Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Sun Sep 20 2026
Day 1
Valid 16Z Sun Sep 20 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...
...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
1158 AM EDT Sun Sep 20 2026
Day 2
Valid 12Z Mon Sep 21 2026 - 12Z Tue Sep 22 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID ATLANTIC REGION AND OVER NEW MEXICO...
...Southern Mid-Atlantic States...
Numerical guidance still has a signal for cold air damming to be
in place from New England southward into the northern Mid- Atlantic
region on Monday with high pressure sprawled across portions of
the Great Lakes into eastern Canada. There could be some elevated
above the stable surface/low levels to support precipitation but
the threat for heavy to excessive rainfall amounts/rates still
looks to be muted. Therefore no significant changes were needed to
the inherited outlook. Forcing and moisture convergence along the
front along the central Appalachians will keep heavier rain chances
and isolated flash flooding a concern there in vicinity of the
boundary between airmasses.
...New Mexico and Nearby Southern High Plains...
There is a low-end risk of excessive rainfall developing across New
Mexico once again as low-level flow of Gulf moisture increases the
amount of moisture available for late day convection.
Precipitable water values should generally be around 1.75 inches
from Texas northward into the Mid- Mississippi Valley with enough
effective bulk shear and instability to result in organized
convection capable of producing heavy rainfall with the associated
risk of flash flooding.
Bann
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Sun Sep 20 2026
Day 3
Valid 12Z Tue Sep 22 2026 - 12Z Wed Sep 23 2026
..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
APPALACHIANS AND NEW MEXICO...
...Ohio Valley and Mid-Atlantic...
Mid level troughing with multiple embedded shortwaves will slowly
track eastward into the OH Valley and Mid-Atlantic where plenty of
moisture should already be in place as a backdoor cold front
approaches. Quite a bit of uncertainty with the details, as a
coastal low should eventually develop offshore either in the Day 3
period or just beyond in the Day 4 period. This would help advect
drier air in from the north over portions of the Mid- Atlantic.
Also the amount of instability, with the backdoor front likely
making a push all the way into the Carolinas (leaving a more
stable airmass to its north and what that implies for rainfall
rates) works against introduction of a Slight Risk area. The
Marginal risk area was maintained given the possibility that a bit
of CAPE could drive some low topped convection and locally higher
rates.
The Slight risk over Pennsylvania and West Virginia was maintained
in deference to an inverted trough which should help focus
convection there. Even though fairly weak, as much as 500 to 1000
j/kg of CAPE could develop. In addition, this area has lower FFG
than the coastal plain given the rainfall expected in the Day 1 and
Day 2 period.,,potentially lowering Flash Flood Guidance further.
...New Mexico...
Models maintained a favorable setup for heavy rain and scattered
to numerous instances of flash flooding ears to be setting up for
New Mexico on Tuesday that appears likely to continue into
Wednesday. Of particular concern is that conditions will likely
get more sensitive with time given possibility of multiple rounds
of convection occurring for over multiple days in a row. There was
not an upgrade to a Moderate risk but it remains a possibility.
Broad troughing over the western US will place New Mexico within
the right entrance region of a strong upper jet..with one stronger
wave moving to the north of Tuesday. At the low level...easterly
upslope flow will feed deep moisture and instability into the
region. This combination of forcing, instability and well over 90th
percentile PWs for September, suggest heavy rainfall and a growing
flash flood risk.
Bann
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Sun Sep 20 2026
Day 4 and Day 5
Valid 12Z Wed Sep 23 2026 - 12Z Fri Sep 25 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHERN
ROCKIES/HIGH PLAINS...
...Southern Rockies/High Plains...
Day 4/Wednesday will be the second day of a significantly wet
period across much of New Mexico, with reasonably high confidence.
PWs are forecast to reach 1.5+ inches, around 2.5-3 standard
deviations above normal, with a likely PW connection to an eastern
Pacific tropical system. This is coincident with deep southerly
flow, as the 250mb jet is basically south to north in response to
an eastern Pacific mid/upper low just southwest of California, and
favorable right entrance region jet dynamics will be in place. Rain
totals for the Tuesday-Wednesday period are explicitly forecast to
be 3-4" broadly, exceeding some lower end ARIs, and easily could
be locally higher. The risk is likely higher-end in areas of
overlap between Tuesday and Wednesday. Unfortunately this may be
over the Sacramento Mountains, which tend to be sensitive given
burn scars. Still anticipate that Moderate Risks may be needed in
future forecast cycles, with locally significant flash flooding
possible.
By Day 5/Thursday, the pattern remains favorable for rainy
conditions, but the bulk of reliable model guidance shows the rain
shifting east a bit into the TX/OK Panhandles and vicinity, with
the EC-AIFS and AIGFS leading the way with the eastern shift. This
is dependent on the evolution of shortwave energy from the eastern
Pacific upper low and east of it. The heavier QPF focusing eastward
would put the rain atop drier antecedent conditions for less
sensitivity for flooding, but any QPF farther west could cause
flooding problems with the lingering wet soils. Will start with a
Marginal Risk to cover both the lower QPF over wetter soils and the
higher QPF over drier soils farther east, but this will need to be
reassessed in future forecast cycles.
...Ohio Valley/Appalachians to Mid-Atlantic...
An upper low will gradually drift across the Ohio Valley Day
4/Wednesday. At the surface, surface high pressure centered over
Quebec will send a cold air damming ridge of high pressure through
the Appalachians and Coastal Plain. This makes the forecast for
heavy rainfall tricky, as instability will be pushed south of the
CAD front. There may be a tongue of instability reaching west of
the Appalachians into eastern portions of the Ohio Valley, and then
some potentially overrunning rain across the Mid-Atlantic.
Meanwhile there is also uncertainty in how far south that front
gets in the Mid-Atlantic, and QPF axes thus vary from Pennsylvania
to North Carolina. Overall, a Marginal Risk was maintained, with
some broadening to include more areas given the frontal
uncertainty.
By Day 5/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, especially eastern
North Carolina. Considered a Marginal for this, but held off
because 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.
...South Florida...
Above average PWs over 2" will persist across the southern Florida
Peninsula and the Keys into Day 4/Wednesday after potentially
several wet days in the short range period. Maintained the Marginal
Risk for heavy downpours that could cause localized ponding or
flash flooding. By Day 5/Thursday the higher PWs look to gradually
push south, so went without a Marginal by then.
Tate
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Sun Sep 20 2026
Day 4 and Day 5
Valid 12Z Wed Sep 23 2026 - 12Z Fri Sep 25 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHERN
ROCKIES/HIGH PLAINS...
...Southern Rockies/High Plains...
Day 4/Wednesday will be the second day of a significantly wet
period across much of New Mexico, with reasonably high confidence.
PWs are forecast to reach 1.5+ inches, around 2.5-3 standard
deviations above normal, with a likely PW connection to an eastern
Pacific tropical system. This is coincident with deep southerly
flow, as the 250mb jet is basically south to north in response to
an eastern Pacific mid/upper low just southwest of California, and
favorable right entrance region jet dynamics will be in place. Rain
totals for the Tuesday-Wednesday period are explicitly forecast to
be 3-4" broadly, exceeding some lower end ARIs, and easily could
be locally higher. The risk is likely higher-end in areas of
overlap between Tuesday and Wednesday. Unfortunately this may be
over the Sacramento Mountains, which tend to be sensitive given
burn scars. Still anticipate that Moderate Risks may be needed in
future forecast cycles, with locally significant flash flooding
possible.
By Day 5/Thursday, the pattern remains favorable for rainy
conditions, but the bulk of reliable model guidance shows the rain
shifting east a bit into the TX/OK Panhandles and vicinity, with
the EC-AIFS and AIGFS leading the way with the eastern shift. This
is dependent on the evolution of shortwave energy from the eastern
Pacific upper low and east of it. The heavier QPF focusing eastward
would put the rain atop drier antecedent conditions for less
sensitivity for flooding, but any QPF farther west could cause
flooding problems with the lingering wet soils. Will start with a
Marginal Risk to cover both the lower QPF over wetter soils and the
higher QPF over drier soils farther east, but this will need to be
reassessed in future forecast cycles.
...Ohio Valley/Appalachians to Mid-Atlantic...
An upper low will gradually drift across the Ohio Valley Day
4/Wednesday. At the surface, surface high pressure centered over
Quebec will send a cold air damming ridge of high pressure through
the Appalachians and Coastal Plain. This makes the forecast for
heavy rainfall tricky, as instability will be pushed south of the
CAD front. There may be a tongue of instability reaching west of
the Appalachians into eastern portions of the Ohio Valley, and then
some potentially overrunning rain across the Mid-Atlantic.
Meanwhile there is also uncertainty in how far south that front
gets in the Mid-Atlantic, and QPF axes thus vary from Pennsylvania
to North Carolina. Overall, a Marginal Risk was maintained, with
some broadening to include more areas given the frontal
uncertainty.
By Day 5/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, especially eastern
North Carolina. Considered a Marginal for this, but held off
because 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.
...South Florida...
Above average PWs over 2" will persist across the southern Florida
Peninsula and the Keys into Day 4/Wednesday after potentially
several wet days in the short range period. Maintained the Marginal
Risk for heavy downpours that could cause localized ponding or
flash flooding. By Day 5/Thursday the higher PWs look to gradually
push south, so went without a Marginal by then.
Tate
