textproduct: Sioux Falls

KEY MESSAGES

- Isolated to scattered storms are ongoing this evening, through development remains very uncertain. Risks of 1" hail and 60 mph will be possible.

- Greater storm potential after midnight and through the overnight. Initial supercells are capable of producing 1-2" hail and wind gusts to 65 mph. Storms gradually grow into broken lines with a few bowing segments. Risks become wind gusts to 70 mph and marginal hail.

- Lingering rain early Tuesday will push the greatest risk of severe storms Tuesday further south. However cooler temperatures and clouds may linger deeper into the day.

- Rain chances persist Wednesday and Thursday, though storm track leads to lower confidence in where rain may fall.

UPDATE

Issued at 918 PM CDT Mon Aug 17 2026

Showers and thunderstorms have moved into the western portion of our CWA. While the convective environment is largely favorable, the forcing from the prefrontal trough is weak and mid-level lapse rates are decreasing. So while there is still potential for an isolated strong to severe storm, the confidence in severe weather with these storms is low. Should a severe storm manage to form threats include hail to the size of a quarter and wind gusts to 60 mph. These storms will continue to weaken and dissipate as they move southeast through the late evening.

Another round of potentially strong to severe thunderstorms are possible through the overnight. An upper shortwave and associated cold front will push into central South Dakota around midnight. Deep layer shear remains strong and the LLJ will be increasing. Though forcing with this round will be stronger, instability, and lapse rates will be decreasing. What this means is most likely upon convection initiation strong to severe supercells are possible with hail 1-2" and wind gusts to 60-65 mph. As the storms progress southeast they will transition to elevated line segments, some with bowing structures. The threat will then transition to a wind threat with gusts of 70 mph possible. For much of the northern part of the CWA, storms will be clear around 7am. For areas along the southern Missouri River and northwest Iowa, storms may linger into mid- morning Tuesday. Lingering clouds through the day and light northerly winds will keep highs in the upper 70s to low 80s.

DISCUSSION

Issued at 132 PM CDT Mon Aug 17 2026

THIS AFTERNOON: Convection risks remain the primary focus in the upcoming 24 hour period, as there continues to be uncertainty on if we can advect sufficient moisture northward, and provide enough focus of lift. Visible satellite this afternoon shows two areas of interest, one area of ACCAS across northeastern South Dakota and west central Minnesota, driven by 700 mb winds over the crest of the Ridge. A second area of ACCAS drifting eastward out of the Badlands area. At the surface, one concern has been a slower arrival of boundary layer moisture as surface dewpoints continue to linger in the upper 50s to lower 60s. Further south, models continue to overestimate dew points in NE/KS, providing some doubt on if the lower 70 dewpoints advertised by "high instability" models can be realized.

The projections into the evening remain slightly uncertain with several focusing mechanisms available. A surface trough extending from NW Minnesota into NE South Dakota and central South Dakota will provide some minor convergence (focused greater over NW Minnesota and eastern North Dakota). A weaker pre-frontal trough in the NE Panhandle into the Sandhills could also serve as a focal point. Finally, a few storms may also develop over the Minnesota River valley downstream of the Buffalo Ridge. Any storm that develops off of these features will likely produced an increased risk for 1-2" hail, but also 60-70 mph winds as MLCAPE pushes 2000+ J/KG and effective shear climbs towards 40 knots. Gradual east southeast storm tracks within the mean flow are likely which could put portions of SW Minnesota and the MO River valley into a higher early evening risk. However the likely overestimation of surface dew points, poor focus of convergence, and weak inhibition, may also work to prohibit convection over a large part of the Tri- State area into the evening.

Further west, additional isolated to scattered convection may form over the western High Plains late this afternoon and evening slightly stronger synoptic lift enters the area. One or more clusters of storms could reach the western CWA by mid-late evening, bringing risks lower risk for hail, but higher risks for a few stronger wind gusts. There continues to be some agreement in mesoscale and lower resolution guidance that a subtle wave crossing the Dakotas could keep scattered convection risks moving through the CWA until at least 4am. Weakening instability, and worsening lapse rates should result in fewer strong to severe storms by daybreak Tuesday. All this to say, coverage of this activity remains highly in doubt, which will keep areal PoPs lower in general.

TUESDAY: Latest guidance continues to trend the greatest risks for severe convection southward on Tuesday. Overnight convection, along with the passing shortwave should stabilize the area considerably by daybreak, with a lingering marginal risks for some crossover hail risks near 12Z. We may however see quite a bit of stratus behind this wave, suggesting cooler than normal temperatures into the afternoon.

WEDNESDAY-THURDAY: Upper troughing moving into Hudson Bay will sharpen the NW flow over the Plains slightly by mid-week. EC/GFS indicates vorticity crossing from the Pacific Northwest through the Northern Rockies early on Wednesday pushing mid-lvl lift towards the region early Wednesday. Differences on the track of this wave keep PoPs lower, with GEFS/GFS further northeast, and the EC leading to more impact through central Nebraska instead. Another wave, along with a weak frontal boundary approach Thursday afternoon. Depending on the amount of inhibition nearby, a few thunderstorms could form along this boundary late in the day.

FRIDAY-SUNDAY: Mid-lvl heights rise and the ridge sharpens over the Rockies as we move into next weekend. This transition of mid-lvl flow could bring one final risk for elevated showers/TSRA early Friday, but generally quiet and dry conditions then develop into the weekend. Temperatures however will fall more under the influence of Canadian high pressure in the Dakotas, suggesting highs may stay in the upper 70s to lower 80s.

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 728 PM CDT Mon Aug 17 2026

VFR conditions to begin the period. Clouds are flowing in from the west ahead of a broken line of multicell thunderstorms. Recent shifts in short term guidance leads to high uncertainty in their evolution. The most likely solution is these storms will continue their southeast progression. Some may yet become strong to severe producing hail to the size of a quarter and 60 mph wind gusts. KHON is the most likely to experience hazardous weather from these storms.

Another round of strong to severe thunderstorms is possible overnight as a stronger wave moves through the area. Any storms that develop will have the potential of producing hail to the size of a quarter and wind gusts to 70 mph. Storms weaken as daybreak approaches, but showers are expected to continue through much of Tuesday morning. Dry conditions expected for the afternoon.

FSD WATCHES/WARNINGS/ADVISORIES

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