FOUS30 KWBC 122011
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 PM EDT Sat Sep 12 2026
Day 1
Valid 1948Z Sat Sep 12 2026 - 12Z Sun Sep 13 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE OHIO VALLEY, MID-ATLANTIC AND SOUTHERN ARIZONA...
20z Update: A Slight risk was added to southern AZ where convection
is developing within an environment of overlapping high CAPE and
PWs. Observational trends and HREF/REFS FFG exceedance
probabilities support a scattered flash flood risk over this area.
Chenard
1630Z Excessive Rainfall Discussion...
Maintained the Slight Risk area in the Ohio Valley and adjacent
portions of the Mid-Atlantic region given the ingredients in place
to focus and support convection capable of producing downpours. 12Z
upper air analysis showed precipitable water values around 1.75
inches in place with the 12Z NAM and HREF both supporting a surge
of precipitable water values of 2 inches or more reaching the
southeast Pennsylvania by early this evening and reach northern New
Jersey and the Tri-State area in the predawn hours Sunday. The
previous forecast had a good handle on this so the changes tended
to be pretty minor. The heaviest rainfall still looks to be focused
farther west through central PA and into the Upper OH Valley, a
convergence of elevated low-level theta-e within the strengthening
area of upper level divergence associated with the approach of a
surface cold front will support convection capable of being
efficient rainfall producers. The 12Z HREF/REFS probabilities for
2 inch amounts in 3 hours remained in the 50 to 60 percent range
from late this afternoon into the evening. Given the lowered Flash
Flood Guidance from recent rainfall in the area going back a few
days...still think this is a "higher-end" Slight Risk for the Upper
OH Valley, with potential for these rates.
Elsewhere...the Marginal Risk areas looked to be in good shape
given the anomalous moisture in the Central Plains as a wave lifts northeastward later today and overnight while the monsoon-fueled
convection ramps up this afternoon with daytime heating from the
Southwest US into portions of the Rockies.
Bann
0830Z Excessive Rainfall Discussion...
...Ohio Valley & Mid-Atlantic...
The broad 500mb trough in the Mid-South will induce healthy upper-
level ascent over the Ohio Valley and Mid-Atlantic through Saturday
night. A warm front lifting north through the Mid-Atlantic 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 between 2.0-2.25" will stream
north along the I-95 corridor from Richmond and D.C. to Baltimore
and Philly by Saturday evening, then into the NYC/Tri-State areas
in the pre-dawn hours Sunday. With instability increasing as the
warm sector drifts north and warm cloud layers likely to be at
least 12,000ft, highly efficient warm rain processes within
developing thunderstorms are likely. The 00Z HREF shows low-to-
moderate chances (20-40%) for 3-hr QPF >2" from the Richmond, VA
area late Saturday afternoon, then from northeast MD into the
Delaware Valley and southern NJ Saturday night. The Slight Risk was
extended north through eastern PA, northern NJ, and into the NYC
metro and lower Long Island. It is important to note for the
northern NJ suburbs, NYC, and Long Island that the Slight Risk is
primarily for late Saturday night with the excessive rainfall
threat lingering into Sunday (Day 2).
Farther west through central PA and into the Upper OH Valley, a
convergence of elevated low-level theta-e within the strengthening
area of upper level divergence in the 200-400mb layer will generate
bands of thunderstorms that track parallel to the mean 850-300mb
wind flow. Soundings are highly saturated from just above the
surface to as high as 300mb (>90% RH). Similar to areas farther
east, warm cloud layers are at least 12,000ft deep and FFGs are
less than 2" in northern WV and western PA. The 00Z HREF shows
worrying signals with moderate-to-high chances (50-80%) for 3-hr
and 6-hr FFG exceedance within the Upper OH Valley, and moderate
chances (40-60%) for 2" of rainfall within 3-hours Saturday
afternoon. This is a "higher-end" Slight Risk for the Upper OH
Valley, with potential for these rates to spills over into north-
central PA early Saturday evening.
...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.
The latest 00Z HREF guidance shows 24-hr QPF >3" probabilities
remain on the low end (10-30%), and are isolated across eastern KS
and western MO. For those reasons, decided to hold on with a
Marginal Risk this forecast cycle.
...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 Central Rockies, but soils
are fairly saturated in parts of AZ and NM following recent
rainfall and provide a slightly more favorable environment for
localized flash flooding.
Mullinax
Day 2
Valid 12Z Sun Sep 13 2026 - 12Z Mon Sep 14 2026
...THERE ARE SLIGHT RISK AREAS FOR EXCESSIVE RAINFALL OVER
PORTIONS OF THE NORTHEAST & MIDWEST...
2030Z Excessive Rainfall Discussion Update...
Northeast...
Rainfall should be on-going as the Day 2 period begins at 13/12Z
over portions of northern New Jersey eastward into southern New
England as mid-level height falls induces surface low
pressure...albeit weak...in proximity to the Southern New England
coastline. This is a bit delayed from the previous suite of model
guidance. As a result...there may be a bit more time for moderate
to heavy rainfall and the Slight Risk was expanded a bit northward
and westward...including more of Long Island and adjacent coastal
areas in addition to the previously mentioned portions of
Connecticut and Rhode Island into Massachusetts. The 12Z HREF
probabilities depicted 45-60%) for 3-hr QPF >3" across the area
during the mid- to late morning hours on Sunday.
Midwest...
The 12Z suite of model guidance tended to show somewhat better
clustering in the QPF guidance as flow over the Upper Midwest backs
in response to height falls coming out of the Rockies, As a
result...the Slight Risk and surrounding Marginal Rusk areas were
adjusted. There may be room for additional adjustment but kept the
changes modest at this point in case the model opinion swings back.
...Southern Rockies & Four Corners...
Monsoonal moisture remains in place as flow aloft strengthens in
response to the mid-level system approaching the international
border late in the period. Current outlook still appears to capture
the area with the greatest chances of heaviest rain based on model
spaghetti plots.
Bann
0830Z Excessive Rainfall Discussion
...Northeast...
The region will reside beneath the right-entrance region of a
strengthening 250mb jet streak, creating strong 200-400mb layer
Q-vector divergence over the Northeast. This is allowing for
unusually strong upper-level frontogenesis atop southern New
England, in addition to the surface warm front that is forecast to
span a strengthening surface low over Long Island early Sunday
morning. This is a continuation of the bands of strong
thunderstorms containing highly efficient rainfall rates from
Saturday night. PWs up to 2.25" are expected and low-level theta-e
advection into the warm front will direct MUCAPE between 500-1,000
J/kg into southern New England.Area averaged soundings across
multiple CAMs members paint a concerning picture; highly saturated
profiles within a sheared environment that could support enhanced
mesoscale rainfall rates >2"/hr within an urbanized corridor. The
rainfall threat moves into RI and southeast MA late morning and
into the afternoon before exiting off the coast late afternoon.
This setup led to the issuance of a Marginal Risk that extends
from northern NJ to as far north and east as southern NH/ME, and a
Slight Risk from the NYC metro on east to the Boston metro and MA
Capes. There is greatest concern for flash flooding impacts over
Long Island and the coastal portions of CT and RI where 00Z HREF
probabilities should moderate chances (40-60%) for 3-hr QPF >3"
across the mid-late morning hours on Sunday.
...Midwest & Central Plains...
A frontal passage and a fast moving 500mb shortwave trough heading
for the Great Lakes will trigger rounds of showers and
thunderstorms from South Dakota and the Nebraska Sand Hills to as
far south as northern KS/MO and as far east as the MS River. As the
first shortwave trough exits east, a strong 500mb closed low will
head through the Northern Rockies, prompting the development of low
pressure in the northern High Plains. This area of low pressure
will work in tandem with high pressure over the Great Lakes Sunday
night to generate a 30-40kt LLJ over the Central Plains. The LLJ
will advect rich moisture and warmer theta-e air into the Central
Plains with the frontal boundary that passed through early Sunday
morning lifting slowly as a warm front Sunday night and into early
Monday morning.
Latest ECMWF guidance shows a pool of anomalous PWs (above the
97.5 climatological percentile) sitting over the Central Plains on
Sunday that lifts north into the Midwest by early Monday morning.
With the LLJ intersecting the frontal boundary, and elevated
instability available, additional strong thunderstorms are likely
to form late Sunday night and into early Monday. While exact
placement of the heaviest rainfall still differs, most guidance is
in agreement that the Midwest will be the focus for potential flash
flooding on Sunday and into Sunday night. The latest WPC QPF has
shifted the heaviest rainfall into the MO River Valley with 1-2"
rainfall amounts (locally higher possible) in northeast KS,
northwest MO, eastern NE, and southwest IA.
...Southern Rockies & Four Corners...
The troughing pattern in the Northwest and the mid-upper level
ridge axis over the Southern Plains will direct additional
monsoonal moisture into the Southwest and Southern Rockies. ECMWF
percentiles suggest PWs will rise to above the 97.5 climatological
percentile Sunday evening as IVT values >300 kg/m/s direct moisture
into the regions. Some locations continue to sport saturated soils,
thus making these areas more prone to potential flash flooding.
While confidence in higher rainfall rates is lower at the moment,
the rgion's sensitive soils and potential for excessive rainfall
rates >1.0"/hr continues to support the Marginal Risk in place.
Mullinax
Day 3
Valid 12Z Mon Sep 14 2026 - 12Z Tue Sep 15 2026
...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDWEST & SOUTHERN ARIZONA...
2030Z Excessive Rainfall Discussion
Adjusted the Slight Risk area a bit as a surface cold front
associated with a deepening upper trough approaches the region.
Expanded the southern boundary of the Slight risk from the
placement shown by the deterministic models given some members of
the ECMWF and GEFS spaghetti plots. The alignment of the upper and
lower level flow becomes favorable for the training of cells and
repeat convection later in the period.
In the Southwest...another round of late-day and evening convection
is expected with monsoon moisture remaining pretty much in place.
Given the PW anomalies...saw little reason to make many changes
here.
Bann
0830Z Excessive Rainfall Discussion
...Midwest...
A strong closed 500mb low, sporing 500mb heights below the 10th
climatological percentile per ECMWF, will work in tandem with a
strong 500mb ridge over the Mid-South to produce an exceptional IVT
throughout much of the North Central U.S.. NAEFS shows the IVT
topping 750 kg/m/s over the MO River Valley and into MN Monday
morning, then strengthening to >1,000 kg/ms/s over the UppeR MS
Valley by Monday afternoon. Strong theta-e advection and anomalous
PWs that surpass the 99th climatological percentile will set the
stage for an active day, consisting of widespread showers and
thunderstorms that contain highly efficient warm rain processes. The
area of greatest concern is along the warm front, closest to the
surface low track, as veering low-level wind profiles ahead of the
approaching cold front support clusters of organized thunderstorms.
Latest WPC QPF shows widespread 1-2" of rainfall from eastern NE
to as far north as the Upper Great Lakes with localized totals
surpassing 3" possible. The inherited Slight and Marginal Risks
remain in good shape, but extended the Slight Risk into southern NE
to account for recent south and west trends in QPF.
...Southwest...
Broad upper-level troughing over the West and upper-level ridging
in the Southeast will continue to support a healthy monsoonal flow
into AZ/NM and as far north as the Central Rockies. NAEFS shows IVT
values still topping the 90th climatological percentile for mid-
September. In addition, the Four Corners region is ideally placed
beneath the right-entrance region of a 250mb jet streak that favors
healthy upper-level ascent. The region has remained fairly active
in recent days and will continue to see daily rounds of scattered
showers and storms over the weekend, helping to further saturate
soils ahead of Monday. QPF is heaviest in southeast AZ at the
moment, and given soils are more likely to moisten rather than dry
out in the coming days, a Slight Risk was issued in ands around the
Tucson area for Monday. A broad Marginal Risk accounts for the
potential of localized flash flooding from central AZ on east
through much of NM and northward into the CO Rockies.
Mullinax
Day 4 and Day 5
Valid 12Z Tue Sep 15 2026 - 12Z Thu Sep 17 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS, MISSOURI VALLEY INTO PARTS OF THE CORN BELT
AND UPPER MIDWEST...
20z Update: Still on track for a favorable axis of
training/backbuilding convection on Tuesday near a stalling front
over the Plains/MS Valley. The relatively consistent AI models and
overall guidance trends support a bit of a southwest shift in the
Slight risk axis with this update, with the risk now extending from
central KS into central IL. The stalling front, persistent
moisture transport into the boundary, sufficient instability, and
robust model QPF output, all point towards at least a locally
higher end flash flood risk. The greatest threat at this time
appears to be from northeast KS, into far southeast NE, far
southwest IA and into northwest MO...where this is considered a
higher end Slight risk. Pending model trends, can not rule out a
focused MDT risk at the event nears.
Not much change was needed to the day 5 (Wed) Slight risk that
stretches from KS into IN. A bit more model spread by this time,
but the overall setup will remain favorable for periods of
training convection. Depending on antecedent rainfall, an embedded
MDT risk is possible as the event nears.
The Southwest will see an uptick in convective coverage Tuesday
into Wednesday as well. At the moment, day 5 (Wed) seems to be the
most active, and a Slight risk may eventually be needed over NM
and eastern AZ.
Chenard
...Previous Discussion...
...Day 4...
The longwave pattern will remain relatively stable east of the
Mississippi with a strong mid and upper ridge axis located across
the southeastern quarter of the U.S. with a formidable area of
surface high pressure dominating the weather pattern for these
areas. Across the Western CONUS, multiple shortwave troughs will
lead to a general trough axis and prevailing southwesterly flow
extending from the Southwest U.S. through the center of the nation
with an expected quasi-stationary frontal placement bisecting areas
from the Central High Plains to points east-northeast through the
Missouri Valley and Corn Belt with the eastern inflection back into
the Lower Great Lakes. This setup will be coincident with a bit of
a Maddox signature which typically precludes more appreciable
heavy rain prospects given the limited forward propagation expected
of a surface front with mean flow generally parallel to the
surface feature leading to training and back- building scenarios
within proximity of the front. Global deterministic continues to
imply multiple smaller shortwave perturbations ejecting northeast
through the main axis of diffluence situated over the above
regions. This will only act as additional regional ascent to aid in
convective genesis, and creating a repeated pattern of heavy rain
prospects as the setup evolves.
Ensembles remain consistent with a mean axis of heavier QPF
situated within the confines of the stationary front through the
course of the forecast period. A textbook bimodal distribution of
the heavier rainfall is forecast with the first enhancement occurring
in the early morning hours Tuesday, then once again in the evening
as the formation of the nocturnal LLJ will lead to enhanced
convergence near the front. Bias corrected QPF aligns a bit further
to the north than the ECENS output, as well as the ML guidance
which has the frontal alignment a bit further south as noted when
assessing the theta_E gradient on each respective model. Heavy
precip is likely given the setup with a pretty stout signature
regardless of which model depiction you assess. Mean QPF of 2-3" is
already forecast within the area of northeast KS, southeastern NE
through northern MO and southern IA. The general max will lie
within 50 miles of either side of the frontal placement, so finding
the exact location of the front will be the key to figuring out
the specifics of where the highest flash flood threat would occur.
For now, the setup is more than sufficient to maintain a broad SLGT
risk over the areas referenced, but did expand more to the
southwest to include Central KS, as well as northeastern extension
up through the southern fringes of the western Great Lakes. This
takes into account some of the known biases of models within the
medium range that trend more in these general directions in setups
like these with a well-defined front and healthy thermodynamic
environments on the flank of the front.
Another period of scattered convective potential across the
Southwest U.S. will lead to yet another MRGL risk in place for the
D4, but will also have to keep an eye for any short term upgrades
given the propensity for repeated action, as well as the
introduction of better kinematics as these smaller shortwaves make
their way around the western periphery of the ridge.
...Day 5...
General persistence for the pattern across the CONUS will lead to
another period of heavy rainfall across the center of the nation
with scattered heavy convective potential lingering over portions
of the Southwest U.S. Consensus is a maintenance of a relatively
stable upper level setup with a lingering quasi-stationary front
bisecting the same areas as the previous period(s). The front will
struggle to lose much latitude unless propelled by stronger cold
pools emanating from the convective threat that occurs over the
previous 48 hrs, continuing into the new D5. There's no real
deviation in the ML and ENS suites in the placement of heavy rain
somewhere between the Central Plains into the western Ohio Valley
with the max crossing through that area in southern IA and northern
MO which would make at least 2 consecutive days of heavy rain in
those zones. Multi-day totals between 3-5" are forecast at range
which is typically coincident with a potential for a significant
flash and hydrologic flood threat when looking back at previous
events in this region of the CONUS. A broad SLGT was maintained
over very similar areas as the previous period with a touch broader
expanse due to some uncertainty in exact frontal placement. The
risk area is expanded a bit southward compared to the previous
forecast to account for some southern frontal motion given the
expected cold pool propagation from previous convection. This time
frame will be one to monitor, once again, as the threat for more
appreciable flooding is at play with this setup.
Scattered thunderstorms will remain forecast across the Southwest
into the Central Rockies thanks to the stagnant upper pattern in
place. Pending more notable shortwave advancements as we move into
the short term, an upgrade is plausible for the period considering
the incessant nature of the heavier precip occurring for several
consecutive periods. For now, the MRGL risk was maintained across
the aforementioned areas with the highest threat centered over
local urban zones, burn scars, and flashier arroyos and small
streams situated over NM/CO/AZ.
Kleebauer
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt__;!!DZ3fjg!5Ml980frefk4Y0E3-WnN8nvf3k3dYHAhBp1Li4hVPIgJ1ygNh46j2BfIaJeUZRjV2r0D5Fvx1JC0w2l6X9sxgPnQJgQ$
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt__;!!DZ3fjg!5Ml980frefk4Y0E3-WnN8nvf3k3dYHAhBp1Li4hVPIgJ1ygNh46j2BfIaJeUZRjV2r0D5Fvx1JC0w2l6X9sxbB788uM$
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt__;!!DZ3fjg!5Ml980frefk4Y0E3-WnN8nvf3k3dYHAhBp1Li4hVPIgJ1ygNh46j2BfIaJeUZRjV2r0D5Fvx1JC0w2l6X9sxNHWpQS8$
$$
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