Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Tue Sep 8 2026
Day 1
Valid 01Z Wed Sep 09 2026 - 12Z Wed Sep 09 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS IOWA...
01Z Update...
Made only minor modifications to ensure the Slight Risk covered the
WPC Mesoscale Convective Discussion #1225 for the area. That
brought a few counties of Nebraska into the Slight and a bit of
Illinois into the Marginal risk area. Overall forecast reasoning
remains similar to what was described in previous Excessive
Rainfall Discussions. Given trends in radar...removed the Marginal
Risk area over Florida. For the Western US...expectation is that
convection should be waning with the loss of daytime heating but
felt it was too early to remove the Marginal Risk areas at this
point.
Bann
16Z Update..
Only a minor expansion to the inherited SLGT Risk area was needed
into northeast Iowa and through far southwest Wisconsin. The
previously mentioned meteorological setup remains valid, and
adjustments were made based on the latest suite of global and CAMs
guidance. The surface front forecast to move through Iowa will
support a line of a progressive and efficient line of
thunderstorms. PWAT values are forecast to exceed 2", and in some
scenarios approach 2.25", and will support heavy rain rates, with
HREF and RRFS probs suggesting high chances (65% and 85%
respectively) of 1"/hr rates. SREF Spaghetti Plots also suggested
greater consensus of 2" amounts over the Day 1 period in northeast
Iowa, which further supported the expansion.
Otherwise, only cosmetic adjustments were needed for the inherited
MRGL Risk area to match the latest run of global and CAMs
guidance.
Blanco-Alcala
...Previous Discussion...
...Iowa...
The surface front currently pushing east into the Northern Plains
will remain very progressive day 1 as it moves into the Upper
Mississippi Valley/Upper Lakes. The progressive nature of the front
will continue to be a detriment to widespread runoff issues.
However, the southwestern portion of this front will slow somewhat
compared to the farther northern portion in the vicinity of eastern
NE /northeast Kansas, east into Iowa. Across these regions there
is a better chance of a period of training of cells late
Tuesday/early hours of Wednesday. A slight risk area was maintained
where current HREF and RRFS neighborhood probabilities are high
for 1, 2 and 3"+ amounts this period.
Marginal Risk areas
..Eastern Arizona into western to central New Mexico...
No significant changes day 1 to the axis of above average PW
values extending from eastern portions of Arizona into western to
central New Mexico. Continued low confidence with qpf details, but
potential for another round of widespread scattered convection
across this area. Locally heavy rain and isolated runoff issues
again possible. Anomalous PW values also expected to push farther
north from Southern CA into Central CA. At the moment, models are
not showing precip developing day 1 in this anomalous axis.
...Central to South Florida...
No changes made to the previous marginal risk across central to
south Florida. PW values are expected to remain anomalous day 1,
2.00-2.25"+ to the south of the frontal boundary across northern to
central FL. HREF and RRFS neighborhood probabilities are fairly
high for 1,2 and 3"+ totals. Areas of heavy rain likely with
isolated runoff issues possible, especially in urbanized regions.
Oravec
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Tue Sep 8 2026
Day 2
Valid 12Z Wed Sep 09 2026 - 12Z Thu Sep 10 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER FAR SOUTHEAST
KANSAS INTO CENTRAL MISSOURI AND THE SOUTHWEST...
21Z Update..
A Slight Risk upgrade was introduced in southeast Arizona and far
western New Mexico in coordination with the local forecast offices. The
early availability of CAMs guidance indicated notable FFG Max
Exceedance probabilities, especially in the RRFS. Anomalous mean
PWATs are expected to exceed the 90th percentile throughout the
period. Scattered thunderstorms will form and be fueled by this
anomalous moisture, and will be locally supported in the upper-
levels, with precip occurring along the eastern periphery of a
ridge, promoting some areas of divergence aloft. A building upper-
ridge should confine precipitation to be in the form of scattered
thunderstorms mostly along southern Arizona, but will have the
potential to bring heavy rainfall.
Elsewhere, the inherited SLGT risk was expanded to include more of
eastern Kansas and central Missouri. Updated HREF & RRFS
precipitation probabilities suggest medium to high chances (~55% &
75% respectively) of rainfall exceeding 3" during the day 2
period. In terms of hourly rates, both HREF and RRFS neighborhood
probabilities suggest medium chances ~50-60% of rainfall exceeding
1"/hr over a 3-5hr period. The general storm motion should be
fairly progressive however, which resulted in maintaining the
Slight.
The inherited MRGL Risk areas were also expanded and introduced
through the Ohio Valley and Lower Great Lakes, as well portions of
the Southern Plains, to capture the latest run of global models
and ensembles. An introduction to a SLGT was considered over the
Ohio Valley given higher QPF totals and higher rain rates per the
HREF, but the expected quick progression of storms was the main
deciding factor to not upgrade to a SLGT. Later runs may
necessitate the need for a SLGT, particularly if storm motions
slow.
Blanco-Alcala
...Previous Discussion...
...Lower Missouri Valley...
A surface cold front expected to continue to sink southward
through the Mid MS, Lower MO and into the OH Valley region day 2.
PW values expected to remain anomalous along this front with values
2"+, supporting potential for widespread moderate to locally heavy
totals. A small slight risk area was added over far southeastern
portions of Kansas into central Missouri for potential of training
of cells in this anomalous PW axis. Much of this area has, however,
been very dry over the past month, which may mitigate potential for
runoff issues. RRFS neighborhood probables are high for 1 and 2"+
totals day 2 in the slight risk area and RRFS probabilities of 3
hour precip exceed FFG values are in the 20-30%+ range at 0000 UTC
Sept 10. These probabilities are not available for the 0000-1200
UTC period, but given the overall favorable pattern along this
front for heavy rains, similar probabilities likely will continue
along the front into early Thursday morning.
Marginal risk areas.
...Sierra, Southeast California, Southwest...
A very broad area of anomalous PW values, 2 to 3+ standard
deviations above the mean, will stretch day 2 from central to
southern California, across the southern Great Basin and into the
Southwest. Latest models do show scattered precip in this high PW
axis from the Sierra into Southeast California and over the
Southwest. The heaviest totals continue across similar areas day 1
and 2 from eastern Arizona into western to central NM where a
marginal risk continues day 2. The marginal risk was expanded
northward from day 1 into the Sierra to capture any locally heavy
precip amounts in the much above average PW axis.
...Northern NY State into Northern Vermont...
The northeastern portion of the front moving south into the Mid MS
Valley/OH Valley region will also remain very progressive to the
east across the eastern Lakes, northern NY State and northern New
England. PW values expected to be anomalous along this portion of
the front also, with values 1.75-2.00"+. No changes to the
marginal risk area for far northern NY state into northern VT where
model consensus is for moderate to heavy precip totals. The
expected very progressive nature of the front will be a detriment
to widespread runoff issues, but the anomalous PW values and
potential for a brief period of training may produce isolated
issues.
Oravec
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Tue Sep 8 2026
Day 3
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...
21Z Update..
Virtually no change was made to the inherited Day 3 outlook other
than cosmetic adjustments and minor tinkering to the SLGT Risk
area to match the latest run of global and ensemble guidance. An
uptick in the multi-model ensemble bias-corrected QPF was noted as
a potential concern, particularly in Kentucky, although there was
a lack of agreement in the global models, and particularly high QPF
in the NAM when compared to the other global models. As such, no
expansion was needed here, although it could be considered in
future updates if CAMs guidance also decide to go the NAM route.
Blanco-Alcala
...Previous Discussion...
...Upper Ohio Valley into the Central Appalachians..
PW values 2+ standard deviations above the mean are forecast along
the west-east oriented frontal boundary sinking south through the
Ohio Valley, Central Appalachians and Mid-Atlantic region day 3. A
slight risk was maintained where there is an overlap of relatively
low FFG values and expected moderate to heavy precip totals. H85
flow parallel to this front will support training potential and
also enhance uvvs in the upslope regions of the Central
Appalachians.
Marginal Risk areas.
...Southwest...
A marginal risk area was continued over portions of the Southwest
for potential for local heavy rains and isolated runoff issues.
Model consensus is for a smaller area of potential heavy rains
compared to the previous two days, with the marginal risk area
drawn for a smaller geographic area over southeast Arizona and
southwest New Mexico. Confidence remains low with qpf details day
3.
Oravec
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Tue Sep 8 2026
Day 4 and Day 5
Valid 12Z Fri Sep 11 2026 - 12Z Sun Sep 13 2026
...THERE IS A MARGINAL RISK FOR FLASH FLOODING FOR MUCH OF THE
SOUTHEAST AND PORTIONS OF THE MIDWEST, PLAINS, AND WEST...
...2030Z Update...
Few changes needed. The previous ERO still covered the few places
where the 12Z suite of guidance still showed the highest QPF totals
(along and ahead of the upper Midwest cold front...Western
US...Southeast US...).
Bann
Previous Discussion...
Large sections of the country are in a Marginal Risk for flash
flooding on both Friday and Saturday. Across the Southeast,
abundant tropical moisture will remain in place under weak
ridging/a rather zonal jet over the Northeast. From the lower to
mid Mississippi Valley east across the Ohio and Tennessee Valleys,
and into VA/NC, the proximity to fronts or the Texas dryline will
generally decide where more organized and concentrated showers and
thunderstorms will develop Friday afternoon into the evening.
Meanwhile, a strong low well up into Canada will push along a cold
front over the upper Midwest/upper Mississippi Valley. While the
front will be more progressive into the upper Lakes, the front may
stall further south near KS/MO. This area near the end of a front
is always a dangerous spot to be as far as flash flooding concerns
go, with abundant moisture, forcing, and slow-moving storms all
concentrated in one area. Thus, this area will be closely watched
in the coming days for a potential Slight Risk upgrade.
Across the West, the monsoon will continue across the Four Corners
region both days. By Saturday, a strong "bowling ball" upper low
will swing across the Pacific Northwest, providing the forcing for
widespread shower and thunderstorm activity over generally flood-
prone areas due to the usual aridity over the area and the
plentiful terrain. Isolated instances of flash flooding are
possible.
Wegman
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Tue Sep 8 2026
Day 4 and Day 5
Valid 12Z Fri Sep 11 2026 - 12Z Sun Sep 13 2026
...THERE IS A MARGINAL RISK FOR FLASH FLOODING FOR MUCH OF THE
SOUTHEAST AND PORTIONS OF THE MIDWEST, PLAINS, AND WEST...
...2030Z Update...
Few changes needed. The previous ERO still covered the few places
where the 12Z suite of guidance still showed the highest QPF totals
(along and ahead of the upper Midwest cold front...Western
US...Southeast US...).
Bann
Previous Discussion...
Large sections of the country are in a Marginal Risk for flash
flooding on both Friday and Saturday. Across the Southeast,
abundant tropical moisture will remain in place under weak
ridging/a rather zonal jet over the Northeast. From the lower to
mid Mississippi Valley east across the Ohio and Tennessee Valleys,
and into VA/NC, the proximity to fronts or the Texas dryline will
generally decide where more organized and concentrated showers and
thunderstorms will develop Friday afternoon into the evening.
Meanwhile, a strong low well up into Canada will push along a cold
front over the upper Midwest/upper Mississippi Valley. While the
front will be more progressive into the upper Lakes, the front may
stall further south near KS/MO. This area near the end of a front
is always a dangerous spot to be as far as flash flooding concerns
go, with abundant moisture, forcing, and slow-moving storms all
concentrated in one area. Thus, this area will be closely watched
in the coming days for a potential Slight Risk upgrade.
Across the West, the monsoon will continue across the Four Corners
region both days. By Saturday, a strong "bowling ball" upper low
will swing across the Pacific Northwest, providing the forcing for
widespread shower and thunderstorm activity over generally flood-
prone areas due to the usual aridity over the area and the
plentiful terrain. Isolated instances of flash flooding are
possible.
Wegman
