Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
715 PM EDT Wed Sep 9 2026
Day 1
Valid 01Z Thu Sep 10 2026 - 12Z Thu Sep 10 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS, MID MISSISSIPPI VALLEY, CENTRAL-SOUTHERN HIGH
PLAINS, AND THE SOUTHWEST...
2230Z Update: Modified a few of the outlook areas, based on the
latest observational trends and most recent CAM guidance (including
recent HRRR trends and HREF/REFS probabilistic data). This
included an expansion of the Slight Risk area into far northeastern
NM, the OK Panhandle, and southwest KS. The highest risk in this
area is essentially in the short term (through this evening), along
a NE-SW PWAT axis where the activity is becoming more widespread/
beginning to conglomerate late this afternoon. The Slight Risk here
incorporates the highest 18Z HREF and SREF probabilities of
1-2"/hr rainfall rates.
Elsewhere, have also expanded the Slight Risk across western KS,
far northeastern OK, and southern-southeast MO; partly owing to the
ongoing convection and convective/mesoscale trends early
(particularly eastern end of the Slight), but also with the latest
CAM trends and HREF/REFS QPF exceedance probabilities with the next
round of (elevated) convection overnight as the LLJ increases.
Later on tonight (after 04Z), the recent RAP runs show Corfidi
Vectors weaken to less than 10kts and become opposed to the
direction of the low-level and 850-300 mb mean wind vectors
(thereby increasing the risk for back-building and cell training).
Hurley
...Previous Discussion...
An axis of above average PW values, 2+ standard deviations above
the mean, expected along the surface cold front pressing generally
southeastward day 1 through the Great Lakes into the Northeast and
southward through the Mid Mississippi Valley and into the Central
Plains. The northeastern portion of the front moving across the
Great Lakes into the Upper Ohio Valley and Northeast will be very
progressive, limiting any widespread runoff issues. Widespread
moderate to locally heavy rains possible along and ahead of this
potion of the front, with the risk level kept at marginal given the
expected progression.
..Central Plains into the Lower Missouri Valley...
Slower movement of the frontal boundary expected from the Central
Plains into the Lower Missouri Valley/Mid Mississippi Valley where
it will become more aligned to the westerly mid to upper level
flow. Simulated radars from the CAMS show general agreement with
the potential for training of cells in a west to east direction in
the 0000- 1200 UTC Thursday time period. HREF and RRFS
neighborhood probabilities for 1, 2 and 3"+ totals remain high in
the slight risk area from southeast Kansas into south-central
Missouri. A small slight risk area was introduced over southeast
Colorado where there is consensus for potentially heavy rains where
low level easterly to southeasterly flow is expected to strengthen
in the early hours of Thursday. HREF and RRFS probabilities of 3
hourly precip exceeding FFG values are in a similar area over
southeast Colorado early Thursday, with probabilities as high as 30
to 40%+ in the 0600-1200 UTC Thursday period.
...Southwest...
No changes made to the coordinated slight risk area across
southeast Arizona into far southwest New Mexico. Additional
scattered convection likely in the above average PW axis. The
slight risk area continues to coincide well with where the RRFS
and HREF show max probabilities of 3 hourly precip exceeding FFG
values. The RRFS is more emphatic than the HREF with these
probabilities, with values as high as 45-60%, compared to the HREF
range of 15-30%. No changes made to remainder of the broad
marginal risk from the Sierra, southward into Southeast CA and
eastward across portions of AZ and NM.
...Southwest Florida...
A weak mid level wave pushing west across South Florida will help
enhance uvvs in the tropical airmass, PW values 2.00-2.25"+. A
marginal risk area was added for Southwest Florida where HREF and
RRFS neighborhood probabilities are high for 1, 2 and 3"+ amounts.
The HREF is more emphatic about heavy rains, showing probabilities
of 3 hour precip exceeding FFG values as high as 40-60% late
Wednesday afternoon. This heavy rain potential may lead to
localized runoff issues, especially in more urbanized areas.
Oravec
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Thu Sep 10 2026
Day 1
Valid 12Z Thu Sep 10 2026 - 12Z Fri Sep 11 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE UPPER OHIO
VALLEY INTO THE CENTRAL APPALACHIANS...
...Ohio Valley & Central Appalachians...
A 300mb jet streak placing its diffluent right-entrance region over
the Ohio Valley and Central Appalachians paired with an
approaching cold front will foster a favorable environment for
developing thunderstorms. PWs within the Slight Risk will contain
anomalous PWs of at least 2.0", which are approaching the 99th
climatological percentile. In addition, MUCAPE >1,000 J/kg will
also be available within a highly saturated environment to support
between 2-3"/hr rainfall rates. Area-averaged soundings show mean
850-300mb winds are generally out of the WSW, and would parallel
the frontal boundary as it moves east, indicating there could be
potential for training. Soils have been drier of late, evident by
NASA SPoRT-LIS 0-10 and 0-40cm soil moisture percentiles. Still,
1-hr FFGs are generally around 2" within the region and given the
potential for training storms, scattered instances of flash
flooding are possible.
...Mid-Atlantic...
Some of these storms in the Central Appalachians and Ohio Valley are
likely to spill over the central Appalachians and track across
northern VA, northern MD, and the Mid-Atlantic I-95 corridor
Thursday evening. Storms should be progressive, but the same
anomalous moisture content will move into the region and some
training could occur. There is also westerly low-level wind field,
which could limit areas flash flood potential given downsloping
effects. Localized flash flooding cannot be ruled out, given the
potential for training storms and storms tracking into a higher
concentration of urban/hydrophobic surfaces.
...South-Central Plains & Mid-Mississippi Valley...
A passing cold front and a pool of anomalous PWs for mid-September
(1.8-2.0" PWs) will be available for thunderstorms as the develop
during the afternoon and linger into Thursday night. Plenty of
instability will be available (1,000-2,000 MUCAPE per 00Z HREF) to
support thunderstorms producing 2-3"/hr rainfall rates. FFGs are
higher in these areas, and given storms will also be generally
progressive, the flash flood threat will remain isolated. A
Marginal Risk for flash flooding remains in place in these regions.
...Southwest...
A Marginal Risk remains in place over portions of AZ/NM and as far
west as the Peninsular Range of southern CA. For southeast AZ and
much of southern NM (along and west of the Sacramentos), the 500mb
ridge axis remains located over central AZ will contain spokes of
500mb vorticity maxima that revolve around its southeast flank,
providing some weak upper-level support aloft. PWs remain elevated
for mid-September with >90th climatological percentile values
along the AZ/NM borders with Sonora and Chihuahua. Farther west,
PWs are above the 97.5 climatological percentile in southern CA
with strong daytime heating causing diurnally-driven thunderstorms
along the higher terrain of the Peninsular Range. Low-chance
probabilities (10-30%) remain present on HREF/REFS guidance there,
so maintained the Marginal Risk for Thursday where areas of complex
terrain have localized flash flood potential.
Mullinax
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Thu Sep 10 2026
Day 2
Valid 12Z Fri Sep 11 2026 - 12Z Sat Sep 12 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN FAR SOUTHERN
ARIZONA...
...Southern Arizona...
The ridge over southern AZ will be remain in place, although small
lobes of 500mb vorticity will move east to west along the US/Mexico
border on Friday. Much of AZ, southern UT, and western CO will
continue to contend with anomalous moisture content with PWs >1.5"
likely throughout much of southern AZ that supports a localized
flash flood threat. REFS/HREF probabilistic guidance does show a
section of far southern Arizona particularly at-risk for flash
flooding. The HREF most notably shows moderate chances (40-50%) for
3-hr QPF surpassing 3-hr FFGs around Santa Cruz county Friday
afternoon. These signals in guidance support an upgrade to a Slight
Risk in far southern AZ.
...Upper Midwest...
A 500mb shortwave trough and an accompanying frontal boundary will
track through the Upper Midwest at the same time a plume of >90th
climatological percentile PWs manifest over the region. Storm
motions should be progressive, although some training thunderstorm
activity closer to the warm front is possible as low pressure
heads for the MN Arrowhead. Rainfall totals could range between
1-2" (locally higher) in northern MN, while rainfall rates could
approach 1.5"/hr as storms track across the region Friday afternoon
and evening.
...Lower MS Valley, TN Valley into the Southern Appalachians...
A 500mb shortwave trough tracking towards the Mid-MS Valley and a
sharp 200mb trough axis aloft is set to generate healthy divergence
aloft over the Mid-South, while copious amounts of moisture
continue to reside over much of the Southeast. A nearby frontal
boundary will trigger widespread thunderstorms that are likely to
contain >2"/hr rainfall rates within the more intense storms. PWs
>2.0" will be common from the central Gulf Coast to the mid-MS
Valley, while PWs in the OH/TN Valleys range between 1.75-2.0",
which are still above the 90th climatological percentile. Storms
are likely to form as far east as the Southern Appalachians and
Carolinas, although the flash flood threat is less likely along the
Southeast coast. Localized flash flooding is possible, especially
in poor drainage areas and within more urbanized communities.
Mullinax
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Thu Sep 10 2026
Day 3
Valid 12Z Sat Sep 12 2026 - 12Z Sun Sep 13 2026
...THERE IS A SLIGHT RISK IN PORTIONS OF THE CENTRAL PLAINS...
...Central Plains...
An approaching 500mb disturbance and a strengthening southerly
low-level jet will interact with a frontal boundary that results in
scattered thunderstorms from the central High Plains to the Mid-MS
Valley. PWs ranging between 1.5-1.75" are generally around the 90th
climatological percentile and ample instability will be supportive
of strong thunderstorms producing >2"/hr rainfall rates.
Uncertainty exists in the placement of the heaviest rainfall, but
the signal supports a scattered flash flood threat Saturday
afternoon and into Saturday night.
...Southeast & Mid-Atlantic...
The broad 500mb troughing setup in the Mid-South will induce
healthy upper-level ascent over the Southeast and Mid-Atlantic on
Saturday. A warm front is forecast to lift north through the Mid-
Atlantic on Saturday that will act as a trigger for storms, while
daytime heating and height falls support widespread showers and
thunderstorms from the Gulf Coast to the northern Mid-Atlantic. PWs
will rise to above 2.0" as far north as the DC/Baltimore area by
Saturday evening with no shortage of instability as the warm sector
drifts north. QPF has been increasing across most guidance,
although the location of the heaviest rainfall is still unclear.
This setup is ideal for a localized flash flood threat,
particularly in the more urbanized sections of the Mid-South and
Mid-Atlantic.
...Florida...
Much of the Sunshine State will remain mired in an environment
containing copious amounts of moisture that will continue to
support scattered thunderstorms containing torrential downpours.
PWs between 2.0-2.25" will be common and storms will be capable of
causing localized flash flooding, especially in poor drainage areas
and in the more urbanized sections of Florida.
...Rockies & Southwest...
A strong 500mb closed-low with heights below the 10th
climatological percentile will work with an increasingly anomalous
PW field to produce scattered thunderstorms with efficient rainfall
over the Northern Rockies. Farther south, the pool of anomalous PWs
that supported daily rounds of excessive rainfall in the Southwest
through the week will stick around and cause additional
thunderstorms that produce locally excessive rainfall rates.
Instability will be lacking over the northern Rockies but soils are
more saturated following recent rainfall and may provide a slightly
more favorable environment for localized flash flooding.
Mullinax
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Thu Sep 10 2026
Day 4 and Day 5
Valid 12Z Sun Sep 13 2026 - 12Z Tue Sep 15 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
UPPER MIDWEST AND THE MONTANA HIGH PLAINS...
...Upper Midwest...
Slight risks of excessive rainfall are in effect for both Sunday
and Monday for portions of the upper Midwest. On Sunday, the Slight
covers from eastern South Dakota and Nebraska east through much of
Wisconsin. On Monday, the Slight continues across much of
Wisconsin, then continues over all of northern Michigan. Abundant
moisture from the Gulf (and the Caribbean before that) will be in
place along a well-established low level jet extending from eastern
Texas straight north through eastern Nebraska. A polar high to the
north will act as an effective barrier to that heat and moisture
from the Gulf pushing any further north. Between the high to the
north and the low level jet, an effective front will act as the
source of lift for all of the moisture running into it.
As the high moves south and east across the Great Lakes, the
southerly low level jet will get to continue north and east behind
the high. Meanwhile a pair of leeside lows developing over the
northern High Plains will allow for further convective development
Sunday night.
Given the aforementioned pattern, expect periods of showers and
thunderstorms across the Slight Risk area through the day on
Sunday, but the much heavier rains will be overnight Sunday in the
Slight Risk area as the nocturnal LLJ both naturally strengthens,
and intensifies further due to the developing pressure gradient
between the lows to the west and high to the east.
On Monday, the lows to the west consolidate into one, which tracks
northeast into Ontario. The trailing front will quickly stall out
from the central Plains (KS) northeast through Wisconsin and
Michigan. This will make for an extended and expansive corridor of
training thunderstorms, especially over Wisconsin and into Michigan
where the best forcing will be at the nose of the low level jet,
which will have turned towards the northeast, resulting in
unidirectional flow favorable for training and backbuilding storms.
...Montana High Plains...
The approach of an upper level trough with 100 kt southwesterly
jet, moisture filtering back westward from the Plains, a high to
the northeast, and a developing leeside low will allow heavy rain
to continue from Saturday night through Sunday. Given expected
rainfall across Montana from previous days, by the time the storms
organizes and produces the most rainfall of any of the prior days,
expect soils over portions of Montana to be quite saturated. Thus,
a widespread 1-2 inches (higher in thunderstorms and lower in
between the storms) is likely to cause widely scattered instances
of flash flooding over much of the Montana High Plains. For this
reason, a Slight Risk upgrade was introduced with this update for
Day 4/Sunday.
Wegman
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Thu Sep 10 2026
Day 4 and Day 5
Valid 12Z Sun Sep 13 2026 - 12Z Tue Sep 15 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
UPPER MIDWEST AND THE MONTANA HIGH PLAINS...
...Upper Midwest...
Slight risks of excessive rainfall are in effect for both Sunday
and Monday for portions of the upper Midwest. On Sunday, the Slight
covers from eastern South Dakota and Nebraska east through much of
Wisconsin. On Monday, the Slight continues across much of
Wisconsin, then continues over all of northern Michigan. Abundant
moisture from the Gulf (and the Caribbean before that) will be in
place along a well-established low level jet extending from eastern
Texas straight north through eastern Nebraska. A polar high to the
north will act as an effective barrier to that heat and moisture
from the Gulf pushing any further north. Between the high to the
north and the low level jet, an effective front will act as the
source of lift for all of the moisture running into it.
As the high moves south and east across the Great Lakes, the
southerly low level jet will get to continue north and east behind
the high. Meanwhile a pair of leeside lows developing over the
northern High Plains will allow for further convective development
Sunday night.
Given the aforementioned pattern, expect periods of showers and
thunderstorms across the Slight Risk area through the day on
Sunday, but the much heavier rains will be overnight Sunday in the
Slight Risk area as the nocturnal LLJ both naturally strengthens,
and intensifies further due to the developing pressure gradient
between the lows to the west and high to the east.
On Monday, the lows to the west consolidate into one, which tracks
northeast into Ontario. The trailing front will quickly stall out
from the central Plains (KS) northeast through Wisconsin and
Michigan. This will make for an extended and expansive corridor of
training thunderstorms, especially over Wisconsin and into Michigan
where the best forcing will be at the nose of the low level jet,
which will have turned towards the northeast, resulting in
unidirectional flow favorable for training and backbuilding storms.
...Montana High Plains...
The approach of an upper level trough with 100 kt southwesterly
jet, moisture filtering back westward from the Plains, a high to
the northeast, and a developing leeside low will allow heavy rain
to continue from Saturday night through Sunday. Given expected
rainfall across Montana from previous days, by the time the storms
organizes and produces the most rainfall of any of the prior days,
expect soils over portions of Montana to be quite saturated. Thus,
a widespread 1-2 inches (higher in thunderstorms and lower in
between the storms) is likely to cause widely scattered instances
of flash flooding over much of the Montana High Plains. For this
reason, a Slight Risk upgrade was introduced with this update for
Day 4/Sunday.
Wegman
