textproduct: Sioux Falls

KEY MESSAGES

- Showers and very isolated thunderstorms will spread north late tonight. Moderate to briefly heavy rain is expected on Friday. The better chance for a few thunderstorms will be on Friday afternoon and evening near and east of I-29.

- Severe weather is not expected with this system. Some locally heavy rain is possible with guidance suggesting about a 10-20 percent chance for 2+" of rain near and east of I-29, likely in a skinny band or two.

- Below normal temperatures will continue through Saturday before warmer conditions return from Sunday onwards.

UPDATE

Issued at 903 PM CDT Thu May 21 2026

The latest water vapor imagery shows a compact wave in eastern WY moving into western NE and western SD. Ahead of this wave southeasterly flow will increase, bringing moisture into the area later tonight. Overall, soundings are nearly moist neutral, leaning towards just a touch of weak instability through the morning, generally less than 500 J/kg CAPE. This suggests a few slightly stronger updrafts to go with the very strong forcing within the incoming PV anomoly and dry slot punching into eastern SD. Right now this lift appears to last about 6-12 hours with the most persistent lift near and 20-30 miles or so on either side of a line from Yankton to Sioux Falls to Marshall. If this weak instability does come to fruition, some locally higher amounts of 1.5"-2" will be likely, although not widespread. Many locations should see about a half an inch of rain with this system.

One thing to keep an eye on will be mid to late Friday afternoon as this system wraps up to the north and dry/cold front moves east through eastern SD. While a very low probability, isolated landspouts may occur along this boundary with some shallow 500-800 J/kg CAPE near and ahead of this front. Generally you would want some fairly light winds across the frontal boundary and we may not see this, but will monitor trends of the winds along this boundary. Higher lapse rates in the lowest couple of km are also fairly shallow.

DISCUSSION

Issued at 230 PM CDT Thu May 21 2026

Scattered clouds will increase late this afternoon ahead of an approaching upper trough and embedded shortwave. Good mixing will allow southeast winds to tap into a modest LLJ. Resultant afternoon winds will be breezy gusting 20-25 mph. Winds decrease slightly after sunset but remain on the breezy side overnight gusting to 20 mph. Highs today will be cooler, in the 60s and 70s. Overnight lows look to fall to 40s. Chances for showers increase late this afternoon and evening as the previously mentioned short wave enters south central South Dakota. Isolated to scattered showers will gradually flow in from the south, expanding in coverage across most of the region by sunrise. Severe weather is not expected.

Beginning very early Friday morning plentiful moisture will advect north thanks to a weak theta e ridge and pool ahead of an approaching cold front. Despite the push of warm and moist air, instability remains low, generally less than 300 J/kg. However, this is enough for a few rumbles of thunder. By Friday mid- morning a region of enhanced vorticity enters central South Dakota and WAA and the LLJ ramp up. The increased forcing will result in scattered showers becoming more numerous and broader in areal coverage. Lightning risks increase as well. Periods of heavier rainfall may occur, up to around 0.25 inch per hour. As the cold front continues to progress east Friday afternoon a region of strong vorticity advection associated with the surface low will work to focus storms into a more organized convective line. A few stronger storms may produce frequent lightning and wind gusts up to 40 mph, but severe weather is not expected. Brief heavy down pours are possible under any of the stronger storms that form. The more organized line looks to set up just to the west of I-29 and progress eastward between 2 and 10 pm CDT. Storms should push east of the region shortly after midnight. Additional light showers are possible as moisture wraps around the low, mainly along and north of Highway 14 Saturday morning. Very little additional accumulation is expected. HREF LPMM indicates rainfall totals of a quarter of an inch up to 1 inch for the majority of the region. Portions of northeast Nebraska and northwest Iowa could possibly see 1.25 inches plus. Guidance has in general been trending totals up the past few runs, if this trend continues some areas may even see up to 2 inches.

Winds continue to increase through Friday afternoon as the SPG tightens further. Peak winds are expected in the late morning to early afternoon. Widespread gusts of 25-30 mph are expected with occasional gusts to 35 mph possible. Winds will decrease after sunset, becoming light overnight. Highs for Friday will be cool in the 50s to low 60s. Lows will be in the 40s.

Saturday, mostly cloudy skies will become scattered through the day. Winds will be light and highs will be slightly warmer in the upper 60s to mid-70s. A few isolated light showers are possible in the very early hours Sunday morning as an upper wave axis crosses through the region. Very little accumulation is expected. Showers end shortly after sunrise. The rest of Sunday will be beautiful day with mostly clear skies and highs in the upper 70s to low 80s. Afternoon southerly winds will be on the breezy side, gusting around 20 to 25 mph.

Late Sunday night into Monday morning another round of showers and thunderstorms are possible as an upper wave and WAA make their way through the region. While convective parameters are generally good, confidence in strong storms is low due to a wide variance in guidance. Showers should clear out Monday late morning. Highs will be in the 80s and 90s with breezy afternoon southerly winds. From Tuesday onward an upper ridge will build over the region. Dry conditions are expected to prevail with highs in the 80s. By Thursday night a more progressive upper wave may move through the region. But details are uncertain at this time.

AVIATION /00Z TAFS THROUGH 00Z SATURDAY/

Issued at 625 PM CDT Thu May 21 2026

VFR conditions will persist through about 6z, possibly 9-12z for areas north of I-90. After this showers and very isolated thunderstorms with deteriorating ceilings will spread north, likely to a widespread IFR or low end MVFR. These lower categories will persist through about the end of the TAF period. Rainfall will be moderate to briefly heavy at times. A brief window form mid to late afternoon near and east of I-29 could see a short period of thunderstorms as the system begins to move out of the area.

FSD WATCHES/WARNINGS/ADVISORIES

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