FOUS30 KWBC 061931
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
331 PM EDT Sun Sep 6 2026

Day 1
Valid 16Z Sun Sep 06 2026 - 12Z Mon Sep 07 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WESTERN IOWA...

16Z Update...

...Western IA...

In collaboration with FSD, OAX, and DMX, a targeted Moderate Risk
was issued for northwest IA for today's Day 1 update. The latest
12Z CAMs have come into better agreement on a narrow swath with
the latest 12Z HREF showing moderate probabilities (40-60%) for
rainfall totals >5" this afternoon and through tonight. This area
coincides ahead of an approaching 700mb vort max over northern NE and
a surface trigger in the form of a stationary front just west of
the MO River. SPC RAP mesoanalysis depicts a highly unstable and
moisture-loaded air-mass in place with 2,000-3,000 J/kg of MLCAPE
and PWs between 1.75-2.0" available. Modeled warm cloud layers
this afternoon across western IA are up to 15,000ft deep and
increasing effective bulk vertical wind shear will support an
environment suitable for self-sustaining convection that can
generate rainfall rates between 2-2.5"/hr. As storms merge and a
cold pool congeals, storms will drift south and east towards
southwest IA where concerns for excessive rainfall are likely to
linger into this evening. With the atmosphere shaping up to
generate torrential rainfall rates within an environment that
contains synoptic-scale support aloft, locally significant flash
flooding may occur in areas that receive >5" of rainfall this
afternoon and evening.

...Northern MT...

The 12Z TFX sounding measured a 1.02" PW, which is in line with
the climatological maximum moving average for early Sept and well
above the 90th climatological percentile. The region lies beneath
the divergent left-exit region of a strengthening 250mb jet streak
aloft, providing healthy synoptic-scale forcing aloft, while the
anomalous PWs over northern MT and MLCAPE topping 1,000 J/kg will
be sufficient to support strong thunderstorms that contain
rainfall rates up to 1.5"/hr. That said, the atmosphere will be
slow to destabilize initially with mostly cloudy skies present
this morning. In addition, storms will be progressive and rainfall
is welcomed given ongoing drought conditions. For these reasons,
the Marginal Risk remains in place. However, this area will
continue to be closely monitored for a potential Slight Risk this
afternoon in the event confidence in areas of scattered areas of
flash flooding increases.

Mullinax

---Previous Discussion---

...Mid-Missouri River Valley...

In collaboration with FSD/Sioux City, SD and DMX/Des Moines, IA
forecast offices, a Slight Risk was introduced with this update for
southeastern South Dakota, the southwest corner of Minnesota, much
of far western Iowa, extreme eastern Nebraska along the Missouri
River, and the northwest corner of Missouri.

Afternoon convection is expected to break out across eastern South
Dakota in response to a ridge riding shortwave in the upper levels
encountering a narrow plume of Gulf moisture riding north up the
Missouri River Valley, the daytime remnants of an overnight low
level jet. Instability will be maximized at this time, with some
guidance suggesting values exceeding 4,000 J/kg. To the east of
this plume of moisture over much of Minnesota and eastern Iowa,
easterly flow from the Great Lakes of significantly cooler and
drier air will create a "wall" of sorts that will prevent the
convection from tracking too far east. Of course, there is inherent
uncertainty as to where this "wall" sets up, but it appears it's
most likely across central Minnesota and Iowa. As a result of the
presence of this wall preventing much eastward progress, the storms
will turn southward following the northbound plume of moisture and instability.

With nightfall, the storms from the afternoon are likely to
diminish or completely dissipate. Most of the day's rainfall is
expected from these afternoon storms. However, the typical
strengthening of the nocturnal low level jet will continue
advecting plentiful moisture and instability into the area
overnight. Guidance is struggling with what forms as a result, but
it seems likely that at least some area of light to moderate
showers and a few storms will redevelop in the same area of the
Missouri River Valley through the overnight, adding rainfall to the
same areas hit earlier that afternoon. A small higher-end Slight
was also added over the northwest corner of Iowa as this is
generally agreed upon in the guidance to be the area where FFG
values are most likely to be exceeded.

...Great Basin...

Few changes were needed for the Slight Risk area which extends from
around Death Valley, CA northeast across southern Nevada and the
Las Vegas metro and into southwestern and central Utah. A narrow
plume of mostly light to moderate rain will develop along a shear
axis and parallel to a developing upper level southwesterly jet
streak. The plume rain may extend southwestward as far back as the
Los Angeles metro, where a Marginal Risk remains in place. The
Slight Risk really highlights a series of areas where FFGs are
naturally low, mainly due to highly vulnerable desert areas not
used to prolonged periods of rain, even if it's of light to
moderate intensity. Since the plume of rain will not be
moving much today, just the cells embedded within it, the Slight
Risk remains in place.

...Southeast...

In coordination with GSP/Greenville-Spartanburg, SC and
CAE/Columbia, SC forecast offices, the inherited Slight Risk was
expanded to include essentially all of South Carolina and into
portions of western North Carolina with this update. Afternoon
convection is likely to break out across the Slight Risk area
during the afternoon, partially from the advance of a back door
cold front over eastern North Carolina, and partially from a
deepening longwave trough in the upper levels extending down into
north Florida. Scattered to numerous thunderstorms will develop, in
the north forced by the back door cold front running into the
mountains, and to the south from the upper level trough and local cold pools.

As the back door cold front continues moving southwestward down the
Piedmont of the Carolinas, it's unlikely to make any progress west
into the southern Appalachians. This will keep the front stuck in
this region, allowing for training and backbuilding storms to
continue into the overnight. The storms will track southward along
the frontal interface. Meanwhile from southern SC into north
Florida, storm interactions with each other will keep the storms
going well into the overnight.

Wegman
 

Day 2
Valid 12Z Mon Sep 07 2026 - 12Z Tue Sep 08 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
NORTHERN MONTANA...

1930Z Update...

No upgrades were made this update with adjustments made primarily
through reviewing the latest WPC QPF and the 12Z HREF/REFS
probabilistic guidance. The biggest change was to expand the
Marginal Risk from central MN all the way to the MO River and
northeast NE. This includes portions of northwest IA, where soils
will be more sensitive following the excessive rainfall that
unfolded on Sunday. Latest guidance shows a frontal boundary
drifting east towards western IA could be the focus for additional
thunderstorms on Labor Day. 12Z HREF probabilities currently show
low chances (15-30%) for pockets of rainfall totals >3", which on
top of saturated soils could pose a localized flash flood threat.

Elsewhere, the Marginal Risks in CO and the interior Southeast were
scaled back given recent drier trends in model guidance. The
inherited Slight Risk in northern MT was adjusted to account for
the latest WPC QPF, but no risk upgrades were made.

Mullinax

---Previous Discussion---

...Northern Montana...

A dynamic setup meteorologically will be in place across Montana on
Monday. A Plains low tracking northward will support advecting
plentiful Gulf moisture westward ahead of the low into western
Montana. This will start Sunday night, but as the Plains low
strengthens, so too will the moisture advection into Monday
morning. Meanwhile a strong pair of upper level shortwaves will
advect east across the mountains. The first will be over western
Montana at the start of the period, then the second will follow
Monday night. The first wave interacting with moisture that will be
over 3 sigma above normal for this time of year will provide the
bulk of the rainfall into western Montana. This first shortwave
will move northeast with the Plains low, but with the second one
off to the west, this second one will prolong light to moderate
rain in this region through the rest of the day Monday and into
Monday night. When added to any rainfall that falls in this area
Sunday night at the start of the storm, Monday's forecast of a
broad 1-2 inches of rain over these normally dry and flood-prone
areas of western Montana and where terrain will locally enhance the
severity of any flash flooding all points to the Slight Risk
upgrade being needed for this day 2 period.

...Marginal Risks...

The plume of moisture to the east of the low center over northern
Minnesota could result in localized flash flooding. However, the
highly progressive nature of the storms towards the east should
limit the flash flooding potential.

Monsoonal moisture continues into portions of the Southwest,
sparking a new round of showers and storms Monday afternoon,
primarily focused into western New Mexico. Isolated instances of
flash flooding are probable.

Afternoon showers and storms are also expected into portions of the
Southeast again on Monday afternoon. Onshore flow should somewhat
limit the instability at the coast. The stalled out front some
50-100 miles north of the Gulf Coast will provide forcing for
storms, but abundant cloud cover should limit storm intensity. With
normally high FFGs in this area, a Marginal Risk appeared a good
balance of these competing factors.

Wegman
$$
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