Severe Thunderstorm Warning 27, NWS Sioux Falls, SD

Area Forecast Discussion
National Weather Service Sioux Falls SD
618 PM CDT Fri May 15 2026

.KEY MESSAGES...

- Thunderstorm risks grow after 5pm for a narrow portion of NW
  Iowa. Hail to 2 inches and 70 mph winds possible. Storms move
  southeast quickly with the risk ended by 9pm.

- Thunderstorms increase in coverage Saturday night into Sunday.
  Severe weather risks including 1.5" hail and 60 mph wind gusts
  will be possible.

- A significant severe weather episode will be possible late
  Sunday afternoon into Sunday night. Very large hail, straight
  line winds, and tornadoes will be possible. This risk may
  extend into Monday.

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

&&

.DISCUSSION...
Issued at 129 PM CDT Fri May 15 2026

THIS AFTERNOON:  Poorly defined "cool" front continues to slowly
sink southeast this afternoon now entering southern Minnesota
and far northwestern Iowa as of 1pm. Southerly winds are slowly
advecting 50 dewpoints into the area ahead of the boundary, but
generally models have been too quick to pull higher dew point
air northward. By late this afternoon a weak impulse in the
700:500 mb layer will arrive into the Tri-State. This increased
lift combined with slight pressure rises to the northwest of the
front should create a bit more convergence sufficient for
convection development in a portion of NW Iowa (along and
southeast of a line from Sioux City to Spencer). Soundings
indicate MLCAPE values may approach 1000- 1500 J/KG, with an
increase in effective bulk shear towards 40+ knots. In any storm
that develops, large hail (potentially 2" or greater) will be
possible as mid-lvl lapse rates break the 8C/KM mark. Inverted V
soundings along the periphery of the CWA boundary in NW Iowa
would suggest downburst potential increases as well. With the
mean wind increasing from the west northwest, storms should
begin to congeal into one or more linear clusters as they head
into central Iowa. The peak timing for development and impact
falls within the 6pm to 9pm timeframe.

TONIGHT: Storms move southeast of the forecast area very early in
the overnight period, leaving quiet and dry conditions overhead.
Soundings show a pronounced low-mid lvl dry layer advecting
southward through the night.

SATURDAY: The aforementioned frontal boundary will be pushed by
convection closer to I-80 by Saturday morning, leaving dry and quiet
conditions in the local area for most of Saturday. A persistent
easterly 850 mb wind fetch is not expected to bring meaningful
moisture northward through the day with the surface boundary
tracking northward towards Highway 20 by mid-evening.  Temperatures
under filtered sunshine peak in the lower 80s, and with
persistent low RH and breezy easterly surface winds fire danger
may be elevated.

SATURDAY NIGHT: Today's guidance has slowed the northward advection
of rich theta-e air northward which will likely keep the area
convection free deep into the evening.  We will be watching areas of
central and southern Nebraska tied to surface low pressure and the
existing warm front that remains in place.  Convection is expected
to develop as a stronger wave ejects into the Plains. One or more
clusters of strong to severe storms will attempt to track north or
northeast along the advancing elevated moisture axis and towards the
MO River valley after midnight.  MUCAPE remains somewhat marginal
with the initial surge of moisture, and mid-lvl lapse rates are only
in the 6.5-7 C/KM range throughout the hail growth zone.
Nevertheless, effective shear may just be strong enough to produce a
few elevated supercells capable of larger hail into Sunday morning.
Tend to feel the straight line wind risk may be lower, but should
the surface front lift northward or boundary layer become only
loosely stabilized, then a few stronger gusts could transport down
through the MO River Valley into NW Iowa into daybreak.

SUNDAY-SUNDAY NIGHT: Significant severe weather potential is
possible Sunday afternoon into Sunday night.  After morning
convection lifts northeast of the CWA early in the morning, guidance
is becoming more confident in a period of shortwave subsidence
moving overhead through a large part of the daytime hours of Sunday.
 While we'll enter a quiet period, moderate to extreme instability
will continue to advect northward into the Tri-State area with some
models hinting at potential for AOA 3000 J/KG by mid-afternoon.
Soundings across the area suggest an EML will hold in place into at
least the mid afternoon hours as we wait for a deeper trough to lift
into the Plains.  This trough should work to push the effective warm
front further north while reinforcing a pre-existing cold front
across central NE/SD. The arrival of deeper synoptic lift after
21Z could lead to explosive convective development by early
evening. With the large reservoir of instability, bulk shear
over 50 knots, and strong low-lvl buoyancy all modes of severe
weather will be likely. Numerous sounding analogs suggest 2-4"
hail in any discrete or initial updraft that forms. Storms
staying discrete is somewhat in question, with guidance hinting
that the potential for quick upscale growth into a linear line
with embedded supercells progressing east northeast instead. The
increase in the low-lvl flow after dark and and slight backing
of surface winds could lead to a much greater potential of QLCS
activity into the late evening and overnight hours as storms
track into Iowa and Minnesota.

As we move past midnight, models suggest the highest severe weather
threat will migrate eastward with the line.  However, there remains
a high degree of uncertainty in the evolution after midnight.
ECMWF/NAM/CMC are holding back the upper trough to the west, and
actually allow deeper instability to lift back northward into Monday
morning. The persistent 40-50 knot LLJ could lead to scattered hail
producing supercells continuing into Monday morning. Still very
uncertain.

MONDAY:  A continued severe weather risks seems plausible on Monday
depending on what happens overnight.  Should the GFS based models
hold true, then both surface based and elevated instability slides
east of the area.  However ECMWF/NAM solutions would pull the
surface front back westward while leaving a large degree of elevated
instability westward.  There's just too much uncertainty to get into
specifics, but Monday could also feature potential for higher end
severe weather especially in areas along or east of I-90.

TUESDAY-THURSDAY: We'll see a distinct cooldown in temperatures
as we head towards the middle of the week. Highs on Tuesday
with a cooler northwest wind only rise into the the upper 50s. A
slow recovery towards the lower to middle 60s is possible by
Wednesday. The pattern through the rest of the week remains
quasi-zonal, suggesting one more more weak disturbances moving
through the flow creating a few rain chances by the second half
of the week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 617 PM CDT Fri May 15 2026

Showers and thunderstorms are ongoing over northwest Iowa at
issuance. These storms have a history of producing large hail and
damaging wind gusts. Storms will move to the east northeast over the
next few hours, clearing the CWA around 16.04Z.

Otherwise, VFR conditions are expected for the duration of this
period. Breezy northwest winds will decrease over the next couple of
hours, becoming light and variable overnight. Saturday morning winds
become north to northeast and increase to 5-10 kts. Through the
afternoon winds become more east and breezy with gusts 16-22
kts that will continue beyond the end of the period.

&&

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

&&

DISCUSSION...Dux
AVIATION...AJP

Hazardous Weather Outlook
National Weather Service Sioux Falls SD
624 PM CDT Fri May 15 2026

IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097-
098-NEZ013-014-SDZ038>040-050-052>071-162330-
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-
624 PM CDT Fri May 15 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...Tonight.

Storm chances return this afternoon and evening, mainly for
northwestern Iowa. Strong to severe storms may produce hail to 2
inches in diameter and wind gusts to 75 mph. Have multiple ways to
receive warnings!

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

Locally elevated to near critical fire conditions are possible
Saturday due to very low relative humidity and light winds.

Thunderstorms return Saturday night into Monday. Strong to severe
storms are possible. Although confidence is low in exact timing and
coverage, greatest risk remains Sunday afternoon and early evening.
Monitor the forecast especially if you have outdoor plans.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed.

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
529 PM CDT Fri May 15 2026

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

0520 PM     Hail             5 WNW Terril            43.33N 95.05W
05/15/2026  M1.00 Inch       Dickinson          IA   Trained Spotter

&&

TRM

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
541 PM CDT Fri May 15 2026

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

0524 PM     Tstm Wnd Gst     4 ESE Arnolds Park      43.35N 95.05W
05/15/2026  E60 MPH          Dickinson          IA   Trained Spotter

            Brief 60 mph wind gust, pea hail, and 0.05
            inch rain before storm took off to the east.

&&

TRM

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
617 PM CDT Fri May 15 2026

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

0600 PM     Hail             6 NNW Hornick           42.31N 96.13W
05/15/2026  E1.00 Inch       Woodbury           IA   Trained Spotter

            Hail covered the ground, with stones as big
            as quarters. No wind and a decent amount of
            rain.

&&

TRM

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
638 PM CDT Fri May 15 2026

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

0619 PM     Hail             3 SSW Anthon            42.35N 95.88W
05/15/2026  E1.00 Inch       Woodbury           IA   Trained Spotter

&&

TRM

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
645 PM CDT Fri May 15 2026

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

0632 PM     Hail             2 SW Correctionville    42.46N 95.81W
05/15/2026  E1.00 Inch       Woodbury           IA   Fire Dept/Rescue

&&

TRM