Flash Flood and Hail in Iowa, June 28, 2026

Area Forecast Discussion
National Weather Service Sioux Falls SD
1251 AM CDT Sun Jun 28 2026

.KEY MESSAGES...

- Isolated strong to severe thunderstorms are possible
  especially west of I-29 tonight. Large hail up to golf ball in
  size and 70 mph will be the main threat. Storms look to
  generally weaken with eastward extent.

- A Heat Advisory has been issued for some areas Sunday into
  Monday, and further adjustments to headlines are possible in
  the coming days.

- Summer heat and humidity is expected Sunday into next week.
  Daily heat indices of 95 to 105 degrees will lead to Major to
  locally Extreme Heat Risk at times. Begin planning now to
  reduce the risk of heat illness and impacts.

- An active pattern through the week leads to periodic moderate
  chances for showers and storms. Uncertainty in the timing of
  rain also brings lower confidence in daytime high
  temperatures.

&&

.UPDATE...
Issued at 956 PM CDT Sat Jun 27 2026

A broken line of strong thunderstorms extends across
most of western South Dakota and into the northern Nebraska
panhandle this evening. These will be the storms to watch as they
gradually lift northeastward through the night aided by a
strengthening low level jet. Latest soundings still show a
strong capping inversion in place over our area through the
night, but the cap will be the weakest along the US Highway 14
corridor in eastern South Dakota especially closer to the James
River Valley late this evening. So this area looks to have the
better chances of wind gusts of 60-65 mph, but this threat still
appears to be isolated due to the overall strong cap.

Farther south along this line over northern Nebraska is a
cluster of thunderstorms with embedded supercell structures that
is capable of quarter to golf ball sized hail. These storms
will be watched closely as well as they lift towards south-
central South Dakota later around midnight as this will be in
an area of higher instability. However, the same capping issues
remain and so this threat for large hail will also remain
isolated. Instability and shear will gradually lessen with
eastward and especially northeastward extent, and thus the
storms will continue weakening as they push eastward heading
into early Sunday morning. However, storms will still be capable
of small hail and wind gusts to 40 mph by the time they reach
the I-29 corridor and points east.

There is an alternate, but less likely scenario where the storms are
able to maintain their strength as they push across the area through
the night and bring 60-65 mph winds and large hail to a wider swath
of the area. Given the strong cap already discussed, this
scenario is unlikely, but if storms can take full advantage of a
potential 40-50 mph LLJ, this can't be entirely ruled out. The
greater threat for severe weather in this alternate scenario
would be near and south of I-90 where there will be greater
instability and the strongest LLJ.

Timing-wise, look for the storms to start moving into the area
around midnight and reach the I-29 corridor by 3-5 am and then
exiting the area off to our east by 7-9 am. One other thing to note
is that though the storms will be quick-moving, some guidance
indicates a narrow, but long swath of a quick 2-4" of rain with
these storms which could result in some minor urban flooding.

&&

.DISCUSSION...
Issued at 1248 PM CDT Sat Jun 27 2026

The first concern will be the potential for thunderstorms tonight.
The current run of models is really pushing for a strong capping
inversion, which strengthens through the night. This suggests that
tapping into the deeper instability may be impossi.ble. Above this
capping inversion some moisture and weak (500 J/kg) instability may
allow for some ACCAS showers and thunderstorms late tonight into
Sunday morning, but any severe threat would be very limited. Will
need to see some fairly large changes in the upcoming data and model
runs for the severe threat to become more prevalent. If a storm can
tap into the lower, deeper instability (2500-3000 J/kg) a very
isolated threat for hail to the size of golf balls is possible.

Very warm air and a continued strengthening capping inversion
barrels north on Sunday, which should shut down convective chances
through about Monday night. Later Sunday night into Monday morning
some patchy elevated showers or storms will be possible but for now
confidence is too low and will keep chances at 20 percent or less.

Concerns for heat ramp up Sunday but one big question mark remains,
stratus. The latest run of the nam has some fairly deep and
widespread stratus that would really bite into the afternoon heat
indices. For now confidence is not high enough to drop the heat
advisory but will need to monitor the trends of this stratus as well
as the trends of the dew points. The nearest 70+ dew point in south
central NE and surface winds are expected to remain southeast into
Sunday morning.

Monday may prove to be the hotter more humid day with less chance
for stratus. Expanded the heat advisory into parts of southwest MN
to cover the potential for more widespread heat indices above 100.
Monday does look to be windy which will help take the edge off of
the heat a touch.

Otherwise, Tuesday through Saturday will have southwest flow aloft
with the stronger upper level ridging just to the east of the area.
This will allow periodic waves to move through the area with off and
on threats for showers and thunderstorms. More than likely this
upper level pattern will keep the extreme heat at bay most of the
days, but still see highs from the upper 80s to mid 90s. But with
these threats for showers and thunderstorms will come "unexpected"
outflow boundaries and stratus at times so there will be some
variability in which days are hottest and which are less hot.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1249 AM CDT Sun Jun 28 2026

A broken line of showers and thunderstorms is moving into
eastern South Dakota to start the period. There have been a few
gusts in excess of 50 kts at times along the line, and this will
continue to be the main threat over the first couple hours of the
period. Storms look to gradually weaken with eastward extent, but
there is still some uncertainty as to how far east these storms will
get before they become consistently sub-severe. With all that said,
the greatest threat of 50+ kt wind gusts will be west of I-29.

MVFR to locally IFR stratus will impact the area in the wake of
these storms, with this stratus deck gradually eroding and
lifting from south to north through the morning and early
afternoon hours. After this stratus deck erodes, VFR conditions
can be expected for the rest of the afternoon through the end of
the period.

Breezy southeasterly winds will continue through the morning hours,
gradually diminishing from west to east heading towards the
afternoon hours today. Winds will turn light this evening across the
area except over portions of northwest Iowa where southeasterly
winds will be sustained around 10 kts through the end of the
period.

&&

.CLIMATE...
Issued at 410 AM CDT Sat Jun 27 2026

Summer heat builds into the area from Sunday through much of
next week. This will lead to a potential for a few temperature
records to be tied, or possibly broken:

Current Record Highs:

Monday (06/29): KFSD(101|2002) KSUX(100|2022)

Current Record Warm Lows:

Monday (06/29): KFSD(77|2020) KSUX(80|1931) KHON(75|2020)

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Heat Advisory from 1 PM this afternoon to 9 PM CDT Monday for
     SDZ062-066-067-069>071.
     Heat Advisory from 1 PM this afternoon to 9 PM CDT this
     evening for SDZ060-061-065-068.
MN...Heat Advisory from 1 PM to 9 PM CDT Monday for MNZ072-080-081-
     089-090-098.
IA...Heat Advisory from 1 PM this afternoon to 9 PM CDT Monday for
     IAZ001>003-012>014-020>022-031-032.
NE...Heat Advisory from 1 PM this afternoon to 9 PM CDT Monday for
     NEZ013-014.

&&

UPDATE...Samet
DISCUSSION...08
AVIATION...Samet
CLIMATE...JH

Hazardous Weather Outlook
National Weather Service Sioux Falls SD
407 AM CDT Sun Jun 28 2026

IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097-
098-NEZ013-014-SDZ038>040-050-052>071-290915-
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-
407 AM CDT Sun Jun 28 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.

Isolated strong to severe thunderstorms will be possible through 6-8
this morning as storms continue to move east. 65 mph wind gusts are
the main threat.

Dangerous heat is expected especially near and south of I-90 this
afternoon and early evening, with heat index values ranging from 95
to 105 degrees, hottest in the Sioux City metro into northwest Iowa.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.

Heat index values will climb into the lower 100s at times through
Monday evening. Plan ahead now to limit risk of heat illness if
spending time outdoors.

If storms are able to develop along a cold front late Monday into
Monday evening, some strong to severe storms are possible. Details
on exact storm location, timing and coverage are still uncertain.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
430 AM CDT Sun Jun 28 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0239 AM     Tstm Wnd Gst     3 E Avon                43.01N 98.00W
06/28/2026  M60 MPH          Bon Homme          SD   Mesonet

            South Dakota State mesonet 3 E Avon.

&&

JC

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
431 AM CDT Sun Jun 28 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0422 AM     Tstm Wnd Gst     3 NNW Sioux Falls       43.58N 96.75W
06/28/2026  M63 MPH          Minnehaha          SD   ASOS

&&

JC

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
437 AM CDT Sun Jun 28 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0339 AM     Tstm Wnd Gst     Canistota               43.60N 97.29W
06/28/2026  M61 MPH          McCook             SD   Public

&&

JC

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
447 AM CDT Sun Jun 28 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0440 AM     Tstm Wnd Gst     2 SSE Lennox            43.33N 96.88W
06/28/2026  M67 MPH          Lincoln            SD   Public

&&

JC

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
458 AM CDT Sun Jun 28 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0427 AM     Tstm Wnd Gst     4 E Irene               43.08N 97.08W
06/28/2026  M60 MPH          Clay               SD   Dept of Highways

&&

JC

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
505 AM CDT Sun Jun 28 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0434 AM     Tstm Wnd Gst     3 W Dell Rapids         43.82N 96.78W
06/28/2026  M59 MPH          Minnehaha          SD   Dept of Highways

            Mesonet station 17 SD RWIS DELL RAPIDS.

&&

JC