Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Fri Sep 25 2026
Day 1
Valid 16Z Fri Sep 25 2026 - 12Z Sat Sep 26 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN HIGH PLAINS AND CENTRAL PLAINS...
...16Z Update...
The Slight Risk was extended northeastward into the Central Plains.
Showers and thunderstorms are expected to redevelop this afternoon
and tonight across the Plains as increasingly diffluent flow aloft
overspreads a moisture plume with tropical origins. By 00z
tonight, strengthening IVT directed towards Kansas will result in
PW values near the top of the climatological distribution for this
time of year according to the NAEFS. As such, the 12Z HREF and
RRFS guidance show increasing probabilities (30-60%) for 24 hour
QPF exceeding 2 inches over the Slight Risk area. Much of this area
has seen heavy rain over the last several days and will be
sensitive to additional rainfall. This scenario for scattered
flooding is supported by CSU First Guess guidance, which
unanimously depicts a Slight Risk extending across the Central
Plains into Southeast Nebraska.
Asherman
...Previous Discussion...
...Southwest to the Central Rockies and Southern High Plains...
Expect some of the overnight activity impacting the region to
remain ongoing beyond 12Z, with some potential for additional
flooding concerns across portions of eastern New Mexico and the
Texas Panhandle into the mid to late morning. Additional heavy
rains should be short-lived however, with the consensus of the
CAMs showing storms diminishing by late morning as the associated
shortwave energy and deeper plume of moisture lift out to the
northeast. However, given the antecedent soil conditions and some
high probabilities in both the HREF and REFS for additional amounts
of 1-2 inches, maintained a Slight extending from southeastern New
Mexico to the southern Panhandle Region. The Slight Risk was also
extended south into parts of West Texas, where activity later in
the day is expected to produce some very localized heavy amounts
(1-2 inches) including areas where 3-hr FFGs are under an inch.
Back to the north and west, a mid-to-upper level shortwave will
lift out of the Southwest, encouraging afternoon and evening
showers and thunderstorms, especially across portions of eastern
Arizona, southeastern Utah, and Colorado. The HREF and REFS
generally agree that the greater potential for localized heavier
amounts will center from southeastern Utah into southwestern
Colorado.
...Central Plains...
Shortwave energy and plenty of moisture emanating from the
Southwest will spread across the region later today into the
evening. While instability will remain weak and soil conditions
less sensitive, deep saturation though the air column will support
efficient rainfall and some portions of the area may see amounts
high enough to raise at least some isolated runoff concerns. Both
the HREF and REFS indicate that areas near the central Kansas-
Nebraska border northeast to the northern Nebraska-Iowa border
could see amounts over 2 inches.
...Northern Mid-Atlantic/Northeast...
Rain along the northern and northwestern side of strengthening
nor'easter will likely begin to spread inland by late today as the
system deepens and begins to track north. While the heaviest
amounts are expected to remain offshore, some coastal areas from
New Jersey to southern New England could begin to see some
localized heavy totals by Saturday morning.
Pereira
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Fri Sep 25 2026
Day 2
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...
...Northern Mid-Atlantic Coast into Southern New England...
Guidance is trending towards a deeper cyclone which retrogrades
toward the coastline on Day 2. Accordingly, QPF has trended
upwards generally along the I-95 Corridor from Philadelphia to
Boston. Compared with the previous forecast, the Slight Risk area
was expanded southwestward into southern New Jersey where model
consensus shows widespread amounts of 1-3" during the Day 2
period.
...Southern to Central Plains...
A Slight Risk was introduced to portions of eastern Nebraska and
central Iowa where a corridor of cell-training is possible late Day
2 along a NW-SE oriented stationary front. Both the HREF and RRFS
suite are in fairly good agreement for 24 hr QPF from this activity to
exceed 2", which could support scattered flash flooding as this
rainfall overlaps with heavy rainfall from the previous day.
The Marginal Risk was also expanded further south where CAMS
signal the potential for locally heavy rainfall near sensitive
soil, although coverage of storms is expected to be lower than
previous days.
Asherman
...Previous Discussion...
...Northern Mid-Atlantic Coast into Southern New England...
As a powerful nor'easter continues to approach and then stall east
of New Jersey and south of Long Island, the potential for heavy
rainfall is likely to increases substantially along portions of the
northern Mid-Altantic to southern New England coasts. The system
will bring a generally prevailing southeast to easterly flow,
supporting a substantial influx of moisture into parts of southern
New England, southern New York, and New Jersey. Increasing
hydrologic concerns are expected. In addition to an increasing
threat for flooding in urbanized and poor drainage areas, this
heavy rain may also exacerbate ongoing coastal flooding. The
Slight Risk highlights the areas where the general model consensus
shows widespread amounts of 1-3 inches during the period.
...Central Plains to the Upper Mississippi Valley...
Rainfall developing late in the Day 1 period, may continue into
late Saturday morning across parts of Iowa and southern Minnesota.
Expect a diminishing threat for heavy to excessive rain by late
morning, followed a potential return later in the day centered farther
to the southwest. While confidence is limited, there remains some
signal in the guidance for storms developing back across across
parts of the central Plains along a strengthening low level jet and
axis of increasing instability and moisture.
...West Texas...
Weak mid-level energy interacting with an uptick in low level
moisture may produce some additional localized heavy amounts,
impacting the more sensitive areas of West Texas around the Davis
Mountains and Stockton Plateau. Therefore, a small Marginal Risk
was added.
Pereira
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Fri Sep 25 2026
Day 3
Valid 12Z Sun Sep 27 2026 - 12Z Mon Sep 28 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
NORTHEAST...
...2030 Update...
As highlighted on Day 2, the nor'easter is trending towards a slower
track overall. Thus, heavy rainfall is expected to linger
overhead, leading to an additional 1-2" of rainfall over much of
the same area. As this overlaps with much of the rain from Day 2,
the Slight Risk was adjusted southwestward to encompass a similar
area.
Asherman
...Previous Discussion...
...Northeast...
The general consensus of the models shows a powerful nor'easter
beginning to weaken as it lingers in the vicinity of the northern
Mid-Atlantic coast. How far inland, if at all, the storm drifts
will help dictate how far beyond the coastal regions its
associated heavy rains will reach. For now, maintained a Slight
Risk over eastern Massachusetts, where confidence in additional
heavy amounts is greater regardless of the track.
...Southwest...
Increasing southwesterly flow ahead of an amplifying trough
digging south across California will begin to tap moisture and mid-
to-upper level energy, supporting an increase in shower and
thunderstorm activity and the potential for localized areas of
flash flooding from Southern California into the Southwest.
Pereira
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Fri Sep 25 2026
Day 4 and Day 5
Valid 12Z Mon Sep 28 2026 - 12Z Wed Sep 30 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE
SOUTHWEST AND FOUR CORNERS REGION THAT EXPANDS INTO THE SOUTHERN
AND CENTRAL PLAINS ON TUESDAY...
2000Z Update: The biggest change from this morning's ERO was made
to the Day 5 Slight Risk. Given the guidance trends, led by several
AI members, the eventual absorption of POLO's remnants into an
amplifying Southwest U.S. upper trough have led to a slower and
wetter trend across the Southwest-Four Corners and into the High
PLains Tuesday into Wednesday. The global ensemble guidance,
particularly the ECMWF (ECENS), all show a notable uptick in QPF
exceedance probabilities from the 00Z guidance suite.
Hurley
Previous discussion below..
...Southwest US...
Tropical moisture from Odalys and Polo will be pulled northward
into the Desert Southwest and when coupled with the afternoon
diurnal instability max will trigger scattered to widespread
convection across the region. An interaction between the eventual
closed upper reflection (ULL) over California and what will be
Polo as it begins moving northward towards Baja over the course of
the weekend into early next week. The ensemble means for Monday
into early Tuesday continue to 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 upper low
swinging into the region. PW anomalies during this period will be
between 2-3.5 standard deviations above seasonal normal for late
September. A Slight Risk for excessive rainfall and flash flooding
has been maintained.
...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 Monday. A broad
Marginal Risk remains in effect for the Dakotas/Minnesota down
into the Central Plains as a continued advection of elevated PWs
streams poleward ahead of a broad upper trough/low reflection out
west. Remnant energy from what is currently Odalys will get pulled
northeastward out of the Pacific prior to the period reaching the
Northern and Central Plains Monday into early Tuesday leading to
quite a larger coverage of convection over the above area. Areal
coverage of convection will increase Tuesday as there will be a
stronger PW surge with the remnant energy from Odalys. 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. A broad Marginal is
in effect for Monday and then a Slight Risk for Tuesday from Texas
to southern Iowa.
New England...
Eastern Massachuesetts continues to be the focus for the highest
QPF on Monday as the coastal low begins to drift away from the
region. 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. 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.
Campbell
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Fri Sep 25 2026
Day 4 and Day 5
Valid 12Z Mon Sep 28 2026 - 12Z Wed Sep 30 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE
SOUTHWEST AND FOUR CORNERS REGION THAT EXPANDS INTO THE SOUTHERN
AND CENTRAL PLAINS ON TUESDAY...
2000Z Update: The biggest change from this morning's ERO was made
to the Day 5 Slight Risk. Given the guidance trends, led by several
AI members, the eventual absorption of POLO's remnants into an
amplifying Southwest U.S. upper trough have led to a slower and
wetter trend across the Southwest-Four Corners and into the High
PLains Tuesday into Wednesday. The global ensemble guidance,
particularly the ECMWF (ECENS), all show a notable uptick in QPF
exceedance probabilities from the 00Z guidance suite.
Hurley
Previous discussion below..
...Southwest US...
Tropical moisture from Odalys and Polo will be pulled northward
into the Desert Southwest and when coupled with the afternoon
diurnal instability max will trigger scattered to widespread
convection across the region. An interaction between the eventual
closed upper reflection (ULL) over California and what will be
Polo as it begins moving northward towards Baja over the course of
the weekend into early next week. The ensemble means for Monday
into early Tuesday continue to 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 upper low
swinging into the region. PW anomalies during this period will be
between 2-3.5 standard deviations above seasonal normal for late
September. A Slight Risk for excessive rainfall and flash flooding
has been maintained.
...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 Monday. A broad
Marginal Risk remains in effect for the Dakotas/Minnesota down
into the Central Plains as a continued advection of elevated PWs
streams poleward ahead of a broad upper trough/low reflection out
west. Remnant energy from what is currently Odalys will get pulled
northeastward out of the Pacific prior to the period reaching the
Northern and Central Plains Monday into early Tuesday leading to
quite a larger coverage of convection over the above area. Areal
coverage of convection will increase Tuesday as there will be a
stronger PW surge with the remnant energy from Odalys. 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. A broad Marginal is
in effect for Monday and then a Slight Risk for Tuesday from Texas
to southern Iowa.
New England...
Eastern Massachuesetts continues to be the focus for the highest
QPF on Monday as the coastal low begins to drift away from the
region. 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. 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.
Campbell
