textproduct: Sioux Falls

KEY MESSAGES

- Isolated to scattered thunderstorms possible after midnight. A couple stronger storms with ping pong ball size hail and 50 mph winds are possible into mid-morning Thursday.

- Hot and humid conditions will persist into the upcoming weekend, with heat index values rising near 100 degrees at times.

- Heat and humidity may lead to additional storm risks, Thursday evening and again Friday evening, though considerable uncertainty remains on development and coverage.

DISCUSSION

Issued at 127 PM CDT Wed Sep 2 2026

THIS AFTERNOON: A mostly quiet afternoon continues in the Tri-State area. A corridor of 40+ knot flow at 850 mb continues to lead to weak elevated convection just south of the CWA. While a low risk, a storm could cross over and enter the CWA into late afternoon. Temperatures will continue to rise through the 80s elsewhere, likely topping near 90 degrees along and south of I-90. Dew points remain elevated either side of 70 degrees leading to heat index values in the upper 90s.

TONIGHT: Models remain a bit split on development later tonight, poorly resolving any subtle vorticity in the mid-lvl flow moving northeast after dark. Some models show a bifurcated LLJ, with one branch centered over NW Iowa and southern MN along a more sustained low-lvl moisture gradient. A second branch may split into central and northern SD after midnight. Convergence on both these features, combined with very weak dPVA could lead to high based showers and thunderstorms after midnight. Severe weather risks continue to remain on the lower side, but MUCAPE lifted off the 850 mb layer could reach as high as 1700 J/KG, with 30-35 knot effective shear potentially suggesting some low end rotation potential. A few stronger wind gust may fall out of this high based activity as suggested by high res CAM data.

THURSDAY: Still some uncertainty in Thursday's forecast as mid-lvl heights begin to rise further over the Plains. Any lingering vorticity is likely to track northeast of the CWA by mid-day, with a surge in low-lvl warm advection pushing 700 mb temperatures as high as 13C over the Tri-state area late in the afternoon. Guidance remains all over the place regarding a stronger shortwave crossing the apex of the ridge mid-day Thursday. Some CAMs suggest a more organized MCS tracking out of eastern ND into central or northern Minnesota in the afternoon bringing a risk for convection forming in the evening along or just east of the CWA. Other CAMs focus on the weaker inhibition on the edge of the CAP, forcing a few thunderstorms along a weak frontal boundary. If no afternoon or evening convection were to develop, then there remains an outside chance for overnight convection along the Ridge/Minnesota river valley after midnight on the LLJ.

FRIDAY-SATURDAY: The mid-lvl ridge axis will remain overhead as we finish the work week end start the weekend. The result will be a continuation of very hot and humid conditions for early September. The combination of 850mb temperatures >10 degrees above normal, along with weak surface flow will push high temperatures into the 90s, with pooling dew points in the upper 60s to lower 70s. The resultant heat index values may top the 95 to 105 range, which could necessitate an advisory for areas along and south of I-90. Those with outdoor activities or sporting events should use extra caution given the heat.

Latest trends keep the risk for convection low heading into the weekend, but not non-zero. A subtle weakness in the flow moving NE through Nebraska late Friday does bring the potential for high based thunderstorms late in the evening and early overnight. While instability and shear will be less favorable, a few gusty storms could be possible. Will maintain a low <30% PoP at this time. The unknown track of any vorticity topping the ridge and/or associated MCS activity lowers confidence Saturday, but a complex of storms is certainly possible with favored areas just north and east of the CWA on the edge of the CAP.

SUNDAY-TUESDAY: No substancial changes to the long term portions of the forecast as the ridge begins to slide eastward and longwave troughing moves closer to the Rockies. The shift towards southwesterly flow aloft should bring an increased risks for precipitation to the Plains depending on the placement of fronts and advection of moisture northward. The increase in clouds and moisture should lower temperatures slightly back towards the lower to middle 80s, still slightly above normal.

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

Issued at 1202 PM CDT Wed Sep 2 2026

VFR conditions are expected into the overnight hours. Scattered diurnal CU field may develop and diminish by sunset. An increase in mid-lvl clouds is anticipated after dark, with some potential for high based showers and thunderstorms Thursday morning. Should this activity develop, then brief periods of MVFR visibility may be possible.

FSD WATCHES/WARNINGS/ADVISORIES

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