Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 PM EDT Thu Sep 24 2026
Day 1
Valid 16Z Thu Sep 24 2026 - 12Z Fri Sep 25 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
SOUTHEAST NEW MEXICO...
...16Z Update...
In collaboration with WFOs ABQ and EPZ, decided to upgrade the
existing Day 1 Slight Risk to a Moderate Risk for portions of
southeast New Mexico given a favorable parameter space for slow-
moving thunderstorms that are more than capable of producing heavy
rainfall, in addition to highly sensitive hydrological conditions
with very saturated soils given recent heavy rainfall over the
past few days. The latest trends in available hi-res guidance
suggest a focused potential for qausi-stationary convection over
parts of Lincoln, Chaves, and Otero counties this afternoon, with
the HREF highlighting neighborhood probabilities over 70% for both
exceeding 3" of rain and 10-year ARIs. Combined with NASA SPoRT
soil moisture percentiles exceeding the 98th percentile, a targeted
Moderate Risk upgrade felt like the prudent move to make in order
to bring attention to this higher end potential.
Elsewhere, the existing Marginal/Slight Risk across portions of the
central/northern Plains were trimmed southward given a lack of
appreciable instability, and generally stratiform rainfall with
only modest rates/intensity. Other changes to the inherited
Marginal and Slight Risk were generally minor and cosmetic only.
Miller/Wegman
...Previous Discussion...
Heavy rain will remain in the picture for much of the southwest,
as mid-level shortwave troughing moves northeast from the Baja
Peninsula, eventually ejecting into the Southern to Central Plains.
Notable low-level southerly-to-southeasterly flow, around 20kts,
will continue to supply abundant moisture from the Gulf into the
Plains and Southwest. PW values are expected to remain around 1.5",
which still remains above the 95th percentile for the area
climatologically, although the highest moisture flux appears more
localized over the southeast parts of New Mexico. Sufficient
instability (MUCAPE ~1300J/kg) will also be present to support
thunderstorms capable of producing locally heavy rain rates. In New
Mexico, per the 00Z runs of the HREF and RRFS, rainfall rates
appear most favorable to exceed 1"/hr, with medium-to-high chances
of exceedance (~50% and ~65% respectively). 3-hour FFG exceedance
probabilities among the two also suggest higher-end chances of
exceedance; around a 65-70%.
Antecedent conditions are also very favorable for continued
instances of flash flooding. Given multiple days of Moderate EROs
over New Mexico and precipitation continuing last night, soils
remain saturated, contributing to very low FFGs. Even modest
rainfall amounts at this point will have the potential to produce
flash flooding. As such a high-end Slight Risk remains in place for
southeast New Mexico, with the surrounding area over the Southwest
maintained in the inherited Slight.
Minor expansions were made to the inherited Slight Risk area to
include northern Kansas. The aforementioned mid-level shortwave
will eventually reach the Central Plains, and with a long fetch of
southerly flow bringing moisture originating from the Gulf, a
broad area of precipitation is expected through the day. While
global models remain bullish with QPF, including localized totals
of 2-3 inches, limited instability should largely favor a more
stratiform and longer-duration rainfall regime. As a result,
hourly rainfall rates are expected to remain modest despite notable
24hr QPF, particularly in Nebraska. This is supported by the
relatively muted HREF and RRFS FFG exceedance probabilities. As
such, the Slight Risk was only expanded to include more of Kansas,
where HREF and RRFS FFG exceedance probabilities appear more
favorable.
Blanco-Alcala
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 PM EDT Thu Sep 24 2026
Day 2
Valid 12Z Fri Sep 25 2026 - 12Z Sat Sep 26 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS A PORTION OF
SOUTHEAST NEW MEXICO...
...2030Z Update...
Moisture, instability, and forcing will all be present from the
southern Rockies to the southern High Plains on Friday, with hi-res
guidance keying in on another round of scattered showers and
thunderstorms that are capable of producing heavy rainfall. Given
the upgrade to a Moderate Risk for southeast New Mexico on Day 1,
and after what will likely be another day of heavy rain on top of
soils that are already well saturated, a targeted Slight Risk was
introduced over a portion of southeast New Mexico for the Day 2
ERO. Even though Friday's convection is expected to produce lower
overall rainfall rates compared to the last several days, the sheer
hydrological sensitivity alone justifies a Slight Risk upgrade.
There was consideration to include parts of the Texas Panhandle and
western Oklahoma with this Slight Risk as models have been showing
a persistent signal for locally heavier QPF, but ground conditions
aren't as sensitive in those areas compared to New Mexico so left
them out for now.
The other consideration for the Day 2 ERO was to possibly
introduce a Slight Risk across parts of eastern Nebraska and
western Iowa. The latest WPC QPF shows a swath of 1-1.5 inches
which is supported by HREF neighborhood probabilities >50% for
exceeding 1 inch in 24 hours. However, much of that rainfall is
expected to fall well downstream of the more robust instability
plume. This should result in more stratiform precipitation and
lower intensity rainfall rates as a whole. As such, opted not to
introduce a new Slight Risk and will instead maintain the inherited
Marginal risk for now.
Only minor adjustments were made to the existing Marginal Risk
across coastal New England which covers the intrusion of the
western flank of the impending nor'easter's precipitation shield
well.
Miller
...Previous Discussion...
...Southwest...
A lingering mid-level disturbance, along with sufficient
instability and moisture influx from the Gulf, will support another
day of showers and thunderstorms across portions of the Southwest.
Antecedent conditions will remain a concern following multiple
rounds of rainfall, particularly across portions of New Mexico
where soils remain saturated and FFGs remain low. While the overall
coverage and intensity of convection may be reduced compared to
previous days, locally heavy rainfall rates will remain possible
where stronger convection develops. Given the increasingly
sensitive hydrologic conditions, even modest additional rainfall
may result in localized flash flooding. The inherited Marginal was
maintained with only minor adjustments.
...Central Plains...
The mid-level shortwave will continue to progress across the
Central Plains, maintaining broad ascent and precipitation from
portions of Kansas northeastward through Nebraska and Iowa. Locally
appreciable rainfall total remain possible, though limited
instability may again favor a substantial stratiform component and
temper rainfall rates. A Slight Risk was considered across portions
of Nebraska into Iowa. However, uncertainty regarding the coverage
and magnitude of heavier rainfall precludes an upgrade at this
time, and the Marginal Risk was maintained. An upgrade may become
necessary in later forecast cycles as a consensus hopefully becomes
more clear, and confidence in mesoscale details increase.
...Northeast...
A Marginal Risk has been maintained for parts of the Northeast as
the details of a potent coastal low continue to be ironed out.
There is still uncertainty pertaining to the time and location of
the low track, but there is a consensus forming in heavy rainfall
occurring along coastal Massachusetts, Rhode Island, and
Connecticut. However, placement of the low along with its central
pressure vary. With global guidance remaining in disagreement,
enough uncertainty was raised to preclude any upgrade at this
time, with continuity of the previous forecast being favored.
Blanco-Alcala
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 PM EDT Thu Sep 24 2026
Day 3
Valid 12Z Sat Sep 26 2026 - 12Z Sun Sep 27 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
NORTHEAST...
...2030Z Update...
The latest 12Z guidance continues to trend this weekend's
nor'easter a bit farther south and west compared to previous runs,
with the low center now tracking closer to the northern Mid-
Atlantic coastline by 12Z Sunday. Unsurprisingly, the resulting
precipitation shield has also ticked to the south and west, though
the heaviest rain is still confined to a swath from Cape Cod to
Long Island to the Jersey Shore. Given the latest QPF, both the
Marginal and Slight Risks were adjusted westward by a county or
two to account for these trends, but otherwise no major changes
were necessary.
The inherited Marginal Risk centered mostly over western Iowa and
northern Missouri was expanded south and west to include eastern
Kansas and northern Oklahoma with this update, mainly due to a
sufficient QPF signal anchored by a plume of 1000+ j/kg of MUCAPE
draped across the central and southern Plains. The QPF signal
becomes more pronounced across Iowa and Missouri and follows a fair
amount of rain that is expected to fall on Day 2/Friday. With the
potential for Day 3 QPF to fall on soils that are now more
saturated, a Slight Risk was considered. However, the best
instability still remains displaced to the west, so opted to hold
off on an upgrade for now.
Miller
...Previous Discussion...
...Northern Mid-Atlantic Coast into Southern New England...
The potential for heavy rainfall increases substantially in the
Northeast in the Day 3 period. The coastal low will bring a
generally prevailing southeast to easterly flow around a low
centered around Long Island, aiding in a substantial influx of
moisture onshore into the Northeast and parts of the Mid-Atlantic.
Current QPF guidance suggest widespread totals of 2-4" with
isolated totals as high as 5" in the 24-hour period. The
significant amount of expected rainfall over the long period into
the urban corridor of eastern MA will likely cause widespread
hydrologic concerns, including flooding in urban areas including
Boston, and poor drainage locations. Additionally, given increased
confidence regarding the heavy rain area, the now "higher-end"
Slight Risk area was maintained as inherited.
...Central Plains...
Further west the focus of the possible heavy to excessive rainfall
remains over the Midwest/Central Plains on Saturday as a shortwave
energy ejects northeastward from the Rockies. The envelope of
solutions span the EC on the north side and the GFS favoring a more
southerly solution corresponding to Iowa and Missouri/Kansas
respectively. A few of the globals remain rather bullish on the
amount of expected rainfall, particularly the ECMWF, while
ensembles continue to focus on this area as well. As such, the
Marginal Risk in Iowa was maintained where flash flood guidance is
lowest after the recent rainfall.
Blanco-Alcala
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 PM EDT Thu Sep 24 2026
Day 4 and Day 5
Valid 12Z Sun Sep 27 2026 - 12Z Tue Sep 29 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF NEW ENGLAND
ON SATURDAY...
New England...
20Z Update: Little change necessary for the D4 and D5 forecasts
across New England with eastern MA continuing to be the focal point
for heaviest rainfall in the D4 with a degrading rainfall outlook
for D5 as our system finally begins drifting away from the region,
but not before one last hit of moderate to locally heavy rainfall
from southeastern ME into southern New England the first part of
Monday. SLGT risk for D4 remains in effect with the risk
encompassing areas where PQPF for >1" remains 40% or greater from
the latest forecast. It will be imperative to keep tabs on the
forecast as we move closer as the behavior of these broad closed
upper level disturbances can be tricky in their final evolution
leading to some short term shifts in the risk areas forecast. For
now, went with a multi-model consensus of heavy rain into eastern
New England, backed by ML outputs and ensemble mean QPF being
heaviest in this corridor.
Kleebauer
..Previous Discussion..
There is a Slight risk of excessive rainfall in proximity to a deep
coastal low that slows its forward speed near southeast New
England. Heavy rainfall will begin to taper off during the period
in terms of areal coverage, rainfall rates and daily rainfall
amounts as the upper system fills. Model consensus has been
building around the idea that the surface system heads inland
compared with pivoting and heading back out to sea. The cumulative
effect of rainfall over the long period into the urban corridor of
eastern Massachusetts will cause hydrologic concerns, including
some flooding in poor drainage locations. For that reason, opted to
keep the Slight risk on Day 4. Opted for a Marginal risk given the
additional amounts shown by the models on Monday...but that may
ultimately depend on the hydrologic state by then.
Bann
...Southwest US...
Sunday will begin to see influence of moisture advection being
pulled off the Eastern Pacific from two different disturbances
(Odalys and Polo) with increasing IVT/PWATs being pulled poleward
into the Desert Southwest. Models initially for D4 are less bullish
in their depictions when it comes to a convective threat over the
region, however there are inferences of some convective development
later Sunday given the continued moisture pull coupling with the
afternoon diurnal instability max. The ECMWF is the most aggressive
with its signature of convection into the region with the
deterministic output suggesting some of the energy from Odalys
being pulled into the Southwestern CONUS as it moves in ahead of a
broad upper trough sliding down the Pacific coast come Sunday
afternoon. Pending the evolution of the vorticity and PWAT
advection, there could be an instance of scattered convection as
far north as southern and central UT for D4 which has led to an
expansion of the MRGL risk into this region to go along with much
of AZ and western NM.
D5, and beyond, is the period to watch as the setup becomes
increasingly favorable for another round of widespread heavy
convection as we begin to see an interaction between the eventual
closed upper reflection (ULL) over CA and what will be Polo as it
begins moving northward towards Baja over the course of the weekend
into early next week. The current model trends at 12z are murky
when it comes to the finer details of exactly when the interaction
between the two features takes place with the ECMWF and GFS
deterministic relaying very different evolutions that will play a
significant role in the final outcome for the period. The ensemble
means for Monday into early Tuesday note a broad expanse of heavier
rainfall increasingly likely for the Southwest CONUS into the Four
Corners area due to the ascent pattern downstream of the ULL
swinging into the region. Regardless of any interaction with Polo,
enough moisture (PWAT anomalies between +2 to +3.5 standard
deviations forecast across AZ/NM into southern UT/CO) and strong
diffluent flow aloft into the region will allow for widespread
heavy convection to occur for Monday afternoon into Tuesday leading
to enough consensus in the means to upgrade a large portion of the
Southwest to a SLGT risk for D5.
If there is further indication of more interaction with Polo with
the progressing ULL on Monday and beyond, a much more significant
excessive rainfall threat is plausible, one that would likely yield
a higher risk over a broader area. This is forecast currently with
some of the ensemble members on both the GEFS/ECENS, as well as the
EC-AIFS ENS which is already quite bullish in its presentation of
the QPF at 5-day leads. Stay tuned for the latest as this setup has
the potential for more substantial impacts pending the
aforementioned interaction.
...Northern Plains/Upper Midwest...
Progressive northern stream trough coupled with prevalent moisture
advection from the southwest will lead to a period of convective
concerns and isolated flash flood potential on Sunday into early
Monday as model outputs target the Northern Plains into portions of
the Upper Midwest for the D4 time frame. The current setup looks to
remain progressive enough to limit flash flood concerns at this
time, so a risk was not added, but wanted to make mention for the
non-zero threat.
A broad MRGL risk remains forecast for the area across the Dakotas
down into the Central Plains as a continued advection of elevated
PWATs streams poleward ahead of a broad upper trough/ULL reflection
out west. Remnant energy from what is currently Odalys will get
pulled northeastward out of the Pacific on D4 with sights on the
Southwest for Sunday into Monday, eventually progressive steadily
northeast into the Northern and Central Plains by Monday into early
Tuesday leading to quite a larger coverage of convection over the
above area. Latest ensemble means are more indicative to widespread
heavier convective potential, especially when you factor in the
stronger PWAT/IVT advection scheme that comes with the remnant
energy from Odalys. A previous case with a similar evolution
occurred a few weeks prior with heavy rain focused over the
Northern Plains that led to some flash flood concerns where
impacted. Mean QPF is currently between 1-2" with locally higher at
5-day leads, a testament to the threat in question as there will
likely be more significant rainfall potential with this setup. As
of now, there is enough consensus to maintain the broad MRGL risk
inherited, but did expand a bit further south and east to account
for the latest trends in ensembles. This setup will likely see some
higher risk potential due to the tropical influence as is
historically the case with these evolutions, so this will be yet
another area to maintain situational awareness for future updates
and potential upgrades.
Kleebauer
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 PM EDT Thu Sep 24 2026
Day 4 and Day 5
Valid 12Z Sun Sep 27 2026 - 12Z Tue Sep 29 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF NEW ENGLAND
ON SATURDAY...
New England...
20Z Update: Little change necessary for the D4 and D5 forecasts
across New England with eastern MA continuing to be the focal point
for heaviest rainfall in the D4 with a degrading rainfall outlook
for D5 as our system finally begins drifting away from the region,
but not before one last hit of moderate to locally heavy rainfall
from southeastern ME into southern New England the first part of
Monday. SLGT risk for D4 remains in effect with the risk
encompassing areas where PQPF for >1" remains 40% or greater from
the latest forecast. It will be imperative to keep tabs on the
forecast as we move closer as the behavior of these broad closed
upper level disturbances can be tricky in their final evolution
leading to some short term shifts in the risk areas forecast. For
now, went with a multi-model consensus of heavy rain into eastern
New England, backed by ML outputs and ensemble mean QPF being
heaviest in this corridor.
Kleebauer
..Previous Discussion..
There is a Slight risk of excessive rainfall in proximity to a deep
coastal low that slows its forward speed near southeast New
England. Heavy rainfall will begin to taper off during the period
in terms of areal coverage, rainfall rates and daily rainfall
amounts as the upper system fills. Model consensus has been
building around the idea that the surface system heads inland
compared with pivoting and heading back out to sea. The cumulative
effect of rainfall over the long period into the urban corridor of
eastern Massachusetts will cause hydrologic concerns, including
some flooding in poor drainage locations. For that reason, opted to
keep the Slight risk on Day 4. Opted for a Marginal risk given the
additional amounts shown by the models on Monday...but that may
ultimately depend on the hydrologic state by then.
Bann
...Southwest US...
Sunday will begin to see influence of moisture advection being
pulled off the Eastern Pacific from two different disturbances
(Odalys and Polo) with increasing IVT/PWATs being pulled poleward
into the Desert Southwest. Models initially for D4 are less bullish
in their depictions when it comes to a convective threat over the
region, however there are inferences of some convective development
later Sunday given the continued moisture pull coupling with the
afternoon diurnal instability max. The ECMWF is the most aggressive
with its signature of convection into the region with the
deterministic output suggesting some of the energy from Odalys
being pulled into the Southwestern CONUS as it moves in ahead of a
broad upper trough sliding down the Pacific coast come Sunday
afternoon. Pending the evolution of the vorticity and PWAT
advection, there could be an instance of scattered convection as
far north as southern and central UT for D4 which has led to an
expansion of the MRGL risk into this region to go along with much
of AZ and western NM.
D5, and beyond, is the period to watch as the setup becomes
increasingly favorable for another round of widespread heavy
convection as we begin to see an interaction between the eventual
closed upper reflection (ULL) over CA and what will be Polo as it
begins moving northward towards Baja over the course of the weekend
into early next week. The current model trends at 12z are murky
when it comes to the finer details of exactly when the interaction
between the two features takes place with the ECMWF and GFS
deterministic relaying very different evolutions that will play a
significant role in the final outcome for the period. The ensemble
means for Monday into early Tuesday note a broad expanse of heavier
rainfall increasingly likely for the Southwest CONUS into the Four
Corners area due to the ascent pattern downstream of the ULL
swinging into the region. Regardless of any interaction with Polo,
enough moisture (PWAT anomalies between +2 to +3.5 standard
deviations forecast across AZ/NM into southern UT/CO) and strong
diffluent flow aloft into the region will allow for widespread
heavy convection to occur for Monday afternoon into Tuesday leading
to enough consensus in the means to upgrade a large portion of the
Southwest to a SLGT risk for D5.
If there is further indication of more interaction with Polo with
the progressing ULL on Monday and beyond, a much more significant
excessive rainfall threat is plausible, one that would likely yield
a higher risk over a broader area. This is forecast currently with
some of the ensemble members on both the GEFS/ECENS, as well as the
EC-AIFS ENS which is already quite bullish in its presentation of
the QPF at 5-day leads. Stay tuned for the latest as this setup has
the potential for more substantial impacts pending the
aforementioned interaction.
...Northern Plains/Upper Midwest...
Progressive northern stream trough coupled with prevalent moisture
advection from the southwest will lead to a period of convective
concerns and isolated flash flood potential on Sunday into early
Monday as model outputs target the Northern Plains into portions of
the Upper Midwest for the D4 time frame. The current setup looks to
remain progressive enough to limit flash flood concerns at this
time, so a risk was not added, but wanted to make mention for the
non-zero threat.
A broad MRGL risk remains forecast for the area across the Dakotas
down into the Central Plains as a continued advection of elevated
PWATs streams poleward ahead of a broad upper trough/ULL reflection
out west. Remnant energy from what is currently Odalys will get
pulled northeastward out of the Pacific on D4 with sights on the
Southwest for Sunday into Monday, eventually progressive steadily
northeast into the Northern and Central Plains by Monday into early
Tuesday leading to quite a larger coverage of convection over the
above area. Latest ensemble means are more indicative to widespread
heavier convective potential, especially when you factor in the
stronger PWAT/IVT advection scheme that comes with the remnant
energy from Odalys. A previous case with a similar evolution
occurred a few weeks prior with heavy rain focused over the
Northern Plains that led to some flash flood concerns where
impacted. Mean QPF is currently between 1-2" with locally higher at
5-day leads, a testament to the threat in question as there will
likely be more significant rainfall potential with this setup. As
of now, there is enough consensus to maintain the broad MRGL risk
inherited, but did expand a bit further south and east to account
for the latest trends in ensembles. This setup will likely see some
higher risk potential due to the tropical influence as is
historically the case with these evolutions, so this will be yet
another area to maintain situational awareness for future updates
and potential upgrades.
Kleebauer
