Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
904 PM EDT Sun Sep 20 2026
Day 1
Valid 01Z Mon Sep 21 2026 - 12Z Mon Sep 21 2026
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDWEST/OHIO VALLEY AS WELL AS THE NORTHERN HIGH PLAINS...
...01Z Update...
Made some additional adjustments based on recent hi-res guidance
and radar trends. Reduced the footprint of the Marginal Risk areas
where additional heavy rain is not expected through the overnight.
Meanwhile, there remains at least some threat for additional heavy
rains or FFG exceedance within the Slight Risk areas, so both were
maintained.
For the Midwest area, did extend the Slight Risk a little farther
southwest into Missouri, reflecting the increasing thunderstorm
coverage in that area and HREF/REFS probabilities that showed localized
amounts over two inches are possible. A relatively greater flash
flood threat remains centered over portions of Ohio and West
Virginia, where localized amounts over 2 inches are likely
overnight for parts of the region according to both the HREF and
REFS. Given the recent heavy rains, these amounts would easily
exceed FFGs, suggesting additional runoff concerns are likely.
Pereira
...16Z Update...
Northern portions of the inherited Slight Risk over the Midwest
(northern Illinois, southern Iowa, northern Missouri) were removed
based on the updated 12Z guidance. Expect that the front will
remain further southward keeping forcing and instability more
focused across central Illinois into east-central Missouri. An
approaching upper-wave will help to trigger a renewed round of
convection later this evening and into the overnight hours. There
is the potential for some training/backbuilding with flow parallel
to the boundary which could lead to some locally heavy totals of
2-4". Confidence in heavy rainfall remains higher further to the
east into the Upper Ohio Valley where pooling moisture and higher
instability will help lead to more robust rainfall rates. Storms
yesterday that continue into this morning have also left wetter
antecedent conditions with 24-hour areal average totals in the 1-2"
range, with localized totals of 3-4". Both the latest 12Z HREF and
RRFS show moderate probabilities for additional totals of 3"+.
Otherwise, the prior forecast remains on track. Expect another
round of storms to develop and meander around the Black Hills
vicinity in the northern High Plains as an upper-low over the
Rockies progresses eastward over the region. Anomalously moist
upslope flow remains in place to support locally heavy rainfall.
Similar to further east conditions are also more sensitive given
rainfall on Saturday.
Putnam
...Previous Discussion...
...Midwest and Ohio Valley...
Overnight convection leading into the start of the Day 1 period
will keep the threat of excessive rainfall lingering from southeastern
Iowa and northern Missouri eastward into portions of the Ohio
Valley.. The threat will slowly wane as the back edge of the
rainfall makes its way from west to east this morning into the
early afternoon. The risk is due to some locally intense rainfall
rates overlapping with compromised Flash Flood Guidance values.
Farther to the east...a separate area of overnight convection had
been moving eastward in proximity to the Ohio River main-stem
towards the higher terrain in West Virginia which also was likely
lowering Flash Flood Guidance leading into the start of the Day 1
period. The HRRR tapers off the convection here during the late
morning and then redevelops the threat of moderate to heavy
rainfall again.
Given the precipitable water plume in the area...there is higher
confidence in both the placement and potential for heavier
rainfall/flash flooding due to antecedent conditions. A bit
higher/anomalous moisture pooling in the valley will also bring a
greater chance of heftier totals in the 2-5" range.
...Northern High Plains...
Maintained a Slight Risk area mainly focused over southwest South
Dakota as a slow moving upper-low over the Rockies will begin to
progress eastward as an open wave later today. Moisture remains
anomalously high and low level flow remains upslope flow. That will
continue to promote efficient rainfall with rain rates of 1-1.5"+
per hour. The region will also be more prone to flooding and runoff
problems where there is overlap with rain that fell on Saturday.
Rockies to the Central Plains... There continues to be a Marginal
Risk of excessive rainfall from the Southern High Plains northward
to the Western High Plains adjacent to the Northern Rockies as deep
moisture remains in place with weak shortwave energy embedded
helping to fuel scattered convection.
...Elsewhere...
A large Marginal Risk area was maintained from the northern Rockies
through much of the Plains and into western Texas. Scattered
thunderstorms embedded within a large plume of anomalously high
moisture will pose the threat for localized heavy rainfall and high
instantaneous rain rates. However, confidence is low on placement,
so a Marginal Risk seems appropriate.
Bann
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Mon Sep 21 2026
Day 1
Valid 12Z Mon Sep 21 2026 - 12Z Tue Sep 22 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL APPALACHIANS...
...Central Appalachians and the Mid-Atlantic...
The Slight risk has been expanded to include a larger portion of WV
and southwest VA. The primary driver of heavy rainfall will be
embedded shortwave energy interacting with a backdoor cold front
and inverted surface trough.
Model guidance indicates a moderately to highly unstable airmass
will develop near the boundary over WV, with 2000-3000 J/kg of
CAPE expected. As a distinct shortwave/MCV currently positioned
over IL/IN progresses into the region, robust convective
development is anticipated. While most model solutions keep these
cells moving steadily across WV, a quick 1-3" of rain over
increasingly saturated soils should result in an additional flash
flood threat.
By this afternoon convection should propagate into southwest VA
along/near the backdoor front. In this region more persistent low
level convergence should increase the duration of convective
elements and locally enhance rainfall totals. While some guidance
pushes the front and best convective focus slightly further south
into northern NC, current model consensus supports southwest VA as
the primary axis for heavy rain. Recent high res models highlight
the potential for localized excessive totals. Both the 00z and 06z
HRRR depict a small but impressive swath of rainfall as high as
3-6". Given the available instability, mesoscale forcing, and
favorable convergence near the front, these elevated totals appear
plausible, justifying the expansion of the Slight risk.
...New Mexico and Nearby Southern High Plains...
Both the HREF and REFS indicate the potential for rainfall locally
exceeding 3" as isolated convection develops this afternoon over
eastern NM and west TX. A widely isolated flash flood risk is
possible.
...South Florida...
A Marginal Risk has been maintained for portions of South Florida
as a plume of anomalous moisture will help to fuel heavy downpours
and potentially lead to some isolated flooding/ponding in urban
areas.
...CA/NV...
Convection underneath a closed low may pose a localized flash
flood risk today across portions of eastern CA into NV.
Instability is forecast to be weak, but with cold air aloft under
the low and PWs forecast to increase over the climatological 90th
percentile, an isolated flash flood risk could evolve.
Chenard
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Mon Sep 21 2026
Day 2
Valid 12Z Tue Sep 22 2026 - 12Z Wed Sep 23 2026
..THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHERN NEW MEXICO...
...New Mexico and West Texas...
A potent, multi-day heavy rainfall event is set to begin across NM
and far southwest TX Tuesday. An upper level trough approaching
from the west will help strengthen an upper level jet,
persistently placing the region in the favorable right entrance
region. Deep southerly flow will transport tropical moisture
northward, while embedded mid level shortwave energy advances north
out of Mexico and into NM by Tuesday night. Low level easterly
upslope flow will also enhance lift in this setup.
PW values are forecast to approach mid to late Sep. record levels.
With sufficient instability expected, this deep moisture profile
will support heavy rainfall rates. Scattered flash flooding is
expected across much of NM and far southwest TX, with a more
concentrated area of slow moving convection possible over southern
NM.
Guidance is in pretty good agreement regarding high QPF potential
over southern NM. The 00z RRFS and 3km NAM depict swaths of 3"
rainfall totals, with the GFS/AIGFS/Gem reg depicting higher QPF
across a similar axis. With a day 3 Moderate risk already in
effect, and the flash flood threat expected to ramp up by Tuesday
night, we opted to upgrade the day 2 to a Moderate as well.
Scattered to numerous instances of flash flooding are anticipated,
some of which could be locally significant. This threat includes
the sensitive burn scar areas within the Sacramento Mountains.
...Ohio Valley and Mid-Atlantic...
An approaching mid-level trough and favorable positioning within
the right entrance region of an upper level jet will support
widespread rainfall across portions of the OH Valley and Mid-
Atlantic. At the surface, a backdoor cold front with strong
easterly flow will enhance low level convergence, further amplified
by a developing inverted surface trough.
Well to the north of the frontal boundary, limited instability
will keep rainfall primarily stratiform, with rain rates remaining
below flash flood thresholds. Considerable model spread remains
regarding where the strongest convergence and sufficient
instability will overlap, complicated by dry air advecting in from
the northeast. While the 12z EC and 00z GFS were previously in
agreement with a heavy QPF axis along the coastal plain from
eastern VA into the Delmarva, the 00z EC has trended further
inland. Meanwhile, the 00z RRFS and REFS favor a further inland
and southward axis stretching from south central VA into central
NC.
Ultimately, the forecast hinges on far south cold air damming
forces the instability axis and also how far inland the surface
trough gets nudged. A localized sweet spot may emerge where
persistent convergence intersects sufficient instability to drive
a heavy rainfall and flash flood threat. Where these ingredients
overlap, the environment will be conducive for some low topped
convective cores and efficient warm rain processes. However given
the current uncertainty, we will keep the risk level at Marginal
for this update, and we will reevaluate later today when we have
the full suite of high res guidance available.
...South Florida...
A Marginal Risk has been maintained for portions of South Florida.
Locally heavy downpours will remain possible with anomalously high
moisture in place, which could lead to some isolated
flooding/ponding in urban areas.
Chenard
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Mon Sep 21 2026
Day 3
Valid 12Z Wed Sep 23 2026 - 12Z Thu Sep 24 2026
..THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF EASTERN NEW MEXICO...
...New Mexico and West Texas...
The heavy rainfall threat established on day 2 will continue and
potentially peak over NM on day 3 as large scale forcing gets even
stronger. A well defined mid level shortwave is forecast to lift
northward across the state, accompanied by robust upper level
divergence. PW values will remain near record levels for this time
of year, with an anomalously deep moisture profile.
With such strong forcing and widespread moisture in place, the
primary limiting factor for heavy convective rates will be the
extent of cloud cover and its impact on destabilization. While
widespread clouds are a concern, daytime heating should allow at
least the eastern edge of the rainfall shield to sufficiently
destabilize, aided by broad easterly upslope flow. Thus while some
areaS may see predominately stratiform rain, a concentrated swath
of intense convective rainfall remains probable.
Guidance continues to show some spread regarding the placement of
this heaviest rainfall axis. The 00z EC remains the furthest west
solution, with the GFS, AIGFS, AIFS and RRFS clustered a bit
further east. The current forecast favors this eastern consensus,
though given we are still three days out, additional spatial
adjustments are probable.
Based on this setup the inherited Moderate risk remains justified,
and it was expanded to encompass a larger portion of eastern NM.
Scattered to numerous instances of flash flooding are
anticipated, some of which could be locally significant. This
threat includes the sensitive burn scar areas within the
Sacramento Mountains.
...Mid-Atlantic...
A complex and somewhat disjointed synoptic pattern will be present
over the Mid-Atlantic on day 3. A mid to upper trough will will
drift slowly eastward from the OH Valley into the Mid-Atlantic
while a separate coastal low develops offshore. Meanwhile, surface
high pressure to the north will advect drier and more stable air
into the corridor between these two systems.
While some instability may materialize directly beneath the cold
core of the mid level low, PW values will have generally decreased
by this time. The bulk of the deeper moisture and instability may
remain offshore with the coastal low.
Despite the intrusion of dry air, there remains enough
uncertainty regarding the exact evolution of these features to
warrant a continuation of the Marginal risk. An embedded Slight
risk may be needed in subsequent updates, contingent upon the exact
track of the upper low, position of the moisture/instability axis,
and the state of antecedent soil conditions from previous days
rainfall.
...Florida...
A Marginal risk continues across south FL where above normal PWs
and instability will continue to pose a localized flash flood
risk.
Chenard
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Mon Sep 21 2026
Day 4 and Day 5
Valid 12Z Thu Sep 24 2026 - 12Z Sat Sep 26 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NEW MEXICO ON DAY 4...
...Southern Rockies/High Plains...
The threat of additional flooding lingers into Thursday given a
favorable upper level pattern for additional showers.Even though rainfall
rates should be diminishing and the bulk of the rainfall will be
shifting into the Texas and Oklahoma panhandles, but the concern in
New Mexico is that any additional rainfall will be falling on wet
to saturated soils. Consequently, introduced a Slight Risk area
within the Marginal Risk area. At the same, refrained from
expanding the Marginal Risk area too far eastward in order to
better assess the potential rainfall amounts/rates that move over
considerably drier ground.
By Friday...some of the convection will be moving into parts of the
central Plains which should also have lowered flash flood guidance
due to rainfall in the preceding days. Introduced a Marginal Risk
area there due to the antecedent conditions and given the low
level feed of moisture being drawn northward from the Gulf. At the
same time, maintained a Marginal Risk in parts of New Mexico.
...Ohio Valley/Appalachians to Mid-Atlantic...
By Thursday, a coastal low will likely be developing in the
Atlantic. Depending on the low position, some guidance shows heavy
rainfall reaching onshore in the Mid-Atlantic on Thursday then spreading
towards southeastern New England on Friday. Models continue to show
the heaviest rain could remain offshore, and because that region
is not very sensitive to heavy rain given the sandy soils if there
are moderate amounts onshore. But this should continue to be
monitored.
Bann
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Mon Sep 21 2026
Day 4 and Day 5
Valid 12Z Thu Sep 24 2026 - 12Z Sat Sep 26 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NEW MEXICO ON DAY 4...
...Southern Rockies/High Plains...
The threat of additional flooding lingers into Thursday given a
favorable upper level pattern for additional showers.Even though rainfall
rates should be diminishing and the bulk of the rainfall will be
shifting into the Texas and Oklahoma panhandles, but the concern in
New Mexico is that any additional rainfall will be falling on wet
to saturated soils. Consequently, introduced a Slight Risk area
within the Marginal Risk area. At the same, refrained from
expanding the Marginal Risk area too far eastward in order to
better assess the potential rainfall amounts/rates that move over
considerably drier ground.
By Friday...some of the convection will be moving into parts of the
central Plains which should also have lowered flash flood guidance
due to rainfall in the preceding days. Introduced a Marginal Risk
area there due to the antecedent conditions and given the low
level feed of moisture being drawn northward from the Gulf. At the
same time, maintained a Marginal Risk in parts of New Mexico.
...Ohio Valley/Appalachians to Mid-Atlantic...
By Thursday, a coastal low will likely be developing in the
Atlantic. Depending on the low position, some guidance shows heavy
rainfall reaching onshore in the Mid-Atlantic on Thursday then spreading
towards southeastern New England on Friday. Models continue to show
the heaviest rain could remain offshore, and because that region
is not very sensitive to heavy rain given the sandy soils if there
are moderate amounts onshore. But this should continue to be
monitored.
Bann
