textproduct: Sioux Falls

KEY MESSAGES

- Showers and isolated thunderstorms expected to develop through the afternoon hours and lift northeast. Brief moderate rain may be possible. This activity will persist into the overnight hours.

- Continued conditional and very low chances for isolated strong storms both this evening, but perhaps again towards daybreak Monday. Low risks for marginal hail, a few strong wind gusts possible.

- After rain ends Monday afternoon an extended period of cooler than normal temperatures will continue through the week with only minor rain chances late Wednesday.

- Warmer temperatures return by next weekend with high temperatures around 10 degrees above normal in the 70s.

UPDATE

Issued at 851 PM CDT Sun Apr 26 2026

The forecast for tonight remains on track. An area of steady rain driven by a wave of mid-level vorticity is continuing to lift slowly northeastward. Farther south in its wake, the air is a bit drier, but more scattered activity looks to develop through the rest of this evening as warm air advection continues. A nose of around 250-500 J/kg of elevated instability is starting to creep into the Highway 20 corridor, and will continue to lift towards the I-90 corridor (staying mainly near and east of I-29 as it does).

Near and after midnight, NAM/RAP soundings indicate around 1000-1200 J/kg of CAPE reaching the Highway 20 corridor, while most other guidance keeps this a bit lower towards the 500-750 J/kg range. As this occurs, a secondary push of mid-level vorticity along with a surface low moving northward across eastern Nebraska into southeastern South Dakota will lead to additional showers and isolated thunderstorms developing overnight. Surface based thunderstorms will become increasingly hard to come by over the next couple of hours as low level air begins to warm a touch just above the surface and begins to cap any potential for surface convection. This will mean the threat for damaging winds will be very low from this point forward.

So with elevated convection expected, hail up to the size of a quarter remains the main threat. This will still be a very isolated threat especially for the rest of the evening as tall and thin CAPE profiles indicate weak vertical motion within any thunderstorm, preventing any hailstones from getting too large. As we head into the late night time frame, the aforementioned secondary push of mid level vorticity moving overhead will cool the mid levels a bit and steepen lapse rates to around 7-7.5 C/km. This could better support hail up to the size of quarters especially if instability shown by the RAP/NAM can be realized, but still, this threat remains low.

DISCUSSION

Issued at 1242 PM CDT Sun Apr 26 2026

THIS AFTERNOON: Scattered showers and embedded thunderstorms continue to develop and lift northward this afternoon. Driven by a combination of moderate dPVA and broad warm advection. Elevated instability is slowly crawling northward, but doesn't seem to extend much past I-90. This may limit overall precipitation rates as this activity moves northeast, but with PWAT values near the 90th percentile, pockets of moderate rainfall will still be possible. Portions of NW Iowa have seen a bit more clearing today, but with clouds increasing, that warm-up and low-lvl lapse rate increase may diminish.

THIS EVENING AND OVERNIGHT: Questions still remain on the degree of elevated instability that can advect northward both this evening and then after midnight. Initial wave of vorticity is expected to track into SW Minnesota very early this evening, leaving a bit of a lull in forcing for a few hours. Trailing instability along with some sharpening mid-lvl lapse rates could still lead to the development of a stronger marginal hailer or two south of I-90 through sunset (focused in a sector from Vermillion to Spencer southward), but focus for development remains uncertain. A few CAMs do also suggest the edge of the warm sector approaches Highway 20, which could support a wind gust transporting downward in any stronger storm.

As multiple MCS develop over the Central and Southern Plains as even stronger upper troughing enters the region, we may begin to see a weak sfc low try to develop over northeastern Nebraska with a northward extended inverted trough. Short term guidance (mainly NAM/RAP) show some persistent northward advection of elevated instability (800-1200 J/KG) ahead of this feature along and east of I-29. Other models (GFS/EC) suggest weaker lapse rates with only 300- 600 J/KG MUCAPE. Thoughts are that while the LLJ begins to veer off to the NW, there may be enough convergence combined with dPVA to induce scattered thunderstorms after 2am. An isolated hailer could be possible, but severe weather risks remain on the lower side.

MONDAY: The aforementioned inverted trough will turn towards a cold front early in the day and surge east. Latest CAMs have sped up the boundary just enough to keep any renewed sfc based convection along or just east of the CWA by late morning. Further west, as the upper trough opens up and passes through the Central Plains and toward the Upper MS River Valley, we'll begin to see a deformation area of rain develop in central SD. This precipitation will drag it's foot eastward during the day, potentially dissipating by time it reaches I-29 by mid-afternoon. Additionally, deepening surface low pressure induced a stronger SPG through the Dakotas, and when combined with strong low-lvl cold advection we should see wind gusts approach the 30 to 40 mph range through the day before settling in the evening. Daytime highs may take place early in the day with falling temperatures in the afternoon. Have adjusted the NBM diurnal temperatures trend to account for afternoon clouds and cold advection.

PRECIPITATION TOTALS: Amounts have tapered off slightly over the past 24 hours, potentially indicating less instability further north, but also some redirection of moisture after dark from convection in the Southern Plains. Nevertheless the probability of an additional 1" of rain breaks the 75% mark in areas along and north of I-90. There are also some stripes of 50% probability of 2"+ of rain towards the Madison, Brookings, and Marshall areas in HREF guidance. Average totals based on the 25/75th percentile likely run between 0.50" and 1.5" with pockets of higher numbers.

TUESDAY-THURSDAY: We'll remain in a dominant troughing pattern for much of the remainder of the work week. Long wave troughing centered over Hudson Bay will keep northwesterly flow aloft in the Tri-state area. The result will be temperatures near to slightly below normal at times, along with a low chance (30%) of showers by Wednesday night into Thursday.

FRIDAY-SUNDAY: The good news is that it appears the mid-lvl pattern may break slightly by the end of the week as long wave ridging moves out of the West Coast and towards the Plains by the weekend. Medium range deterministic and ensemble guidance still remains a bit split in this potential, with quite a bit of spread in ensemble guidance. NBM 25/75th spread approaches 10-15 degrees by Sunday, suggesting a range from the mid 60s (should ridge be slower to advect east), to the upper 70s and 80s by Sunday (if ridge move east faster). At this point the NBM mean in the lower 70s seems appropriate.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 653 PM CDT Sun Apr 26 2026

Rain continues to lift northward across the area, bringing MVFR to IFR conditions along with it for the most part. Some lingering VFR ceilings remain primarily in parts of northwest Iowa and southwest Minnesota, but ceilings will be dropping area-wide through tonight, becoming IFR for a good majority of the area by daybreak Monday.

The area of steady rain that's moving through to start the period will move out of the area late this evening, with more scattered showers left behind through the rest of the overnight hours and into Monday morning. A few thunderstorms will also develop with this activity near and east of I-29, with the best chances into northwest Iowa.

Winds will increase through the day Monday as a cold front moves through and turns winds northwesterly. Gusts by the afternoon will be around 30-40 kts across the area, with a few gusts locally up to 45 kts in south-central South Dakota.

FSD WATCHES/WARNINGS/ADVISORIES

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