Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
830 PM EDT Thu Oct 8 2026
Day 1
Valid 01Z Fri Oct 09 2026 - 12Z Fri Oct 09 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS CENTRAL
FLORIDA...
01Z Update...
With showers and thunderstorms on-going across portions of the central
and northern Florida peninsula...no fundamental changes were made
to either the Slight Risk or the surrounding Marginal risk area
even though there were a few minor adjustments based on latest
radar imagery.
Bann
Previous Excessive Rainfall Discussion
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
428 AM EDT Fri Oct 9 2026
Day 1
Valid 12Z 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...
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 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 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Fri Oct 9 2026
Day 2
Valid 12Z Sat Oct 10 2026 - 12Z Sun Oct 11 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
SOUTHEAST...
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST
CALIFORNIA AND FAR SOUTHERN NEVADA...
...Southeast (Isaias)...
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
428 AM EDT Fri Oct 9 2026
Day 3
Valid 12Z Sun Oct 11 2026 - 12Z Mon Oct 12 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE SOUTHWEST...
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.
Lamers
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Fri Oct 9 2026
Day 4 and Day 5
Valid 12Z Mon Oct 12 2026 - 12Z Wed Oct 14 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
ARIZONA INTO UTAH ON DAY 4...
Day 4 (12Z Mon - 12Z Tue)...
The threat for widespread convection continues for another period
as lingering elevated PWATs leftover from Rachel and the general
advection regime maintains a signal for scattered flash flood
prospects across the Mogollon Rim up through UT. Recent ensemble
trends have converged on a general maximum located within the
terrain of both AZ/UT with a northern inflection into southeastern
ID and southwestern WY. Instability fields coincide with a remnant
moisture source leading to typical diurnal flare up of convection
during peak diurnal destabilization. There's also assistance from a
maturing upper trough over the Western U.S. with the latest
guidance inferring on the trough becoming closed off over the Great
Basin leading to enhanced upper level diffluence downstream of the
eventual upper level low (ULL). This would put the aforementioned
areas within the area of concern for regional ascent to couple with
the steadily favorable thermodynamic posture left behind from
Rachel. The terrain over central AZ up into southwestern and
northern UT are the areas highlighted by an areal average of 1-2"
precipitation on the latest forecast. Deterministic outputs are
closer to 3-4" for the max in these zones with the best threat
located over portions of the Wasatch, Uinta's, and the canyon
regions in southwestern UT. This provided enough of a signal to
warrant a broad SLGT risk issuance over the above areas with a
MRGL risk encompassing, including areas of NV, southern ID, into
western CO as convective threat remains prevalent with the
lingering elevated moisture.
Day 5 (12Z Tue - 12Z Wed)
We'll finally see a decay in the regional moisture field come
Tuesday with ensemble anomaly plots showing PWATs dipping closer to
the seasonal average, perhaps a bit above normal, but closer to +1
standard deviations instead of the 2.5-3 deviations in place the
periods prior. This alone will decrease the risk of more
appreciable precipitation over the Great Basin into the
Southwestern U.S, but the upper level pattern will keep the period
active as our ULL over the west will continue to dictate the
precipitation pattern. Our eventually closed upper reflection will
slowly shift eastward through the Great Basin with strong mid and
upper level forcing mechanism positioned for much of the interior
west come Tuesday through at least Wednesday morning. The
positioning of the low over NV into eventually UT is textbook for
scattered to widespread convective potential with the primary
driving being the continued upper forcing ahead and underneath the
ULL circulation. Models are trending in the direction of locally
heavy rainfall prospects in the area of UT with the signal most
notable over the northern half of the state thanks to being on the
northern periphery of the upper low where ascent is maximized.
Totals reflected currently are between 1-2" which when coupled with
what will fall the periods prior will need to be assessed for
potential concerns for flash flooding as grounds may very well be
saturated leading in.
Given the growing signal of the ULL evolution and expectation of at
least scattered convection under the influence of the low over the
interior west, a broad MRGL risk has been added for the D5 time
frame. The MRGL is very similar in outline to the previous period
with a chance for an upgrade as we move closer, especially if
guidance maintains continuity or increases magnitude of the
rainfall as we move closer.
Kleebauer
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Fri Oct 9 2026
Day 4 and Day 5
Valid 12Z Mon Oct 12 2026 - 12Z Wed Oct 14 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
ARIZONA INTO UTAH ON DAY 4...
Day 4 (12Z Mon - 12Z Tue)...
The threat for widespread convection continues for another period
as lingering elevated PWATs leftover from Rachel and the general
advection regime maintains a signal for scattered flash flood
prospects across the Mogollon Rim up through UT. Recent ensemble
trends have converged on a general maximum located within the
terrain of both AZ/UT with a northern inflection into southeastern
ID and southwestern WY. Instability fields coincide with a remnant
moisture source leading to typical diurnal flare up of convection
during peak diurnal destabilization. There's also assistance from a
maturing upper trough over the Western U.S. with the latest
guidance inferring on the trough becoming closed off over the Great
Basin leading to enhanced upper level diffluence downstream of the
eventual upper level low (ULL). This would put the aforementioned
areas within the area of concern for regional ascent to couple with
the steadily favorable thermodynamic posture left behind from
Rachel. The terrain over central AZ up into southwestern and
northern UT are the areas highlighted by an areal average of 1-2"
precipitation on the latest forecast. Deterministic outputs are
closer to 3-4" for the max in these zones with the best threat
located over portions of the Wasatch, Uinta's, and the canyon
regions in southwestern UT. This provided enough of a signal to
warrant a broad SLGT risk issuance over the above areas with a
MRGL risk encompassing, including areas of NV, southern ID, into
western CO as convective threat remains prevalent with the
lingering elevated moisture.
Day 5 (12Z Tue - 12Z Wed)
We'll finally see a decay in the regional moisture field come
Tuesday with ensemble anomaly plots showing PWATs dipping closer to
the seasonal average, perhaps a bit above normal, but closer to +1
standard deviations instead of the 2.5-3 deviations in place the
periods prior. This alone will decrease the risk of more
appreciable precipitation over the Great Basin into the
Southwestern U.S, but the upper level pattern will keep the period
active as our ULL over the west will continue to dictate the
precipitation pattern. Our eventually closed upper reflection will
slowly shift eastward through the Great Basin with strong mid and
upper level forcing mechanism positioned for much of the interior
west come Tuesday through at least Wednesday morning. The
positioning of the low over NV into eventually UT is textbook for
scattered to widespread convective potential with the primary
driving being the continued upper forcing ahead and underneath the
ULL circulation. Models are trending in the direction of locally
heavy rainfall prospects in the area of UT with the signal most
notable over the northern half of the state thanks to being on the
northern periphery of the upper low where ascent is maximized.
Totals reflected currently are between 1-2" which when coupled with
what will fall the periods prior will need to be assessed for
potential concerns for flash flooding as grounds may very well be
saturated leading in.
Given the growing signal of the ULL evolution and expectation of at
least scattered convection under the influence of the low over the
interior west, a broad MRGL risk has been added for the D5 time
frame. The MRGL is very similar in outline to the previous period
with a chance for an upgrade as we move closer, especially if
guidance maintains continuity or increases magnitude of the
rainfall as we move closer.
Kleebauer
