Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Wed Sep 16 2026
Day 2
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...

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, with origins from the tropics, will eject out of
the Southwestern U.S, pivoting up through the Central Rockies and
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 by the end of the forecast period. 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 as the disturbance migrates across SD.

Model outputs continue to be bullish on 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. Assessment of local forecast
bufr soundings indicate a traditional tall, skinny CAPE signature
with a saturated column typical of tropical-like air being pulled
poleward. These types of environments become more privy to enhanced
rainfall rates and more efficient warm rain processes that can
cause issues for areas further north. This setup is more favorable
for at least scattered flash flood prospects over the Northern
Plains into the Upper Midwest with the primary threat likely
occurring over the terrain areas of SD, extending east-northeast
into the area of I-90 and north, especially within close proximity
of the center of the disturbance. A high-end SLGT is now forecast
across southwest to south-central SD with a chance for an expansion
into eastern zones of the state if the general storm motion allows
for the heavier rainfall to make it east of the longitude of the
Missouri River which some models are depicting. Something we will
keep an eye  in subsequent forecasts.

...Southwest U.S...

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
and San Juan's 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 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt