textproduct: Sioux Falls

KEY MESSAGES

- Dangerous heat and humidity this afternoon will lead to heat index values of 100-110 degrees south of I-90. The warmest conditions will be along and north of Highway-20.

- Chances for strong to severe storms return to the area by late afternoon mainly along and south of Highway-20. Golf balls and damaging winds to 65 mph will be the main hazards.

- Wildfire smoke will progress through the area today with some near surface impacts expected especially north of I-90. Be prepared for temporary reductions in visibility and air quality. Any sensitive groups should monitor their local air quality.

UPDATE

Issued at 426 AM CDT Mon Jul 20 2026

Taking a look across the area, we're continuing to watch showers and thunderstorms develop across southwestern and southcentral MN mostly in response to increasing lift with an approach shortwave and nocturnal LLJ. While the better lift is just to our northeast, the increased shear has spurred a few isolated showers and thunderstorms across Cottonwood and Jackson counties as of 3 am. While severe weather is not expected, pockets of moderate rain and an occasional stroke of lightning will continue over the next hour or so before things exit our area. Otherwise, the main focus continues to be on the heat today. Higher dew points in the 60s to mid 70s along with building heat this afternoon will lead to heat index values in the low 100s south of I-90 with the potential for up to 110 degree across the Highway-20 corridor and portions of the lower Missouri River Valley.

With this in mind, went ahead and upgraded portions of our Heat Advisory to an Extreme Heat Warning. Both of these products will be in effect from 12pm to 8pm. Lastly, could see a few stronger storms develop by late afternoon mainly along and south of the Highway-20 corridor as a cold front progresses southeastwards through the region. The main threat would be golf ball sized hail and damaging wind gusts up to 65 mph. However, with most guidance continuing to trend initial development southwards; the window for any stronger activity would be very narrow (2-4 hours) if anything at all. Lastly, with wildfire smoke following closely behind the cold front; could see temporary reductions in visibility starting this morning. Any sensitive groups should start to monitor their local air quality.

DISCUSSION

Issued at 229 PM CDT Sun Jul 19 2026

A few light showers persist across the area this afternoon. These showers are also accompanied by a mid level cloud deck which has kept temperatures a bit cooler than previously expected. However, sunshine remains across parts of south central South Dakota. Thus, temperatures have warmed to the upper 70s, 80s, and a few locations pushing into the 90s. With dew points remaining in the 60s to low 70s, heat indices have warmed to the 80s and 90s with a few locations hitting 100F. With some diurnal warming left to go, should still see the heat indices nudge a little higher, allowing for the Heat Advisory to remain in effect until 9PM this evening. Another round of showers and thunderstorms remains possible late tonight into tomorrow morning (11PM to 6AM). These storms will develop along a cold front in North Dakota and move southeastwards through the evening hours. These storms may grow into a line as they push southeastwards. However, there remains some uncertainty in whether these storms will push into southwest Minnesota or stay just outside the forecast area. These storms look to be focused on the nose of the low level jet where convergence and the strongest warm air advection (WAA) will lie. The bulk of the hi-res guidance and the ensembles also support the storms remaining in this location. Still have left light chance to chance PoPs across southwest Minnesota in case lingering showers and storms pass through the area.

The thermal ridge will push directly over the forecast area on Monday, pushing 850 mb temperatures up to +25C to about +30C. Mixing these temperatures down to the surface will result in widespread high temperatures up to the 90s to low 100s, hottest along the Missouri River Valley. With dew points remaining in the 60s to low 70s, heat indices will warm up to 100F to 105F south of I-90. As such, have issued another Heat Advisory for this area. The Advisory is in effect from noon to 8 pm. The previously mentioned cold front will be pushing through the area throughout the day. While this front will cool temperatures, the main cold advective push from the front will come in the late afternoon and evening timeframe, thus allowing for hot and humid conditions to continue. This front will serve as the focal point for additional showers and storms between 4PM and 10PM. The front will push through rather quickly, limiting the severe weather threat to near and south of highway-20. Storms look to remain more discrete as the develop and quickly race out of the area along the boundary. The moisture pooling along the boundary will push CAPE values up to 2,000 to 2,500 J/kg. Hodographs show deep layer shear values on the order of about 45 knots. Hodographs are generally straight with slight backing aloft. This should allow for large hail up to the size of golf balls. DCAPE will also be high around 1,500 J/kg, resulting in the potential for damaging winds to 70 mph. Since these storms are discrete and look to only impact a small area, should only see isolated impacts from severe storms. Once the front pushes through, temperatures will fall to the 60s to low 70s overnight.

Tuesday will be a cooler day with the post frontal airmass in place. High temperatures will be seasonable in the 80s with dew points falling to the 50s. While this would usually make for a pleasant Summer day, smoke will filter in behind the front which could affect sensitive individuals. Fortunately, it looks like the highest concentration of smoke will come Monday night into Tuesday morning and be slowly decreased thereafter. These seasonable conditions will persist for the rest of the work week as northwest flow persists aloft. Next chances for rain look to come on Thursday and Friday as a shortwave trough pushes through the area. The ensembles support this as they show a 40-60% chance for rainfall amounts exceeding a tenth of an inch for this period of time. Instability looks to be quite limited, so severe storms are not expected at this time. Upper level ridging looks to potentially make a comeback next weekend, leading to a return of above average temperatures. This potential remains uncertain so will keep an eye on how the pattern evolves over the next week.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 628 AM CDT Mon Jul 20 2026

Mainly VFR conditions are expected this TAF period. Taking a look across the area, a few scattered showers continue to progress through portions of southwestern MN with the LLJ. Should see this activity stay clear of our TAF sites for now. Otherwise, a cold front will progress through the area bringing the increased potential for a bit of wildfire smoke. While some visibility reductions will be possible, most areas should stay in VFR thresholds. Otherwise, a few showers and thunderstorms will be possible near and south of the Highway-20 corridor. With guidance continuing to trend southwards with initiation decided to leave any mention out of the KSUX TAF for now. Otherwise, southwesterly winds will become more northwesterly behind the previously mentioned front to end the TAF period.

FSD WATCHES/WARNINGS/ADVISORIES

SD...Heat Advisory from noon today to 8 PM CDT this evening for SDZ050-063>069. Extreme Heat Warning from noon today to 8 PM CDT this evening for SDZ070-071. MN...None. IA...Heat Advisory from noon today to 8 PM CDT this evening for IAZ001>003-012>014. Extreme Heat Warning from noon today to 8 PM CDT this evening for IAZ020>022-031-032. NE...Extreme Heat Warning from noon today to 8 PM CDT this evening for NEZ013-014.