textproduct: Sioux Falls

KEY MESSAGES

- Hot and humid conditions will persist through at least Saturday. Afternoon temperatures in the 90s will push heat index values near 100 at times. Outdoor events should plan accordingly to the heat risks.

- Occasional thunderstorms will be possible again Saturday and Sunday, though remain highly unpredictable on location. A few stronger storms could be possible, the the widespread risk of severe weather remains low.

- A shifting pattern may bring higher rain chances, slightly cooler temperature, and a severe weather risk early next week.

UPDATE

Issued at 901 PM CDT Thu Sep 3 2026

Winds tonight will become light and variable. Despite light winds over night lows will remain warm in the 60s to low 70s. Areas of patchy fog are possible in the early morning hours, mostly along and north of I-90. The majority of the fog should remain confined to low areas and waterways. Fog should burn off by mid-morning Friday. In addition, isolated thunderstorms are possible Friday morning mostly over northwest Iowa. Confidence is low as guidance is split on storm formation and soundings indicate a general lack of moisture. However, some weak mid-level Theta-e advection may destabilize things enough that convection could form along the nose of the strengthening LLJ. Severe weather is not anticipated at this time, however, below 850 mb a deep dry layer exists. Evaporative cooling may result in a few stronger wind gusts to 50 mph. Storms should be short lived and end by late morning.

Through the day Friday the upper ridge will increase in amplitude. As it does so a lee cyclone located over eastern Wyoming will drift north into eastern Montana. This will drag north a surface frontal boundary currently draped across the northern Great Plains and into the Great Lakes region. The mid- levels will warm into the upper 20 degrees C, which is at 99% of the climatological average for this time of year. Good mixing up through approximately the 850 mb layer will boost high temperatures at the surface into the 90s with a few 100s possible through the James River Valley and along the southern Missouri River Valley. Dew points will once again climb into the 60s and low 70s, with Heat Index values in the 90s to around 100. Currently a Heat Headline appears unnecessary as the higher values will brief and not widespread. Still, caution is needed with any activities outside.

Friday night into Saturday morning another round of thunderstorms is possible over central South Dakota. Modest moisture advection in the mid-levels coupled with weak convergence and a strengthening LLJ may initialize scattered thunderstorms. Some of these could be strong to possibly severe with steep mid-level lapse rates, 1500 J/kg of MUCAPE, and 30-35 kts of deep layer shear. Similar to many of the storms recently these will be weakly forced. Therefore confidence on location and coverage of storms is low.

DISCUSSION

Issued at 1158 AM CDT Thu Sep 3 2026

THIS AFTERNOON: Another hot and humid early September day as temperatures are rising into the 90s. With dew points in the upper 60s to lower 70s, heat index values will climb between 95 and 100 degrees. Mid-lvl vorticity that has been strung across the CWA continues to produce high based showers and isolated storms. While updrafts themselves are not significant, high LCLs and significant DCAPE may continue to lead to pockets of stronger winds as they track northeast and pulse. As low-lvl temperatures increase and vorticity stretches out and moves east, this activity should dissipate by mid-afternoon. Some localized areas may have an impact on their high temperature due to the accompanying cloud cover.

TONIGHT: Shortwave ridging arrives for the overnight hours, with skies clearing this evening. However we'll remain very warm in the low-lvl and any surface wind will result in low temperatures in the 70s. Convection risks continue to stay on the lower end, with any focus for convection tied to convergence on the LLJ to the east of the CWA. We may have to watch for some fog focused in areas along and north of Highway 14.

FRIDAY: A variable wind early will give way to southerly winds quickly on Friday. With highly anomalous 925:850 mb temperatures remaining in place, we'll again see high temperatures rise into the middle to even upper 90s. The focus for the warmest temperatures may be in the James River valley where more significant impacts from the drought have continued. In these areas both HREF and the NBM indicate a 40-60% probability of reaching 100 degrees. Heat index values will top out near 100 degrees for a couple hours mid- afternoon, though the duration may not be long enough to warrant an advisory attm. Outside of daybreak convection in the far eastern CWA, significant capping should limit the potential for afternoon and evening convection.

Convection risks may increase slightly by daybreak Saturday as some guidance hints at the redevelopment of LLJ centered through central South Dakota. As has been the normal, very isolated strong convection could be possible, and with steep mid-lvl lapse rates, thin stable boundary inversion, and considerable dry air in the sub- cloud area, additional heat bursts may be possible.

SATURDAY-SUNDAY: A bit more uncertainty lingers for the daytime hours of Saturday. Any morning convection could generate mid- lvl vorticity that could cause persistent lingering iso-sct storms into the early afternoon hours. Should convection form and linger, then it may mess with the afternoon temperature forecast. Currently anticipating another hot and humid day with highs in the 90s, and a few hours in the mid-late afternoon with heat index values either side of 100. Anyone with afternoon/evening outdoor activities may want to consider adjusting the timing of their plans due to the heat.

The broad ridge remains planted overhead on Sunday, with some potential for mid-lvl vorticity to ride over the top of the ridge Saturday night and Sunday. These waves are highly unpredictable, and lead to spurious MCS development that may or may not develop. Typically these MCS would track along the periphery of the EML (sometimes tracked by the +10 to +12C temperature line). This could cause a weakening MCS to track southward into the Tri-state area early in the day.

MONDAY-WEDNESDAY: Upper troughing digging into the western CONUS will suppress the Central Plains ridge and push it slightly further to the east. The resulting southerly mid-lvl flow will pull stronger return flow northward and lead to an increase in PoPs for the early portions of the week. Depending on the positioning of the surface fronts and upper jet, an increased severe weather signal could develop. This signal seems to also be hinted at by the AI learning models for Monday and Tuesday.

AVIATION /00Z TAFS THROUGH 00Z SATURDAY/

Issued at 628 PM CDT Thu Sep 3 2026

VFR conditions and mostly clear skies will prevail for this period. Winds will become light and variable after sunset. Early Friday morning patchy fog is possible, mostly along low areas and water ways. Any fog should burn off by mid-morning. Winds become easterly through the morning and remain light through the end of the period.

FSD WATCHES/WARNINGS/ADVISORIES

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