Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 PM EDT Sat Sep 5 2026

Day 1
Valid 16Z Sat Sep 05 2026 - 12Z Sun Sep 06 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN NORTH
CAROLINA INTO THE VIRGINIA TIDEWATER...

...Southern Ohio Valley through Carolinas...

16Z Update: Complex from this morning is making a steady
progression into VA at this hour with a relatively progressive
signature in the storm motion over the past 3 hrs. This steady
progression will likely maintain itself over the course of the
afternoon and evening as reflected via a majority of the CAMs and
even the global interpretation. This has resulted in a slight
decrease in the probs for >3" across portions of southern VA into
NC with the strongest signal still prevalent over the Tidewater of
VA down through eastern NC. This is still the thought for where the
best flash flood threat would arise, but sandier soil composition
in eastern NC will limit more substantial flash flood prospects and
tend to favor isolated to widely scattered instances in more urban
settings areas inland, west of Pamlico Sound. Sufficient buoyancy
and deep moisture presence over the area outlined in the SLGT
maintains a suitable environment overall with the 12z sounding out
of KMHX signaling a moist boundary layer and strong SBCAPE output
~3500-4000 J/kg. This would propose any convective threat to be
pretty prolific once matured with cells capable of local 2-3"/hr
rates which tend to tip the scale for flash flood prospects in that
area of the Carolina's. Considering the signal still prevalent,
maintained general continuity of the SLGT with a small change in
the northern fringe of the risk given the slightly more progressive
attribute of this afternoon's convection in the VA.

Across the Ohio Valley, signals for scattered heavy rains over IN
down through southern OH/WV and KY remain with the frontal
approach adding some surface based enhancement as it migrates
through the area. Trailing mid-level vorticity around the backside
of the trough will aid in upper support favorable for convective
prospects through the peak hours of instability this afternoon and
evening before sinking southeast into TN and the Carolina's by the
back end of the forecast. The MRGL risk was more than sufficient
for this threat as modest probabilities for >2" (25-40%) exist over
the the aforementioned areas.

Kleebauer

..Previous Discussion..

Persistent west-northwesterly flow regime aloft will see a change
starting today as large scale closed low/trough over northeastern
North America retrogrades today with stronger north to south push
of drier Canadian air crosses the Great Lakes into the Ohio Valley
by the end of the day. The lingering deep layer moisture plume and
conditionally unstable air mass will start to shift through the
Upper Ohio Valley and through the Central Appalachians with some
ongoing WAA convective line SE OH, E KY and WV at the start of the
forecast period. This 'back-door' cold front will also continue a
southward push through the Mid-Altantic and interact with the Ohio
Valley wave toward peak diurnal period in the afternoon across VA
into N NC interacting with the higher theta-E air and higher
moisture over the Carolina coastal Plains.

Total PWats to 1.75-2" will support hourly rates of similar range
though forward propagation and interaction with the sea-breeze
boundary will likely result in the greatest scattered totals
ranging from 2-5". HREF probs of 3"/24hrs (mainly coastal) are
around 50% but also across sandier, higher FFG values. So scattered
incidents of flash/rapid inundation flooding are possible. Overall little
change was made for the low-end Slight Risk for this cycle.

Gallina

...Gulf Coast...

16Z Update: Current forecast MRGL across the Gulf Coast and the
southern half of FL remains on track with the morning model
consensus maintaining a consistent signature of scattered heavy
rain signals with potential for local areas exceeding 3" in the
hardest hit locales. Areas of note for the D1 will be over the
western side of LA with a rapid development of convection after the
18z time frame that will lead to flash flood concerns in more
urbanized footprints, especially those along I-10 in southwestern
LA (Lake Charles to Lafayette area).

Across FL, expecting rates to be between 2-3"/hr on average
leading to localized flash flood concerns for areas south of I-4.
HREF neighborhood probs for >3" are greater than 60% for a large
portion of South FL with a sharp decline in probs for >5" meaning
heavier totals likely in the 3-5" range which historically is
sufficient for isolated flash flood potential over that area of the
Sunshine state.

Kleebauer

..Previous Discussion..

Return flow along the southern edge of the synoptic ridge with weak
embedded waves will continue to support enhanced regions of
convective activity at diurnal peak. Broad zone of 2-2.25" total
PWats and narrow, skinny profiles supporting 1500-2000 J/kg of
MLCAPE will be sufficient to support stronger cells with intense
rates over 2"/hr. Guidance is in fairly good agreement for a few
areas of enhanced confidence for increased coverage and
organization. The larger/broader shortwave will be progressing
westward from S MS to SE TX with enhanced 2-3"/24hr probabilities
across the saturated areas of SE TX into SW/S LA, as well as, a
secondary maxima expected across S AL and western Panhandle of
Florida. Model trends show some isolated to scattered activity
between across northern Florida, but minor enough to remove from
the Marginal Risk.

Overall coverage and increased easterly flow across the Bahamas
into Southeast Florida should also see an increase in convective
activity with increased probabilities through much of the southern
half of the peninsula, favoring the eastern side due to the
slightly increased convergence.

Gallina

...Upper Rio Grande Valley through Central High Plains...

16Z Update: Little change was necessary for the inherited MRGL risk
across portions of the Southwest U.S. into the Central High Plains.
Models remains consistent in their presentation of heavy rainfall
likely confined to the terrain with a few cells drifting off the
terrain and impacting the neighboring valley's in AZ/NM before
dissipating. Heaviest rainfall will occur over the Central High
Plains of northeastern CO into southwest NE and northwestern KS.
Modest probabilities for locally >2" exist over southwestern NE
and the northeastern corner of CO leading to a solidly MRGL flash
flood concern this evening.

Kleebauer

..Previous Discussion..

Day 1 period will see a lull in available moisture and convective
convergence relative to prior and future days. However, the return
flow around the western edge of the synoptic ridge. Low level
flow/upslope through the Rio Grande/Pecos River Valleys will
overlap with weak southwesterly flow across NW Mexico. Mountain
circulation will likely drive initial convection and pose a risk
mainly for known/prone burn scars across much of NM. However,
deeper layer flow will continue to transport moisture northward
through the Central High Plains. Weak convective clusters are
expected to grow upscale after 00z across NE CO/SW NEB/W KS, where
increasing pooled moisture could support slower clusters with
internal cell mergers. HREF probs of 40-50% for 2" and 20% spots of
3" in an area of similar FFG values. As such, have expanded the
Marginal Risk of Excessive Rainfall north-northeast to encompass
the area up to the southern SandHills of NEB.

Gallina


Day 2
Valid 12Z Sun Sep 06 2026 - 12Z Mon Sep 07 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL PORTIONS OF THE
GREAT BASIN AND SOUTHEASTERN U.S...

...Great Basin....

20Z Update: Changes to the previous SLGT were to extend the risk
down into portions of the Mojave Desert with emphasis on the area
encompassing Death Valley and surrounds. Trends are for locally
heavy convection to occur near those areas by Sunday afternoon with
rainfall rates between 0.5-0.75"/hr possible in the stronger cell
cores. These areas are susceptible to flash flooding in these types
of environments and projected rates, so wanted to cover for the
threat given the susceptibility.

Elsewhere into UT, extended the SLGT to include much of Central UT
towards the Uinta Range where models have trended higher in their
QPF signals as we approach the time frame of interest. Anytime you
can advect PWAT anomalies of +3 or greater into the region, the
prospects for flash flooding become much greater over this area of
the country as past events would concur. QPF between 1-2" is
forecast from southwestern UT up through Central UT, including the
more favorable slot canyon areas in Zion and Bryce Canyon National
Parks. HREF probs for these totals are solidly >60% everywhere in
the current SLGT risk issued, a signal that typically correlates to
the SLGT risk threat.

Kleebauer

..Previous Discussion..

An upper low centered along the Pacific Northwest coast early in
the period is expected move inland, reaching the northern
Intermountain West by the close of the period. Meanwhile, a series
of shortwaves moving along the base of the associated trough will
track across southern California before lifting northeast through
the Great Basin. In addition to this energy, southwesterly flow
will tap tropical moisture associated with Maria, supporting an
impressive PW anomaly axis (+3 std dev) extending from southern
California to central Utah. This moisture along with large-scale
ascent afforded by the aforementioned shortwaves and upper jet
forcing is expected to set the stage for widespread shower and
thunderstorm activity. However, limited instability remains a
potentially factor with regards to rainfall rates. A Slight Risk
remains where models continue to signal where moisture, lift, along
with orographic effects will support an enhanced threat for
heavier amounts. Only minor adjustments were made to the previous
Slight Risk area extending from southern Nevada and northwestern
Arizona to central Utah, including a small shift to the north that
reflects the latest trend in the guidance. Remaining in this area
are some of the more flash flood prone areas in southwestern Utah.

Farther east, more typical monsoon moisture will support diurnal
activity, with the potential for isolated flash flooding across
southeastern Arizona, New Mexico, and southern Colorado.

Elsewhere, a broad Marginal Risk was maintained to the north,
covering portions of the Northwest, northern Rockies, and High
Plains. The previously noted upper low/trough is expected to
support widespread activity. Instability west of the Rockies will
remain quite limited, however rates may become sufficient to raise
isolated concerns, especially across recent burn scars.

...Southeast...

20Z Update: The current forecast was maintained for areas
encompassed in the SLGT risk over SC into the northern coast of GA.
The SLGT was expanded to the south over much of southeastern GA
down into northeastern FL as trends remain favorable for a bit more
progression of the outlined front, below. HREF neighborhood and EAS
probabilities took a notable jump in the latest run with the latter
now indicating a 40-60% prob for at least 1", something notably
more prevalent with SLGT risks across the Southeastern CONUS. Areal
instability will run between 2000-3000 J/kg SBCAPE, on average with
PWAT firmly >2" across the area situated along and south of the
front. Low-level convergence pattern near the coastal plain from
areas south of Charleston, SC down into Jacksonville will
ultimately aid in convective initiation Sunday afternoon with a
steady progression southward as we move through the period.
Heaviest rainfall will begin over SC/GA and migrate into northern
FL by late-afternoon, carrying into the evening. In coordination
with the local Jacksonville WFO, a the SLGT risk was expanded
southward to account for the threat and trends in the latest CAMs.

Kleebauer

..Previous Discussion..

Energy dropping into the base of a broader-scale
trough centered along the East Coast is expected to support a weak
wave developing along a moisture-laden front extending from the
coast near the South Carolina-Georgia border back across the
Southeast. Models continue to signal heavy amounts near the coast,
but also some potential for heavy rains and potential flash
flooding extending as far north as Upstate South Carolina and
western North Carolina as deep moisture is drawn northwest by the
developing wave. Reflecting the overnight model trends, the Slight
Risk was shifted a little farther south, but also extended
northwest along the South Carolina-Georgia border.

Pereira

...Montana...

Broad upper trough with several mid-level shortwaves ejecting
northeast through the Northern Rockies and High Plains of MT will
create a period of heavy rainfall potential in the form of
scattered thunderstorms, some strong to severe in their maturation.
PWAT anomalies based on the latest NAEFS show a +2 to +3 standard
deviation located over much of central and eastern MT with the
maxima located along the northern tier of the state, north of the
Great Falls to Glasgow line. Relevant instability on the order of
1500-2500 J/kg MUCAPE along with the elevated PWATs, ample mid-
level forcing, and increasing bulk shear will be sufficient for
locally heavy rainfall threats within any convective genesis during
the Sunday evening and overnight time frame. Rates between 1-2"/hr
at peak intensity will offer the threat for localized flash flood
concerns, especially in any urban footprint and flashier small
streams littered across the region. One caveat to a more
substantial risk is the progressive cell motions anticipated as the
mean flow will allow for storm motions to average at least 30kts to
the north-northeast after their initiation. This should limit more
widespread flash flood chances, however this setup is close to
something more prominent compared to the, "run of the mill" threat
we see this time of year. A MRGL risk was maintained for the area,
however this is an area of interest for an upgrade if the signal
becomes more aggressive within future CAM forecasts. The risk
remains on the upper threshold of MRGL for the time being.

...Northern Plains to the lower Missouri Valley...

Multiple shortwaves will round the ridge and enter into the
Northern Plains and Missouri River Valley between NE/IA during the
course of the D2. Elevated PWATs and a prevailing LLJ in the
morning and following evening hours will offer the opportunity for
locally enhanced convective patterns with any thunderstorms capable
of rainfall rates between 1-2"/hr at their peak intensity. FFG's
across western IA into eastern NE remain on the higher side and the
antecedent soils are generally dry, even after being impacted by
convection earlier this morning. In any case, the prospect for 2-4"
of rainfall within the general area above seems more substantial
than normal considering the latest HREF and NBM probabilities >60%
for at least 2" across the area. Considering the antecedent soils
in the area and total rainfall being split between two time frames
in the period, opted to maintain the MRGL risk, but a future SLGT
is possible with a target mainly for that Missouri River basin
where flash flooding concerns in several larger cities/towns along
the river are heightened compared to areas away from the river.

...Southeast Texas...

Isolated to scattered thunderstorms are forecast across Southeast
TX, including the Upper Texas coast into the Piney Woods area that
is still recovering from impacts from Edouard. Area FFG's are
considerably lower than normal thanks to saturated grounds post
storm, so the threat for flash flooding in any cells producing
>2"/hr rates is higher leading to a localized flash flood threat.
In coordination with the local Houston WFO, a MRGL risk was added
over the aforementioned areas.

Kleebauer


Day 3
Valid 12Z Mon Sep 07 2026 - 12Z Tue Sep 08 2026

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST, NORTHERN ROCKIES, NORTHERN PLAINS, UPPER MIDWEST, AND
THE SOUTHEAST...

20Z Update: The previous MRGL risk issuances across the CONUS were
generally maintained outside a few minor adjustments overall. The
general synoptic pattern will remain active over parts of the
northern tier of the CONUS, as well as the Southeast and Southwest,
respectively. QPF magnitudes in placement maintained a suitable run
to run consistency that alleviated any reason to make noteworthy
changes. Considering the consistency in the guidance, maintained
continuity to the previous forecast(s).

Kleebauer

..Previous Discussions..

...Northern Rockies to the High Plains...
An upper low/trough will continue to translate east, reaching the
northern High Plains/southern Canadian Prairie by Tuesday morning.
Mid-level energy and upper jet forcing interacting with notable PW
anomalies (+2 std dev) will contribute to widespread precipitation,
with some orographically-focused heavier amounts. Rainfall rates
will likely be checked by limited instability, but may be
sufficient to cause isolated runoff concerns, especially in
vulnerable areas.

...Northern Plains into the Upper Mississippi Valley...
Moisture will continue to deepen across the region as southerly
flow intensifies ahead of the previously noted upper trough moving
out of the West. This is expected to fuel showers and storms
developing ahead of a strong cold front moving across the northern
Plains, with heavy amounts possible, especially across portions of
northern Minnesota.

...Southwest...
Additional showers and storms along an axis similar to the previous
day are expected across portions of eastern Arizona, western to
central New Mexico, and southern Colorado - where PW anomalies will
remain around 2 std dev above normal.

...Southeast...
Deep moisture will remain centered along a front that will continue
to sink slowly south across the Southeast. PWs around 2 inches,
along with weak mid-to-upper level forcing, will help accentuate
diurnal activity -- raising the potential for heavy amounts and
possible flash flooding across parts of the South Carolina
Low Country, southern Georgia, and North Florida.

Pereira


Day 4 and Day 5

Valid 12Z Tue Sep 08 2026 - 12Z Thu Sep 10 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHWEST/CALIFORNIA, PLAINS, GREAT LAKES, AND NORTHEAST...

...Southwest...
An axis of above average PW values will stretch from the Southwest,
northeastward through the Central Plains and into the Upper
Mississippi Valley and Upper Lakes region. Scattered monsoonal
convection likely to continue across the Southwest day 4,
supporting locally heavy rains and isolated runoff issues. By Day
5, PW values expected to become increasingly anomalous across the
Southwest, southern to central California, and into the Great
Basin. This is the result of the re-amplification of the mid to
upper level trof off the northern to central California coast that
will be capturing tropical moisture from in the vicinity of Marie
and advecting it northeastward. Scattered convection in this
anomalous PW axis, 2-3+ standard deviations above the mean, will
support locally heavy rains and isolated runoff issues from the
Sierra, into southeast California, the Desert Southwest and
portions of the Southern Rockies.

...Upper Mississippi Valley into the Upper Lakes...
A frontal boundary is expected to be progressive day 4 moving from
the Upper Mississippi Valley into the Upper Lakes. The
southwestern portion of the front will become oriented more
southwest-to-northeast along the southern edge of the westerlies.
At this time the low level inflow into this front is also expected
to become more parallel to the boundary, supporting potential for a
period of training along the front Tuesday night into early
Wednesday. The heavy rain threat continues into Day 5, focused from
the western central High Plains eastward to the mid-Mississippi
Valley where PW values will be +1 to +2 sigma. Some organized
convection is possible, with heavy rain and isolated runoff issues.

...Northern New York State into Northern New England...
The northern and eastern extension of the front over the Central
Plains to Mid MS Valley region is expected to remain progressive
day 5 as it pushes from the Great Lakes into the Northeast. There
is the potential for organized frontal convection in an axis of PW
values 1.75-2" moving eastward across NY State into New England. A
marginal risk was depicted where FFG values are relatively low
across northern NY State into Northern New England. The very
progressive nature of the front will be a detriment to any
widespread runoff issues.

Fracasso/Oravec


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt