Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
839 PM EDT Tue Sep 15 2026
Day 3
Valid 12Z Thu Sep 17 2026 - 12Z Fri Sep 18 2026

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTH-CENTRAL PLAINS, UPPER MIDWEST, & SOUTHWESTERN U.S...

...Northern Plains & Midwest...

21Z update... Once again, the guidance depicted a modest northward
shift in the QPF footprint across the Plains and Upper Mississippi
Valley. This lowered the likelihood of excessive rainfall and
flash flooding concerns along the southern periphery of the
Marginal Risk that was in effect. Therefore about 1-2 tiers of
counties were trimmed out across NE/MO/IL/OH while adding 1-2 tiers
of counties across SD/ND/MN/WI. Additionally, the axis of
orientation of the heaviest rainfall became more SW-NE and so did
the adjustments of the Slight Risk area to reflect that change.

Campbell

Our broad upper ridge will continue to flex over a large portion of
the CONUS as we head into Thursday with the northern fringes of
the height field extending into the Upper Midwest and expanding as
far west as the Central High Plains. As we move through Thursday, a
robust shortwave ejecting out of the Southwestern U.S. will pivot
up through the Central Rockies, rounding the northwestern flank of
the ridge and entering into the Northern Plains as we head into
Thursday evening. This shortwave will also provide a significant
PWAT increase as noted via both the the ensemble anomaly plots and
IVT climatological percentiles approaching the 95-99th percentile
up across South Dakota into southern portions of MN. Given the
strength of the forecast shortwave ejection, regional forcing will
increase significantly as the disturbance approaches leading to
more organized convective clusters to materialize by Thursday
afternoon and evening, lingering through the overnight hours to
Friday morning. Model outputs have become increasingly bullish for
a swath of 2-4", locally higher occurring within the confines of
the shortwave impulse as the environmental conditioning and ascent
pattern remain favorable within proximity to the disturbance. This
setup is becoming more favorable for at least scattered flash flood
prospects over the Northern Plains into the Upper Midwest with the
primary threat likely occurring over those urbanized areas in the
zones referenced. Considering the source region of this moisture
advection pattern, we will be monitoring the necessity for a higher
risk potential over the area of SD into MN for the activity
occurring Thursday night into early Friday morning. For now, a
broad SLGT risk remains forecast for the region with a MRGL
surrounding.

...Southwest U.S...

21Z update... Minor expansions of the Slight Risk was made along
the northern boundary as convection is expected to track further
north than previous cycles have indicated. The areas of higher
terrain across southwest CO will be the most sensitive.

Campbell

Prevailing southerly flow will maintain a steady plume of elevated
moisture across the Southwestern CONUS, but the shortwave
disturbance associated with the proliferation of the previous
period's convection will vacate to the northeast allowing for a
gradual de-escalation of the convective magnitude over the region
in time for Thursday. That said, lingering PWAT anomalies ranging
from the 90-99th percentile of climo will allow for another busy
afternoon of widespread convective potential over much of NM and
eastern AZ leading to an elevated flash flood concern, once again.
Precip totals between 1-3" are forecast for the Sangre de Cristos
down through much of the terrain focused in the interior areas of
NM. This allowed for a high-end SLGT forecast to remain across
these aforementioned areas of NM into southern CO. Pending the
impacts from the prior period, a higher risk is also non-zero and
could be added in future forecasts as sensitive grounds could
enhance the threat for flash flooding further within the region.

Kleebauer

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