textproduct: Sioux Falls

KEY MESSAGES

- Isolated to scattered thunderstorms possible during the late afternoon through early overnight hours (6pm-12am) mainly south of I-90 with the primary risks of up to Ping-Pong ball size hail (1.5") and 60 mph winds.

- Temperatures remain 10+ degrees above normal into mid-week, but expect a slight cooldown back to normals by next weekend. - Rain risks return Friday into next weekend (20-40%), however severe weather chances are very low.

UPDATE

Issued at 358 AM CDT Mon May 25 2026

Now that most of our lingering activity has pushed into central IA, quieter conditions have returned to the forecast area. Looking into Memorial Day, most areas will wake up to a warmer start with temperatures mainly in the low to upper 50s. As mixing resumes, southerly to southeasterly winds and lingering warm air advection (WAA) aloft will help temperatures quickly climb into the upper 80s to low 90s by early afternoon. Similar to yesterday, an influx of warm and moist air will help dew points climb into the upper 50s to low 60s with the highest dew points focused across northwestern IA. This combined with some weak lift from an approaching wave and a strengthening LLJ will lead to isolated to scattered showers and thunderstorms developing along the Missouri River Valley from 6pm- 12am.

With the wave riding the mid-level ridge into our area, should see most of this activity gradually lift northeastwards throughout the evening towards the I-90 corridor before gradually weakening. Focusing on the severe component, once diurnal heating erodes the cap (warmer air aloft); developing thunderstorms will have access to about 1500-2500 J/kg of CAPE. However, with meager shear (20-25 kts) values; not expecting any long-term development like yesterday. If anything developing updrafts will be very pulsy. Nonetheless, steep mid-level lapse rates between 7-8 degrees C/km and the potential for storms to become more cold pool dominant will foster an environment for large hail up to Ping-Pong Balls (1.5") and up to 60 mph wind gusts which are the primary hazards with any stronger activity. Lastly, the CAMs are starting to show additional potential for "Popcorn" showers by daybreak on Tuesday morning with the nocturnal LLJ. Ended up adjusting our POPs to reflect the scattered potential.

DISCUSSION

Issued at 1211 PM CDT Sun May 24 2026

THIS AFTERNOON/EVENING: Temperatures continue to rise towards and into the 80s this afternoon. Initial mid-lvl vorticity tracking across South Dakota this morning will keep a narrow channel AC focused along the Ridge and Minnesota River valley early this afternoon. Increasing southwesterly surface flow will continue to pull modest boundary layer moisture northward, but some of the high-res CAMs are overdoing current moisture at mid-day. Nevertheless, we should eventually see more consistent mid/upr-50 dew points arrive by late afternoon. The arrival of slightly enhanced mid-lvl lapse rates will push MLCAPE values towards the 1500-2200 J/KG layer by late afternoon. The main question will then become "where does initiation occur?". Some preference today for a weak area of surface convergence and low- lvl jet convergence towards the Highway 14 corridor and into the Minnesota River valley after 4pm. With inhibition weakened further south towards the MO River, additional isolated/scattered development could take place by early evening further west along a weak dry line near the James River valley, but this development is much less certain and more conditional based on weaker convergence. It's the arrival of mid-lvl vorticity in the 00-03Z window that may help development further develop and linger into the early overnight hours before low- lvl flow veers and pushes convection eastward.

Regarding hazards, the increase in effective shear towards the 30-40 knot mark support a few supercells, with analogs suggesting 1.5-2" hail potential. A few wind gusts to 60-70 mph may be possible given steep low-lvl lapse rates, but only modest DCAPE. The tornado risk remains very low. The west-east slow storm movement through the Highway 14 corridor this evening could lead lead to minor flash flood risks with HREF PMM data suggesting 1-2" per hour rates. Storm risks lower quickly after midnight with a quiet remaining overnight forecast. MEMORIAL DAY: Even warmer temperatures are expected for Memorial day as winds remain southwesterly. Highs are expected to rise into the upper 80s to lower 90s. One thing to note will be a rise in afternoon dewpoints towards the lower 60s that may push heat index values into the 90s. Will also need to monitor the potential for isolated convection to form Monday evening across portions of north central/northwestern Iowa into northeastern Nebraska, given the slightly dirty mid-lvl flow remaining in the Plains. Several CAMS (more NAM based), all suggest low-end development by late afternoon to early evening, potentially driven by weak low-lvl convergence on the developing LLJ. Shear remains rather weak but instability and mid-lvl lapse rates of more than sufficient. Could see a few stronger storms with localized downburst winds and severe hail if trigger mechanism can be determined. This activity may quickly weaken after sunset, but continue to drift north into the overnight hours.

TUESDAY-THURSDAY: The middle of the week favors dry and mild conditions as a mid-lvl ridge builds through the Plains and extends northward into Central Canada. Residing under the ridge of this quasi-blocking pattern would suggest very weak flow aloft, a slight influence from east southeasterly surface winds, and dry conditions with afternoon CU.

FRIDAY-SUNDAY: A broad mid-lvl ridge over the CONUS remains in place through next weekend. However, we begin to see eastward shifts in the the dominant surface high over the Great Lakes, allowing a better channel of low-lvl moisture to move through the Mid/Upr MO river valley. What develops is a nearly daily low (20-40%) risk for shower and thunderstorms, mostly within the peak diurnal timeframe. Shear and instability in this pattern will be quite weak, so non- severe thunderstorms are favored. Temperatures are also favored to drop closer to seasonal average in the middle to upper 70s.

AVIATION /06Z TAFS THROUGH 06Z TUESDAY/

Issued at 112 AM CDT Mon May 25 2026

Mainly VFR conditions with occasional MVFR to IFR vsby will persist this TAF period. Taking a look at radar, we're continuing to see scattered showers and thunderstorms around northwestern IA this morning. While these developing storms could persist over the next few hours, should this activity move out of our area by 09-10z. Nonetheless, this should affect any of our TAF sites. Otherwise, light southerly winds will become more southeasterly into Monday with marginally breezy conditions possible. There is some moderate confidence in a return of showers and thunderstorms by the evening hours so added in a few PROB30 groups at KFSD & KSUX. Lastly, could see a few hours of LLWS through Monday morning to end the TAF period.

FSD WATCHES/WARNINGS/ADVISORIES

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