974
FXUS63 KFSD 182256
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
556 PM CDT Fri Jul 18 2025

.KEY MESSAGES...

- Strong to severe storms are possible this evening and night.
  Storms look to develop along and north of Highway 14 late this
  afternoon. Initial storms track southeast, with renewed
  development near I-90 late in the evening.

- Large hail up to 2 inches, damaging winds to 70 mph, a brief
  tornado, and brief locally heavy rainfall are all possible.
  Storms look to exit the area by 3-4 am.

- Saturday looks to be a mostly dry day with low risk (20%) for
  thunderstorms near the Missouri River/Highway 20 late in the
  day.

- Additional convection develops Sunday night into Monday and
  slide east, some of which could be strong to severe.

- An active pattern persists aloft Monday through Friday next
  week which looks to bring daily chances for showers and
  storms. Machine learning guidance suggests that the storms
  could be strong to severe.

&&

.DISCUSSION...
Issued at 211 PM CDT Fri Jul 18 2025

THIS AFTERNOON:  We`re continuing to see mid-lvl ACCAS drift through
the region at mid-afternoon as 850:700 mb winds remain veered to the
southwest. Satellite and surface observations show a surface low
west of Valentine with an elongated frontal boundary lifting
northward through central South Dakota and then curving eastward
just north of Highway 14.  Stable stratus layer continues to
indicate fairly widespread stability through the region, with narrow
corridor of lower stability forming along the Highway 14 corridor
near surface boundary.  30 knot surface gusts into this boundary are
serving to enhance low-lvl convergence and should further
destabilize the atmosphere by 5pm. Aloft, forcing mechanisms
have been more difficult to pickup, but WV showing clear
shortwave leaving eastern Montana and moving into northwestern
SODAK this afternoon. This wave should help develop convection
late this afternoon or early evening.

FRIDAY EARLY EVENING and NIGHT: A modestly unstable environment
remains in place south of the aforementioned boundary, with
expectations of 1500 to 2500 J/KG MLCAPE forming through the
evening. A stubborn EML will remain problematic and may delay
development further south towards I-90 towards the late evening
hours. While there has been considerable bouncing around of CAMs
today, a few more consistent signals for convective development
near or north of Highway 14 late this afternoon continue,
focusing at the apex of increases surface and deep moisture
convergence. Eventual storms will be shifting east and southeast
along the edge of the EML into western and southwestern
Minnesota into early/mid evening. Effective shear of 40 to 45
knots (especially mid-upper column) should support a few
rotating supercells capable of golf ball or larger hail. As the
evening progresses onward, additional development may begin to
form near the I-90 corridor in the 8-10pm timeframe as low-lvl
flow begins to increase. While hail will be the primary risk in
convection through midnight, modest cold pool development along
with intrusion of mid-lvl dry air could force convection
towards a wind risk as storms form deeper into the night and
shift into southern South Dakota and Iowa. One aspect of this
event we`ll have to monitor is the eventual ramp-up of low-lvl
flow and rapidly increasing 0-1km helicity, especially as the
boundary layer remains mostly mixed and LCLs stay low deep into
the night along and south of I-90. This could increase the
tornadic risk towards late evening and early overnight.
Convection will drift southeast and should exit by 3am.

SATURDAY: High pressure sinks into the region early Saturday, likely
allowing stratus to develop and patchy fog through mid-morning.
Convective chances may begin to redevelop by mid-late afternoon
Saturday, with MLCAPE plots suggesting the greatest potential
along and south of the Missouri River and Highway 20 areas.
Increasing mid-lvl flow and approaching vorticity could spell
some increased severe weather risks from late afternoon through
mid-evening.

SUNDAY: Mid-lvl heights increase early Sunday, which should keep
convection in the West River areas through most of Sunday. Further
east, high pressure will keep conditions cooler.  Nocturnal
convection risks increase late Sunday night into Monday as the LLJ
increases and a mid-lvl shortwave moves through the Plains.  A low
(marginal) risk of severe storms will be possible.

MONDAY-FRIDAY:  The pattern through most of next week remains highly
unsettled with a building, but flat mid-lvl ridge moving over the
Central CONUS. This typical mid-Summer MCS pattern should bring both
warmer temperatures but also surging surface dew points. Multiple
days of severe weather risks may be upon us next weeks, mostly
focused late in the evening and through the overnight hours.
Regarding dew points, it`s most likely the case that NBM dew points
are too low given vegetation growth, recent rainfall, and moisture
convergence.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 542 PM CDT Fri Jul 18 2025

No significant changes from the previous TAFs. A mix of VFR and
MVFR ceilings remain in place this afternoon. Isolated to
scattered showers continue across the area late this afternoon.
Beginning to see convection fire near and north of US Hwy 14
(with the strongest cell at the time of this discussion near
KATY). Expect convection to continue to develop this evening and
move southeast, with additional development near I-90 later
tonight. Confidence in timing/spread of convection remains low
as CAMs continue to handle current development poorly. Storms
could be strong to severe with all modes possible.

Otherwise, guidance continues to show stratus and/or fog
developing after storms roll through tonight, leading to MVFR to
IFR or lower conditions. Improvement is expected through the
late morning and early afternoon hours.

Isolated to scattered showers and storms may develop once again
tomorrow afternoon, with the greatest chances south of I-90 and
toward US Hwy 20. Will omit from KSUX for now given low chances
and low confidence.

&&

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

&&

$$

DISCUSSION...Dux
AVIATION...SG