Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 PM 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
358 PM EDT Sun Sep 20 2026
Day 2
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 southern Mid-Atlantic...
A Slight Risk has been introduced for portions of the central
Appalachians covering eastern West Virginia and adjacent
southwestern Virginia. Storms overnight Sunday may continue into
part of the day 2 period Monday morning. In addition, moisture and
instability will remain in place through the day as the cold front
hangs further north along the mountains. Forcing from the front as
it pushes southward to the east along the mountains and also
approaches from the northwest will lead to additional storms during
the day and into the overnight hours. The now available CAMs
support rainfall totals of 2-5" with low to moderate HREF
probabilities of this rainfall exceeding FFGs. Antecedent
conditions remain wet with 24-hour areal average rainfall of 1-2"
(locally as much as 3-4") over central West Virginia according to
MRMS estimates. Additional storms will be possible further east
along the front into southern Virginia and northern North Carolina.
However, FFGs are comparably much higher across this region.
...New Mexico and Nearby Southern High Plains...
There is a low-end risk of excessive rainfall developing across New
Mexico and western Texas as low-level flow of Gulf moisture
increases the amount of moisture available for late day convection.
Some locally heavy totals in the 1-3" range will be 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.
Putnam/Bann
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 PM 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 NEW
MEXICO AND WEST TEXAS...
...New Mexico and West Texas...
Models maintained a favorable setup for heavy rain and scattered
to numerous instances of flash flooding setting up across much of
New Mexico Tuesday and continuing into Wednesday. 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 levels...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. The latest guidance shows the potential for locally heavy
totals as much as 2-4 inches particularly from the Rio Grande
Valley east through the Sacramento Mountains and into the adjacent
Plains. This would be more than enough to lead to at least
scattered instances of flash flooding. The potential for a Moderate
Risk remains especially if the CAM guidance agrees on similar if
not higher totals. For now, there is more confidence in more
widespread and significant impacts into Wednesday following the
rainfall on Tuesday.
...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. Although
QPF has trended up quite a bit particularly along a frontal
wave/inverted trough in vicinity of Chesapeake Bay, expect that
the lack of instability and cooler air in place will lead to a
mostly steady, long-duration rainfall. The Marginal risk area was
maintained given the possibility that a bit of CAPE could drive
some low topped convection and locally higher rates. A targeted
Slight Risk over the central Appalachians was removed. Despite more
sensitive antecedent conditions, expect the environment to be much
cooler and more stable as the front finally pushes further
southward after hanging up along the Appalachians, which should
keep rates down and any additional flooding concerns isolated.
...South Florida...
A Marginal Risk has been maintained for portions of South Florida.
Locally heavy downpours will remain possible with anomolously high
moisture in place, which could lead to some isolated
flooding/ponding in urban areas.
Putnam/Bann
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
