FOUS30 KWBC 160737
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
337 AM EDT Sat May 16 2026

Day 1
Valid 12Z Sat May 16 2026 - 12Z Sun May 17 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER TO MID OHIO VALLEY AS WELL AS PORTIONS OF THE CENTRAL PLAINS...

...Lower to Mid Ohio Valley...

Areas of convection that will form along a warm front at the nose
of a low-level jet are expected to develop during peak heating
today across the lower to mid Ohio Valley. While the main branch of
the low level jet remains oriented south to north over the Plains,
the side branch over the Ohio Valley will be oriented southwest to
northeast. This will promote training storms with backbuilding
convection, while the primary storm movement is towards the
east. Training storms are most likely to develop in the Marginal
Risk area. Despite very dry soils over the region, which will limit
the flash flooding risk, some urban areas are threatened with the
possibility of training storms, so the Marginal remains in place.

...Central Plains...

During the early evening hours, showers and thunderstorms will
develop along the nose of a robust low level jet transporting
abundant moisture and instability north from the Gulf into the
central Plains. A warm front at the leading edge of the warm, moist
air mass will interact with a strengthening upper level shortwave
and a developing surface low over southwest Kansas to support the
convective activity. While some of the convection will develop into
a highly progressive bowing segment, it's likely that the southern
end of the bow will get "stuck" and remain largely in place as
additional convection backbuilds westward, generally along the
Iowa/Missouri border. Repeating rounds of convection are possible,
and likely to be of somewhat greater vigor than the ongoing
convection across Iowa. Any overlap of the convection this evening
with the rainfall footprint of the ongoing storms present over 
central Iowa could also locally increase the flash flooding risk.
However, at the moment any overlap should be minimal, as the
strongest storms with the greatest flash flooding risk are expected
along the Missouri/Iowa border, which did not see any meaningful
rain last night. Thus, the Marginal remains in place as soils in
the region remain very dry.

...Elsewhere...

In between the areas of storms over eastern Missouri/western
Illinois, a "break" is likely to remain, separating the footprints
of the two areas of storms. With minimal rainfall expected along
the Mississippi River near St. Louis, the inherited Marginal was
removed, leaving two separate Marginal Risk areas.

Wegman


Day 2
Valid 12Z Sun May 17 2026 - 12Z Mon May 18 2026

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

A robust low level jet feeding plentiful moisture ahead of a
developing low over the Plains will provide the moisture and
instability needed for numerous thunderstorms across a large
portion of the upper Midwest from far eastern Nebraska northeast
through the Arrowhead of Minnesota. The storms are likely to be
their strongest Sunday night. This will coincide with the diurnal
strengthening of the low level jet. Clusters of showers and storms
will develop with increasing forcing ahead of a slow moving upper
level trough over the High Plains. The storms are likely to train
as several rounds of upper level energy support several rounds of
storm clusters. Much of Iowa saw beneficial rains overnight
tonight, such that by Sunday night, the soils will still have some
moisture left over. Urban concerns, especially in and around the
Twin Cities could also locally increase the flash flooding threat.
Significant dry time in between the storms should limit the
flooding threat such that the inherited Marginal still looks good,
and few changes were needed.

Wegman


Day 3
Valid 12Z Mon May 18 2026 - 12Z Tue May 19 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF  
THE CENTRAL PLAINS THROUGH THE UPPER MIDWEST...

An upper level trough that has been "stuck" over the northern High
Plains for a few days will finally begin to eject across the
northern Plains on Monday. This will occur because a strong
shortwave on its southern edge near the 4 Corners region will help
to nudge the trough eastward. This strong shortwave will run into
plentiful Gulf moisture that will continue streaming northward
across the middle of the country, as it has been doing for the past
several days. A cold front on the northwestern boundary of that low
level jet will provide ample surface forcing for the storms. The
initial meeting of the warm, moist air mass to the east and the
cooler, drier air mass over the northern Plains will be along the
Missouri River from far northeastern Kansas north across much of
western Iowa. This will be the corridor where the heaviest rainfall
totals are expected on Monday through Monday night. This is the
same region that will have been hit by multiple days of scattered
heavy rainfall. By Day 3/Monday, soils should be much closer to
saturation. Thus, with the greatest rainfall amounts areally
expected on Monday, the flooding threat increases into the Slight
Risk category. Any storms that form will continue to rapidly
progress northeastward into the upper Mississippi Valley. This area
too will have seen multiple days of storms by Monday. Thus,
expecting wetter than normal soil conditions in most areas by
Monday, the Slight Risk area was expanded northward to cover the
upper Mississippi Valley to the western tip of Lake Superior. 

There remains some uncertainty as to where the axis of heaviest
rainfall will set up. Thus, the surrounding Marginal covers
extensive real estate east of the Slight from northern Illinois
north through much of the U.P. of Michigan. While expected rainfall
totals are lower in this region, greater urban concerns are present
in this area, which supports the Marginal Risk.

Wegman
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 * Origin: Capitol City Online (618:250/10)