textproduct: Sioux Falls

KEY MESSAGES

- Avoid burning Wednesday. Any fire will be able to spread rapidly given critical fire weather conditions. A Red Flag Warning is in effect for much of the area from 12 pm to 8 pm CDT.

- Scattered thunderstorms will move across the region Wednesday night through Thursday with the passage of a strong cold front. Some storms may become strong to severe, capable of producing large hail and damaging winds. There is a Level 2 of 5 risk for severe thunderstorms Thursday afternoon and evening for areas along and east of I-29.

- A return to more seasonable temperatures Friday into the weekend with highs in the mid 50s to lower 60s. While not a wash out, there is a 30 to 60 percent chance for showers and thunderstorms during this period.

DISCUSSION

Issued at 148 PM CDT Tue Apr 21 2026

Wednesday will be hot, dry, and windy as a strong 500 hPa ridge moves overhead. The 12Z HREF Grand Ensemble is showing a 60+ percent chance of high temperatures greater than 85 degrees across most of SD and parts of southwest MN with similar probabilities of 90 degrees or higher along and west of the James River Valley. Some locations could approach record values for the day. Afternoon southerly wind gusts will reach 40 to 45 mph given deep boundary layer mixing. This heat and wind, combined with very dry fuels and low humidity will result in critical fire weather conditions. See Fire Weather discussion for additional details concerning the newly issued Red Flag Warning.

Attention then shifts to a large closed 500 hPa low centered over the Northern Rockies Wednesday night through Thursday. A strong leading short-wave is progged to lift northeastward across the Northern Plains during this time. Forcing aloft, combined with an eastward advancing surface cold front/dry line will provide the focus for thunderstorm initiation. Ahead of the surface boundary, dewpoints will rise into the upper 50s and lower 60s, resulting in MUCAPE up to 1500 J/kg.

Some thunderstorm activity is possible Wednesday night west of I-29 with the approaching boundary, but the severe threat is lower. Main concern is for areas along and east of I-29 Thursday afternoon/evening. While this area is highlighted by a SPC Day 3 Level 2 of 5 severe weather risk, much will depend on the longevity of a strong 700 hPa thermal cap, widespread low cloud cover ahead of the front, and the timing of the front itself. Right now, the best chances for severe storms exist across southwest MN and northwest IA from 4pm into the early evening. The primary hazards would be hail to the size of quarters and damaging wind gusts to 60 mph.

Passage of the cold front Thursday night will usher in a more seasonable airmass Friday into the weekend with highs from the mid 50s to lower 60s. Aforementioned 500 hPa low continues to wobble across southern Canada into early next week, allowing other short-waves to traverse the Northern Plains. Predictability in the strength and timing of these waves is low, so NBM chance POPs seem reasonable at this time.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1232 PM CDT Tue Apr 21 2026

VFR conditions will be the rule through 22.18Z with the primary impact to aviation operations being changing wind direction and speed. Outside of some cirrus, skies will be mostly clear/sunny. Light north-northeast wind this afternoon will gradually turn to the east-southeast this evening and overnight. By Wednesday morning, expect winds to quickly shift to the south and increase with gusts 25 kts or higher by the end of the forecast period. Looking ahead to Wednesday afternoon, southerly winds will strengthen further with gusts over 30 kts possible.

FIRE WEATHER

Issued at 148 PM CDT Tue Apr 21 2026

Hot, dry, and windy on Wednesday. Temperatures will soar into the upper 80s to mid 90s with afternoon minimum relative humidity bottoming out between 15 and 20 percent. The warmest and driest areas will be along and west of I-29. Model soundings show a deep boundary layer with around 35+ kts to mix to the surface by mid-afternoon. This will result in widespread southerly wind gusts from 40 to 45 mph. Putting this together, most of the forecast area is within the Hot Dry Windy Index (HDWI) exceeding the 90-95th percentile. As a result, issued a Red Flag Warning for the entire forecast area from 1200 pm CDT to 800 pm CDT Wednesday. The exception being areas along and southeast of a Spirit Lake to Sioux City IA line where minimum relative humidity is expected to bottom out between 25 and 30 percent. This area will need to be watched for a possible inclusion in the Red Flag Warning. Burning should be avoided on Wednesday as any fire will be able to spread rapidly.

FSD WATCHES/WARNINGS/ADVISORIES

SD...Red Flag Warning from noon to 8 PM CDT Wednesday for SDZ038>040-050-052>071. MN...Red Flag Warning from noon to 8 PM CDT Wednesday for MNZ071- 072-080-081-089-090-097-098. IA...Red Flag Warning from noon to 8 PM CDT Wednesday for IAZ001- 002-012. NE...Red Flag Warning from noon to 8 PM CDT Wednesday for NEZ013- 014.