Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 AM EDT Sun May 17 2026

Day 1
Valid 12Z Sun May 17 2026 - 12Z Mon May 18 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS TO THE UPPER MIDWEST...

A pair of shortwave troughs tracking northeast on the downwind side
of a slow moving longwave trough over the Intermountain West will
provide upper level forcing for several rounds of thunderstorms to
impact a large area from Kansas through the upper Great Lakes. A
fully open Gulf and associated moisture plume will continue to
advect northward up the Plains and Mississippi River. This will
keep PWATs consistently above 1.25 inches, with many areas above
1.5 inches. This influx of moisture will support and instability
will support both storms capable of heavy rainfall, as well as 
development and redevelopment of storms in this favorable weather
regime. For the most part, due to fast upper level flow, the storms
will likely organize into fast moving lines/segments, which should
work to keep rainfall totals manageable in any one area. The
greatest area of concern stretches from southeastern Nebraska east
along the Iowa/Missouri border, where multiple inches of rain have
fallen overnight last night. Thus, any backbuilding or holdups in
any lines of storms could have outsized flooding impacts. A short-
fused Slight may be needed once again in this general area if
rainfall totals today come up further.

For southeastern Louisiana, the inherited MRGL was removed. Nearly
all of the guidance shows only light showers expected to impact
that area, with only a one or two showing one isolated storm.
Further, high FFGs should preclude any flooding even if that one
storm does form.

Wegman

Day 2
Valid 12Z Mon May 18 2026 - 12Z Tue May 19 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL
PLAINS TO THE UPPER MISSISSIPPI VALLEY...

Relatively few changes were needed to the outlook for this likely
busiest day of the upcoming three. A strong shortwave making up the
base of the longwave trough over the Rockies will finally eject out
over the Plains on Monday. As that shortwave interacts with
plentiful Gulf moisture streaming north up the Plains, renewed
lines of showers and storms are expected to develop across the
Slight Risk area. Since the storm motion will be towards the
northeast, largely parallel to the moisture and instability plume
emanating from the Gulf, backbuilding will be a greater
possibility than in previous days. Several of the high resolution
model guidance show that there could be multiple rounds of storms
to impact the area from northeast Kansas through southwestern Iowa,
especially in the late afternoon through the first half of the
overnight. This area is currently getting very hard-hit with heavy
rain from a very slow-moving MCS that has been producing multiple
inch per hour rainfall rates and numerous instances of flash
flooding. Thus, the soils have become very saturated in some areas.
Following potential heavy rain during the day 1/Sunday period,
much more rain is expected in this region again on Monday. A
higher-end Slight covers the aforementioned region from northeast
Kansas through the southwest corner of Iowa. Should multiple inch
per hour rates materialize again in this same region, a Moderate
Risk upgrade will need to be considered with future updates.

Wegman

Day 3
Valid 12Z Tue May 19 2026 - 12Z Wed May 20 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SOUTHERN
PLAINS THROUGH THE LOWER OHIO VALLEY...

The inherited Slight Risk for the southeastern half of Oklahoma was
removed with this update. Guidance has been understandably
struggling with the placement of expected rainfall/convection
across this region. Since there's excellent agreement that the axis
of heaviest rainfall will shift well south of the area expected to
be hit with heavy rain in days 1 and 2, the region of greatest
impact is an area where soils have dried out and will likely be
very receptive to beneficial rainfall. Since guidance is in poor
agreement on where the storms will be most persistent, instead
broad brushing much of the Marginal Risk area with anywhere from a
half inch to 1.25 inches, have opted to leave the large Marginal in
place. None of the guidance particularly favors southeastern
Oklahoma for any heavier amounts that anywhere else, especially
further south into Texas. Expect that should a new Slight be needed
with a future update, it will be over a portion and not into
Oklahoma, which sits at the back/northwestern edge of the heavier
rain axis.

Wegman


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt