaias. There are three main
areas of concern (thus 3 separate Slight Risk areas) -- one across
northern MS-western TN and far NW AL, associated along and just
west of track of Isaias's circulation which by that time will be
going through extratropical transition with sharpening mid-level
deformation on the western side. The other areas include the south
and east facing slopes of the southern Appalachians with the strong
upslope south-southeast flow (850 mb wind anomalies some 5+
standard deviations above normal) along with the high-PW
environment east of Isaias's remnant low (PWs ~2.5 standard
deviations above normal per the GEFS). Over this area, the ERO risk
a bit more concerning with much of the guidance indicating amounts
between 2.5-5.0+ inches on day 4, which is why we included a 25%
area or "high-end Slight" in the ERO. This area will need to be
monitored for a potential upgrade to Moderate Risk in later
outlooks.
Lastly for Day 4, included another separate Slight Risk along the
Carolina coast, given the high PW environment (PWs up to 2.25-2.5")
along with an uptick in deep-layer instability with strengthening
trajectories advecting off the Gulf Stream.
Day 5 (12Z Sun-12Z Mon) -- The Eastern U.S. will not the only part
of the country that will be affected by a remnant tropical
cyclone. Much of the global guidance, led by both the GFS, ECMWF
and their AI versions, show the remnants of Rachel having an impact
across parts of the Southwest into the Lower Great Basin. The
latest GEFS/ECENS/CMC ensemble output show the highest QPF
exceedance probabilities across southeast CA-southern NV into
southwest UT and western AZ, where PW anomalies peak between 4-5
standard deviations above normal while 850 mb wind and 850-700 mb
moisture flux standardized anomalies reach +5. Most certainly
sufficient multi-model support for at least a Slight Risk (which is
supported by the GEFS first-guess ERO), however for now have
refrained from including anything higher given (a) the event still
over 4 days out and (b) the degree of instability (and thus
rainfall rates) is too much of a unknown at this point.
Hurley
