textproduct: Sioux Falls

KEY MESSAGES

- Isolated showers and storms may develop through the early morning hours mainly over northwest Iowa and nearby areas of Minnesota, South Dakota, and Nebraska. Severe weather is unlikely, though the strongest storm may produce a wind gust to 50 mph and/or small hail.

- Heat indices climb to around 105 on Monday mainly south of I-90. Locally higher heat indices are possible along and south of the Highway 20 corridor. A Heat Advisory is in effect until 8 pm Monday for areas south of I-90 into portions of southern Minnesota.

- Next threat for showers and thunderstorms will be on Thursday. An isolated storm could be severe.

UPDATE

Issued at 932 PM CDT Sun Jul 26 2026

Storms have struggled to develop late this afternoon and thus the threat for severe weather has diminished and will remain quite low through tonight. That said, weaker, elevated convection may develop late tonight (mainly after midnight) for areas in the quadrant near and east of I-29 and near and south of I-90. Severe weather chances with this activity are quite low, but still can't rule out gusts to 40-60 mph and small hail if something can get going and be able to sustain itself.

Otherwise, the main story continues to be the heat heading into the day on Monday. We will be getting a boundary pushing south through the day, helping to dry out the dew points for areas north of I-90 well before peak heating. For areas south of I-90, moisture pooling ahead of this boundary will result in dew points once again climbing into the middle to upper 70s and thus allowing for heat indices to quickly climb to the 100s potentially as early as 9 to 10 am. Blended in the 27.00Z HRRR into the NBM for dew points through the early afternoon to get them a bit higher. All that said, did expand the Heat Advisory farther west and north and extended the time to start it now through 8 pm Monday. The 18Z REFS ensemble does indicate around a 40-60% chance of heat indices rising above 105 degrees along and south of the Highway 20 corridor Monday afternoon, so future upgrades may be considered.

The air temperature forecast gets a bit complicated especially north of I-90 with the boundary moving through as wildfire smoke filters in behind it. This smoke reduced visibilities over parts of North Dakota and Montana today, and though it may thin out a bit as it arrives here, it may still prevent the warmest temperatures from being realized. Still blended in some CONSAll for max temperatures to get them a little bit higher than what the NBM had, but was not as aggressive as some of the other models had. Look for highs Monday in the middle 90s along Highway-14, middle to upper 90s along I-90, and then around 100 degrees along the Missouri River Valley where the boundary moves through last. This smoke may also cause issues for mainly sensitive groups, so be sure to check in with the latest air quality conditions before heading out and about Monday afternoon and evening.

DISCUSSION

Issued at 118 PM CDT Sun Jul 26 2026

The thunderstorm threat is still very low confidence for late this afternoon into tonight. Quite a bit of cloud cover has lingered into the early afternoon which is slowing heating. And, as opposed to yesterday, the mid to upper level wave today is a bit weaker and may not help to organize activity as efficiently, so will need to find a way to break the cap. For now these chances look best south of I-90 into northwest IA where the deeper moisture resides. South central SD shows the weakest cap late afternoon while parts of northeast NE, far southeast SD and northwest IA see a moderately increasing LLJ during the evening hours, which could focus some thunderstorm development.

Winds turn more northerly tonight into Monday which should allow dew points to settle into the 60s and lower 70s. This will likely keep heat indices in the 90s and thus heat headlines will not be necessary. One exception may be across parts of northeast NE and northwest IA where heat indices should creep above 100 again with the higher dew points. Will have a heat advisory for these locations.

Monday and Monday night should be quiet as more stable air settles in. One exception may be later Monday night in central SD as some upper level support spreads from western SD into that area. Just some small chances west of the James for now.

Tuesday and Wednesday will be on the northern edge of the upper level ridging which will keep some westerly flow aloft in place. This should allow for a couple of weaker waves to move through. While precipitation is unlikely, some occasional cloud cover and slightly suppressed temperatures will likely keep the more extreme heat and humidity at bay, so heat headlines for both of these days looks unlikely.

A stronger wave moves into the area Thursday and should be the next better chance for showers and thunderstorms. Should see some isolated severe storms with this activity.

Temperatures settle back to near normal Friday into next weekend as northwest flow aloft should bring much more comfortable air into the region with little chance for showers and thunderstorms.

AVIATION /06Z TAFS THROUGH 06Z TUESDAY/

Issued at 1253 AM CDT Mon Jul 27 2026

Shallow fog is beginning to develop as shown on area cameras mainly in eastern South Dakota to start the period. Look for more fog to develop through daybreak mainly near and north of I-90 as temperatures cool to the dew point in our very moist atmosphere under light to calm winds. For the TAF sites, greatest confidence for this to develop was at KFSD as KHON may begin to dry out due to a boundary moving into the area before fog can develop there. Smoke will move into the area behind this boundary and could bring patchy MVFR visibilities to parts of south-central South Dakota this afternoon.

In terms of storm chances through the period, a line of weakening thunderstorms may push into south-central South Dakota this evening, bringing locally 30-40 kt wind gusts with it as it pushes southeastward. Otherwise winds will be fairly light through the period, out of variable directions before sunrise this morning and after sunset this evening, with northeasterly winds sustained between 5 to 12 kts during the daytime hours today.

FSD WATCHES/WARNINGS/ADVISORIES

SD...Heat Advisory until 8 PM CDT this evening for SDZ050-063>071. MN...Heat Advisory until 8 PM CDT this evening for MNZ081-090. IA...Heat Advisory until 8 PM CDT this evening for IAZ001>003- 012>014-020>022-031-032. NE...Heat Advisory until 8 PM CDT this evening for NEZ013-014.