Flash Flood Warning 1, NWS Sioux Falls, SD

507 
WGUS73 KFSD 170649
FFSFSD

Flash Flood Statement
National Weather Service Sioux Falls SD
149 AM CDT Sun May 17 2026

IAC149-SDC127-171015-
/O.CON.KFSD.FF.W.0001.000000T0000Z-260517T1015Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Plymouth IA-Union SD-
149 AM CDT Sun May 17 2026

...FLASH FLOOD WARNING REMAINS IN EFFECT UNTIL 515 AM CDT EARLY THIS 
MORNING FOR SOUTHWESTERN PLYMOUTH AND EAST CENTRAL UNION COUNTIES...

At 149 AM CDT, Doppler radar and automated rain gauges indicated 
thunderstorms producing heavy rain across the warned area. Between 2 
and 3 inches of rain have fallen. The expected rainfall rate is 1 to 
1.5 inches in 1 hour. Additional rainfall amounts of 0.5 to 1.5 
inches are possible in the warned area. Flash flooding is ongoing or 
expected to begin shortly.

HAZARD...Flash flooding caused by slow moving thunderstorms.

SOURCE...Radar and automated gauges.

IMPACT...Flash flooding of small creeks and streams, urban areas,
         highways, streets and underpasses as well as other poor
         drainage and low-lying areas.

Some locations that will experience flash flooding include...
  Elk Point.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Turn around, don't drown when encountering flooded roads. Most flood 
deaths occur in vehicles.

Be especially cautious at night when it is harder to recognize the 
dangers of flooding.

Be aware of your surroundings and do not drive on flooded roads.

&&

LAT...LON 4266 9666 4273 9671 4277 9636 4265 9634
      4263 9654

FLASH FLOOD...RADAR AND GAUGE INDICATED
EXPECTED RAINFALL RATE...1-1.5 INCHES IN 1 HOUR

$$

SG

352 
WGUS73 KFSD 170808
FFSFSD

Flash Flood Statement
National Weather Service Sioux Falls SD
308 AM CDT Sun May 17 2026

IAC149-SDC127-171015-
/O.CON.KFSD.FF.W.0001.000000T0000Z-260517T1015Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Plymouth IA-Union SD-
308 AM CDT Sun May 17 2026

...FLASH FLOOD WARNING REMAINS IN EFFECT UNTIL 515 AM CDT EARLY THIS 
MORNING FOR SOUTHWESTERN PLYMOUTH AND EAST CENTRAL UNION COUNTIES...

At 308 AM CDT, Doppler radar and automated rain gauges indicated 
thunderstorms produced heavy rain earlier this morning across the 
warned area. Between 2.5 and 3.5 inches of rain fell with these 
thunderstorms. Additional rainfall amounts up to 0.5 inches are 
possible in the warned area overnight. Flash flooding is ongoing or 
expected to begin shortly.

HAZARD...Flash flooding caused by slow moving thunderstorms.

SOURCE...Radar and automated gauges.

IMPACT...Flash flooding of small creeks and streams, urban areas,
         highways, streets and underpasses as well as other poor
         drainage and low-lying areas.

Some locations that will experience flash flooding include...
  Elk Point.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Turn around, don't drown when encountering flooded roads. Most flood 
deaths occur in vehicles.

Be especially cautious at night when it is harder to recognize the 
dangers of flooding.

Be aware of your surroundings and do not drive on flooded roads.

&&

LAT...LON 4266 9666 4273 9671 4277 9636 4265 9634
      4263 9654

FLASH FLOOD...RADAR AND GAUGE INDICATED

$$

SG

400 
WGUS73 KFSD 171008
FFSFSD

Flash Flood Statement
National Weather Service Sioux Falls SD
508 AM CDT Sun May 17 2026

IAC149-SDC127-171018-
/O.EXP.KFSD.FF.W.0001.000000T0000Z-260517T1015Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Plymouth IA-Union SD-
508 AM CDT Sun May 17 2026

...FLASH FLOOD WARNING WILL EXPIRE AT 515 AM CDT EARLY THIS MORNING 
FOR SOUTHWESTERN PLYMOUTH AND EAST CENTRAL UNION COUNTIES...

The heavy rain has ended. Flood waters are expected to recede this 
morning. Use extra caution if traveling before daybreak, as flood 
waters are harder to see in the dark. Please continue to heed 
remaining road closures. Turn around, don't drown.

&&

LAT...LON 4266 9666 4273 9671 4277 9636 4265 9634
      4263 9654

$$

SG

Area Forecast Discussion...UPDATED
National Weather Service Sioux Falls SD
1120 PM CDT Sat May 16 2026

.KEY MESSAGES...

- High based sprinkles to weak storms remain possible near
  Highway 20 into tonight with isolated lightning strikes.

- Thunderstorms increase in coverage tonight into early Sunday.
  Weakening storm clusters may produce marginally severe hail
  and brief 60 mph wind gusts between 10pm and 5am.

- A significant severe weather episode is possible Sunday
  afternoon into Sunday night. Very large hail, straight line
  winds, and several tornadoes, possibly strong, will be
  possible. The main timeframe for severe storms is between 3 pm
  to midnight on Sunday.

- If you have outdoor plans Sunday afternoon through Monday, the
  time is now to prepare and have alternative plans in place.

&&

.UPDATE...
Issued at 932 PM CDT Sat May 16 2026

Convection has developed across I-80 this evening, along the
effective warm front. Despite this convection, a few weak storms
have developed along part of the highway-20 corridor in far
northeast Nebraska. these storms will have a lightning threat with
them but are not expected to be severe.

Looking further south, latest hi-res models show convection that has
developed across northwest Kansas/southwest Nebraska transitioning
into an MCS and pushing eastwards along the effective boundary. This
MCS should keep the bulk of the strong to severe storms south of the
forecast area. That said, still seeing the 850 mb front and
associated moisture are on track to push northwards this evening and
through the night. CAPE values could reach up to 1,000 J/kg with
effective shear values on the order of 40 knots will continue the
chance for strong to severe storms. Large hail to the size of a
quarter (1 inch) and damaging winds to 60 mph will be the main
hazards. Instability will be waning through the night so any chance
for strong to severe storms should slowly wane through the overnight
hours.

Attention then turns to the severe weather set up for Sunday. Some
rain showers may persist from the overnight timeframe into Sunday
morning. Though any shower chance will be waning as subsidence from
the backside of the departing shortwave trough takes over. There
continues to look to be widespread low level stratus that blankets
the area tomorrow morning. Much steeper mid level lapse rates will
also push into the area from the south, creating a capping
inversion. While these factors could limit severe storm potential,
another shortwave trough is poised to encroach on the area
tomorrow afternoon. At the same time, the latest hi-res guidance
is in decent agreement in low level stratus clearing out for
locations south of I- 90 and east of the James River as a drier
air is advected northwards within the incoming surface trough.
The latest guidance does suggest that the effective warm front
will push northwards despite the overnight MCS. This warm front
will make up one part of the inverted trough. The other
component is an incoming cold front from the west. There looks
to be elevated convection Sunday morning and lasting into the
afternoon hours just behind this boundary mainly west of the
James River. This cold front will intersect the increasingly
unstable warm sector. Convergence and surface vorticity along
this boundary look to be enough forcing for ascent to overcome
the cap in place. Thus, convection is expected to develop.
Soundings continue to show hodographs with looping low levels in
place. 0-1km shear on the order of about 20 knots and 0-3km
shear with a magnitude of close to 40 knots will yield higher
storm relative helicity (SRH) values and tornado potential.
0-3km SRH values look to exceed 200 m2/s2 and may exceed 300
m2/s2. Storms will quickly develop in this volatile of an
environment along the cold front. Supercells look to be the main
storm mode as convection develops. Large hail up to 2+ inches
is expected as the mid level will be dry along with sufficient
shear in the mid levels. Tornadoes are also on the table with
these supercells, especially as the hodographs show nearly
streamwise vorticity in the lowest 0-1km layer and a larger
streamwise component up to about 2-3 km. 0-3km CAPE looks to be
impressive potentially exceeding 100 J/kg if full mixing can be
achieved in the warm sector. With the cold front pushing
eastwards, these storms look to quickly grow upscale into a
line. This will transition the main hazard from large hail to
damaging winds and embedded tornadoes. 0-3km shear will be
sufficient around 35 to 40 knots and oriented perpendicularly
off the squall line. DCAPE values will be impressive, exceeding
1,000 J/kg as well. This squall line will quickly race eastwards
on an east/northeastward trajectory, bringing the better
environment northwards with it. Storms look to exit the area
during the evening timeframe. The overall timeframe for severe
weather looks to be between 3pm to about midnight. Locations
along to just south of I-90 and east of the James River have the
highest chances for seeing the initial development of severe
storms after 3 pm. With the more unstable environment continuing
to push northwards as the afternoon goes on, the overall
parameter space looks to widen to parts of east central South
Dakota, southwest Minnesota, and northwest Iowa. Any chance for
strong to severe storms will come to an end by midnight though
it could be a little earlier depending on how quickly the storms
move eastwards. Light showers may persist through the night.

What was described above is the most likely scenario for tomorrow.
The things that can alter the forecast is if morning low level cloud
cover does not clear out as expected and the overnight MCS ends up
pushing the effective boundary further to the south, preventing it
from returning as far northwards tomorrow. Will have to see how
things evolve tonight to get a better idea of how the warm
sector will evolve tomorrow. Please continue to monitor the
latest forecast for the most up to date information.

Strong to severe storms remain possible on Monday but this will be
highly dependent on Sunday's storms. If the cold front is pushed
southwards more quickly than currently anticipated, then a
lower severe weather chance can be expected. If the boundary's
location remains on track, the strong to severe storms will be
possible over portions of northwest Iowa. Tough to say which is
the most likely scenario at this time.

&&

.DISCUSSION...
Issued at 101 PM CDT Sat May 16 2026

THIS AFTERNOON: We continue to see a pleasant Saturday afternoon
across the region with a light to easterly wind prevailing.
Temperatures are gradually rising towards the 80s, but humidity
remains very low in the 15 to 20% range. Weak warm advection to our
south will begin to stream elevated ACCAS overhead, perhaps with a
sprinkle or two by early evening. This activity is based well
aloft AOA 600 mb, with soundings showing minimal potential for
deepening. Any surface based convection will remain well south
of the CWA with the frontal boundary stuck north of I-80.

TONIGHT: Easterly low-lvl flow will keep the risk for convection
lower through the evening hours as a gradual northward advancement
of the surface front begins to our south. We'll begin to see
convection form along and south of this boundary extending from
western Nebraska into western Iowa late this evening. This
activity will be pulled northward by a stronger shortwave
lifting out of the Central Rockies into the Plains. The
advancement of a low- lvl front and strong theta-e advection
will bring the risk for elevated thunderstorms to the Tri-State
area after 10pm with one more clusters of storms (or potentially
an MCV) that form to our southwest tracking towards the area
through daybreak. MUCAPE plots and soundings ahead of this
convection continue to look marginal, with mid-lvl lapse rates
around 7 C/KM and storms likely to be strongly elevated or
marginally severe. At this point the greatest severe weather
risk would be south of I-90 and perhaps more focused along the
MO river and Highway 20 corridors. One thing to watch would be
the potential for any MCV that develops to bring wake low
conditions, or strong evaporative cooling winds northeast as
convection wanes. Some high-res guidance supports pockets of 50+
mph winds moving northeast with the weakening convection.

SUNDAY-SUNDAY NIGHT:

The key takeaways for Sunday continue to be:

1. Convective debris, stratus, and an EML may be problematic and
could make this forecast conditional.

2. If storms get rooted into the boundary layer, there is no
shortage of spin potential into the early overnight and tornadic
activity becomes an increased concern.

3. If discrete mode is short lived and storms turn linear, then
duration of threat in any one area may be intense but brief.

The mesoscale picture may be quite cloudy to start Sunday.
Considerable stratus will be present in the area and we'll only
get a better pictures of where the cold front/inverted trough
and associated warm front will be positioned through the
morning. Most of the morning is likely to remain quiet, with
some renewed convection possible in SW South Dakota/western
Nebraska on the nose of the upper jet entering the Plains. Some
CAMS do have this mostly elevated convection brushing the NW
edge of the CWA by 18Z. Further east, it may take into the mid-
afternoon hours for clouds to thin and the warm front to jump
northward (if it does). A stubborn EML will also remain within
the 600-700 MB. Guidance is still very bullish on the rapid
northward advection of 2000-3000 MLCAPE northwards towards
Highway 14 and west to Highway 281 by 21-00Z. The increasing
CAPE will be aided by the arrival of 8.5-9 C/KM 700:500 lapse
rates, and once the next wave of mid- lvl vorticity arrives,
scattered convection may quickly develop along the frontal
boundary stretching near Hwy 281 after 20Z. A second area of
convection may form further south along the dry line
intersection in eastward Nebraska and track northeast into the
evening. Some uncertainty persists on how discrete storms will
remain or if they will quickly turn linear as they expand and
slide east/northeast into the evening and early overnight.
Supercells with very large hail (2-3") and an increased tornadic
risk will be possible initially, but if linear trends develop,
line segments with straight line winds/embedded tornadoes may
become a possibility as storms track east northeast towards I-29
and MN/IA. The degree of low- lvl helicity/shear combined with
increasing 0-1km CAPE approaching 100-200 J/KG and critical LCL
values along and slightly east of I-29 is particularity
concerning heading into the evening and continues to support the
10-15% tornado probabilities by SPC. In fact, STP values push
5+ along portions of I-29 between SF/SC by 00Z. Most of the
recent CAMs support the linear lines moving east of the CWA by
midnight with lower risks into daybreak Monday.

MONDAY: There is better agreement in today's guidance pushing the
effective boundary east of the CWA by Monday morning. That said,
incoming shortwave energy may pull elevated instability and
increased lapse rates back west of the boundary by mid-late
afternoon. This could place areas of NW Iowa under the risk of
elevated hailers for a period of time. For NW Iowa, this could
also be the time where the most beneficial rainfall may also
take place. Further west, significant cooldown is expected as
temperatures struggle to climb through the 50s.

TUESDAY-FRIDAY:  Quiet and cooler than normal temperatures are
expected through the middle of the week as highs climb back through
the 60s by Thursday. Another trough moves through the Plains by the
end of the week however a lack of deep moisture prevents any
significant severe weather risk.  The latest AI learning guidance
suggests less than 5% probabilities of any severe storm at this
time.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1108 PM CDT Sat May 16 2026

Focus on the aviation forecast is for convection initially
tonight, then low clouds with MVFR ceilings developing on
Sunday, then additional thunderstorms during the afternoon
hours. Isolated showers and thunderstorm are expected to the
south of I-90 during the overnight hours on the north side of
the much stronger area of thunderstorms well to the south in
Nebraska. Generally lower confidence on these
showers/thunderstorms hitting KFSD/KSUX, so have handled with
PROB30. With the primary convection being farther to the south,
does look to be a lull in the precipitation on Sunday morning,
as the low stratus develops over the area. The focus then turns
to the potential for severe thunderstorms to develop during the
afternoon and continue through early evening. Confidence is
lower on exact locations, especially farther northwest into less
instability, so handled with PROB30, except at KSUX, where
greater confidence is in place for storms evolving into a line
in the late afternoon.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.

&&

UPDATE...Meyers
DISCUSSION...Dux
AVIATION...SRF

Hazardous Weather Outlook
National Weather Service Sioux Falls SD
343 AM CDT Sat May 16 2026

IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097-
098-NEZ013-014-SDZ038>040-050-052>071-170845-
Lyon-Osceola-Dickinson-Sioux-O'Brien-Clay-Plymouth-Cherokee-
Buena Vista-Woodbury-Ida-Lincoln-Murray-Cottonwood-Nobles-Jackson-
Pipestone-Rock-Dixon-Dakota-Beadle-Kingsbury-Brookings-Gregory-
Jerauld-Sanborn-Miner-Lake-Moody-Brule-Aurora-Davison-Hanson-McCook-
Minnehaha-Charles Mix-Douglas-Hutchinson-Turner-Bon Homme-Yankton-
Union-
343 AM CDT Sat May 16 2026

This Hazardous Weather Outlook is for northwest Iowa, west central
Iowa, southwest Minnesota, northeast Nebraska, central South
Dakota, east central South Dakota, south central South Dakota and
southeast South Dakota.

.DAY ONE...Today and tonight.

Locally elevated to near critical fire conditions this afternoon due
to very low relative humidity.

Isolated and possibly severe storms may develop along US Highway 20
between 4 and 8 PM today. Large hail to 2 inches and 60 mph wind
gusts are possible if storms develop.

More widespread thunderstorms are expected tonight into early Sunday.
Strong to severe storms may produce large hail to 2 inches and 60 mph
wind gusts from 10 PM to 5 AM.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

A significant severe weather event is possible Sunday afternoon into
Sunday night. Very large hail, damaging straight line winds, and
tornadoes are all possible. Prepare now if you have outdoor plans and
have alternate plans in place. Review your severe weather plans.

Strong to severe storms are possible Monday for portions of the area.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed at times Saturday afternoon through
Monday.