Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
407 AM EDT Tue Sep 15 2026
Day 2
Valid 12Z Wed Sep 16 2026 - 12Z Thu Sep 17 2026

...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS, MISSOURI AND OHIO VALLEYS, AND THE SOUTHWEST
U.S...

...Central Plains to Ohio Valley...

The upper ridge centered over the Southeastern U.S. will begin to
shift more to the west and center over the Lower Mississippi Valley
by mid-week while also strengthening with global deterministic
forecasting 500mb heights encroaching on 600dm for the center of
the ridge. This will allow for increasing heights to the north as
the ridge flexes and provides a greater influence in the
latitudinal positioning of our frontal boundary situated over the
Central U.S. extending into the Ohio Valley. Wednesday morning will
see continued heavy rain over northern MO as the previous period's
activity lingers and migrates eastward before dissipating with the
loss of the LLJ influence as we move forward in the period. By the
late-afternoon and evening hours, expectation is for front to stall
and eventually lift north slowly as move into the evening hours.
Scattered convection will occur over the Ohio Valley during the
peak of diurnal heating with an increase in coverage anticipated
until the evening hours. A relatively buoyant environment and
elevated PWATs situated over the area will allow for some isolated
to perhaps widely scattered flash flood prospects across IL/IN/OH
with the current forecast depicting localized areas of 1-3" where
impacted by convection. Most notable areas will be along the
positioning the front due to a greater propensity for cells to
anchor and sit over the region, or train with the mean flow
aligning a bit more parallel to the front leading to the threat.
The greatest concerns are being referenced from central IL into IN
where CAMs are more aggressive with the signal given better
instability fields and PWATs overhead providing a more suitable
environment for higher rates for flash flooding potential.

By the evening, another shortwave from the Central Plains will
rotate around the northern periphery of the ridge and help initiate
a nocturnal round of convection in-of IA into northern IL leading
to another scattered flash flood concern around the vicinity of the
I-80 corridor. Models have backed on a more prolific opportunity
compared to the past few nights leading into the forecast, but the
threat is still worthy of a SLGT risk when you add the antecedent
moist soils that will be focused across the intended area of impact
for Wednesday evening into early Thursday morning. Some of the
guidance is still relatively aggressive in local 2-3" totals along
the area extending from Des Moines to just west of the Chicago
metro, including those suburbs in the Chicago metro. NBM probs for
>2" remain between 20-40% over the region of interest, a noteworthy
signal for at least a localized threat of flash flood concerns,
especially within those more urbanized settings referenced. A broad
SLGT risk extends from the Missouri River between southeast NE,
northeast KS to points east covering much of IA, northern MO,
through the northern-third of IL. The SLGT extends east into the
central portions of IN to account for the previously referenced
convective threat downstream of the nocturnal setup.

...Southwest...

Another period of scattered to widespread convective activity is
forecast for much of the Southwestern U.S. with a broad axis of
heavy rainfall prospects likely to occur over the eastern two-
thirds of AZ into almost all of NM and southern CO on Wednesday.
Guidance continues to reference a more robust shortwave ejection
out of Mexico providing further mid and upper level ascent that
can exacerbate an already formidable environment in place. The
latest model 500mb vorticity forecasts are noting a potential MCV,
or organized vorticity field lifting north out of the southern
Baja over the next 48 hrs that will likely materialize from an area
of convection that spurred up this evening. PWAT anomalies are
forecast to reach firmly into the +2 to +3 standard deviation
range, perhaps even flirting with upwards of +4 standard deviations
as the latest ECENS output reflects areas of NM approaching
monthly max PWATs when assessing the climatological percentiles.
Ensemble means remain very aggressive for Wednesday afternoon into
early Thursday morning with some models printing out 2-3", locally
higher, in spots across central and southeastern AZ into western
and northern NM, areas where flash flood concerns will be elevated
from the repeated nature of this pattern, as well as the location
of the more sensitive burn scar locales within each state. Latest
HREF probs for >2" between the 12-00z time frame Wed/Thu are
upwards of 70-90% for portions of southeastern and south-central AZ
centered over the Mogollon Rim, as well as over the Sacramento and
Sandias Mountains in NM, a testament to a growing threat for more
widespread flash flood concerns when you consider the environment
advecting into the region. This threat maintains a higher end SLGT
risk forecast with a chance for even an upgrade further over
portions of AZ and NM with the primary area of interest within the
Mogollon Rim down into the mountains of southeastern AZ, and
perhaps into the Sacramento Mountains to account for the still
sensitive burn scars in place near Ruidoso.

Kleebauer


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