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


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