textproduct: Sioux Falls

KEY MESSAGES

- Isolated showers and non-severe storms across the Missouri River Valley drift eastward through late morning.

- Patchy fog with visibility briefly below two miles east of I-29 lifts through the mid morning.

- Today's storm potential remains the focus of the forecast. While exact storm coverage remains uncertain, isolated to scattered severe storms are possible after 4pm, with greatest risk after 7pm. Main hazards produce 65 mph winds and two inch hail. Tornado risk remains low but not zero.

- Lingering morning rain Tuesday may limit the risk for strong storm redevelopment during the afternoon and evening. Greatest storm potential remains south of I-90 toward the Missouri River and US Highway 20.

- Expect temperatures near or slightly below normal from Wednesday through Saturday. Severe weather and widespread rain is not likely.

UPDATE

Issued at 329 AM CDT Mon Aug 17 2026

Scattered showers and storms developed just after midnight in central SD with the LLJ. These continue to move to the southeast this morning with possibly some additional development. Severe weather is not expected. Showers/storms may linger into mid morning across the James Valley.

Further east where high clouds are more scattered, patchy fog has developed with brief and isolated visibility below two miles. Fog mixes out through the morning.

Main focus of the forecast remains on the severe weather threat. Models are struggling to resolve the surface front and coincident mid level wave. No major changes to guidance or the forecast this morning. Lowered dew points by a couple of degrees for this afternoon and evening; despite southerly flow and modest moisture return, believe that Td in the mid 70s will be hard to come by. Bumped up the wind potential in strongest storms to 65 mph, based on latest runs of hi-res guidance and some ensembles. This risk may be higher if any clusters can grow upscale into more linear segments. DCAPE values overall are a little on the low side, but some deterministic guidance shows some pockets of values over 1000 J/kg. Also increased the potential hail size to 2 inches as models have increased 0-6km shear to over 40 knots. Additional details in the Discussion below remain valid.

One big question for Tuesday is how much convection we see overnight into Tuesday morning. 17.00z hi-res guidance seems to be keying off of another wave and the LLJ moving through the area behind the surface front through the overnight hours into the Tuesday morning commute, more so than previous runs. Instability should wane quickly through the overnight, so severe weather threat is low after midnight/1 AM CDT. However, morning storms and convective debris will impact severe potential later in the day with the greatest risks toward the MO River Valley and US Hwy 20, although some guidance is a bit further north.

DISCUSSION

Issued at 1252 PM CDT Sun Aug 16 2026

THIS AFTERNOON: A pleasant Sunday afternoon across the region and northerly winds continue to push lower dew point air southward and temperatures rise towards the 80 degree mark. The only convective focus into the evening hours will be near the Black Hills region.

TONIGHT: High pressure sinks southward this evening and overnight, allowing a quick drop in temperatures. As we reach the midnight hour, gradually increasing low-lvl warm advection over central SD and northern NE should induce a mid-lvl cloud deck. There may be sufficient lift to produce isolated to scattered high based showers and thunderstorms along the MO river, with a secondary wave dropping out of ND also producing a pocket of elevated convection. Latest trends keep PoPs focused along or west of the James River valley through 12Z, with no severe weather expected.

MONDAY/MONDAY NIGHT: Strong to severe convection risks remain the biggest story for Monday afternoon and evening. We may start the day with quite a bit of mid-lvl clouds and a few showers near the James River valley. Should a small area of vorticity continue crossing northern SD, then associated warm advection may push eastward towards I-29 by mid-day bringing a low risk for sprinkles to high based showers.

By the afternoon hours we should begin to gradually become more unstable as winds turn southwesterly. The degree of instability remains in question partially due to the fact that 70 dewpoints have been pushed well south of the CWA, and southerly boundary layer flow only commences by late morning. Secondly, lingering mid-lvl clouds could also lower temperature potential. We'll begin watching a front slowly pushing into south central and northeastern SODAK and western MN by late afternoon which may, along with any weak pre- frontal trough, provide enough convergence to develop isolated to scattered convection after 4pm. Greater convection risks may develop later in the evening or early overnight as guidance continues to show weak vorticity in the northwesterly flow entering the region inducing a stronger LLJ response. It's also possible that the aforementioned front may further slow down. It should be said though, that this northwesterly flow continues to remain problematic for models to resolve.

From a parameter perspective, MLCAPE upwards of 1800-2800 J/KG (dependent on T/Td trends and mixing in the afternoon), combined with effective shear AOA 35 knots should produce at least potential for multi-cell/supercellular development capable of golf ball hail and 60 mph winds in the evening. Some storm clustering towards straight line wind risks could also develop given the unidirectional mid-lvl flow. Tornadic risks will remain low, but not- zero, especially if storm development holds off until after 00z when better boundary layer moisture arrives into the Tri- State area and very low- lvl flow begins to increase with the developing LLJ. This could push 0-1 SRH upwards of 100 M2/S2 by 00-03Z. Overall storm coverage remain uncertain given somewhat marginal forcing initially and perhaps some potential for weak inhibition holding on.

Will continue with elevated PoPs through the overnight hours given the potential for increased forcing as noted earlier. The severe weather risks should lessen after 10pm, as mid-lvl lapse rates weaken. Though a few gusty storms may continue into Tuesday morning.

TUESDAY: There remains quite a bit of uncertainty regarding Tuesday's forecast. Should convection persist overnight, this may drive any effect boundary south of the CWA. However some guidance continues to hint that any strong convection may develop quickly in the morning near the MO river and Highway 20 and drive southward effectively limiting the afternoon potential in the CWA. Temperatures Tuesday cool back into the 70s.

WEDNESDAY-SATURDAY: A larger trough dropping into Hudson Bay by mid- week will push a passing trough eastward through the region on Wednesday. Cooler temperatures and lower instability should keep the risk of severe storms low. Into the weekend, we'll continue to look for near to slightly below normal temperatures, with no focused area of high PoPs given a uncertainty of waves in the returning northwesterly flow aloft.

AVIATION /06Z TAFS THROUGH 06Z TUESDAY/

Issued at 1044 PM CDT Sun Aug 16 2026

VFR conditions begin the TAF period. Latest satellite shows remaining thunderstorms across southwestern South Dakota and northwestern Nebraska that continue to push southeastwards. While these storms will avoid the forecast area, another round of showers and a few thunderstorms remain on track to develop late tonight mainly west of the James River. Patchy valley fog also remains possible but confidence is too low to include in a TAF at this time. Another round of showers and thunderstorms remains on track to develop tomorrow afternoon and evening. Coverage of these storms is still uncertain but starting to come into a clearer picture. Have included a PROB30 group in KFSD's TAF as confidence is highest in this terminal seeing showers and storms. Chances for showers and storms will be ending at the end of the TAF period.

FSD WATCHES/WARNINGS/ADVISORIES

SD...None. MN...None. IA...None. NE...None.