textproduct: Sioux Falls

KEY MESSAGES

- Afternoon thunderstorms will primarily pose a heavy rain risk, with localized rates of 2-3" per hour possible. A brief window for gusty winds, small hail, and even an isolated funnel over NW IA and SW MN is possible this afternoon.

- A front will bring a risk for isolated to scattered severe thunderstorms Monday afternoon and evening. If storms develop, risk of 2" hail and 60 mph winds will be possible.

- An active weather pattern will keep intermittent thunderstorm chances in the forecast from Tuesday to Thursday. With a conditional risk for severe storms Tuesday afternoon and evening.

DISCUSSION

Issued at 124 PM CDT Sat Aug 15 2026

THIS AFTERNOON: Mid-lvl vorticity continues to lift northeast through the Tri-state area early this afternoon. Scattered showers and thunderstorms tied to this wave will continue to slide east northeast into the afternoon hours. Most of the strongest activity has been tied to the NW quadrant of the wave, aligned with an approaching cool front moving in from the northwest. Some of the convection has produced rainfall rates greater than 3-4" per hour, with radar estimates suggesting 2-4" of rain has fallen in isolated pockets in Miner county. Convection risks will remain focused mostly on heavy rain this afternoon, though if sufficient clearing can develop in SW Minnesota and NW Iowa, then a small risk of 50 mph winds, small hail, and also brief funnels could be possible given the weak and chaotic surface flow. The heavy rain risk will persist in any thunderstorm given deep warm cloud layer, and PWATs above the 90th percentile of climatology.

TONIGHT: The front clears the CWA by late evening, with skies briefly clearing. Those skies may only stay clear for a short duration of time as increasing mid-uper forcing could bring high lvl clouds back into the region. If these clouds don't develop, then overnight lows could fall into the upper 50s to lower 60s, inducing some fog potential.

SUNDAY: A dry day is anticipated for Sunday as winds remain northerly. Temperatures rise into the lower 80s in most locations, with a bit of mid-high lvl cloud cover in the region through the day and diurnal CU in the afternoon.

MONDAY: Northwesterly flow will persist into the new work week and is expected to bring a frontal boundary into the Dakotas on Monday. An initial surge of warm advection Sunday night into Monday could bring scattered showers and thunderstorms eastward out of central South Dakota/Nebraska by daybreak. This activity may slowly fade through the morning allowing stronger diurnal heating to occur. The frontal boundary arriving in the afternoon will then be watched for convection risks. Should convergence along this boundary be strong enough to overcome lingering inhibition, isolated to scattered storms may develop by 3-5pm as soundings show a relatively uncapped environment through eastern SODAK. Upwards of 2000-2500 J/KG MLCAPE ahead of the boundary, may combine with modest effective shear approaching 35 knots to help produce a few severe storms with 2" hail and 60 mph wind potential. The overall coverage of storms remain most uncertain given unknown impacts of morning clouds.

TUESDAY: Monday's convection may dictate the placement of the frontal boundary by Tuesday. Some guidance pushes the front and boundary layer moisture south of the CWA, with other models holding the effective boundary closer to I-90. The NW flow will likely bring another wave into the Plains in the afternoon, which could trigger isolated to scattered strong late afternoon and evening storms once again.

WEDNESDAY-FRIDAY: With the mid-lvl ridge centered back over the Four Corners region we'll continue to see northwesterly flow aloft through the Plains for the second half of the week. This problematic flow pattern will keep a rain risk into the region Wednesday and potentially Thursday, before the ridge sharpens and pushes more stable air into the Plains into next weekend. Expect lower than typical confidence in specific forecast details during the middle of the week. Temperatures through the second half of the week will run near to slightly below their seasonal normals which are now in the lower 80s.

AVIATION /18Z TAFS THROUGH 18Z SUNDAY/

Issued at 1211 PM CDT Sat Aug 15 2026

IFR to MVFR ceilings continue to move eastward, tied to a disturbance aloft that is moving northeast. A frontal boundary continues to move towards the James River valley region and will turn winds to the west northwest later this afternoon and evening.

Scattered showers and thunderstorms will move eastward into early evening, with ceilings rising to VFR levels in most areas. This activity may not be widespread as we get deeper into the afternoon hours, with most of the focus east of I-29 by 21Z.

Overnight, skies may briefly clear, but mid and upper clouds will spread northeastward after dark. With light winds, can't rule out patchy fog, but feel the increased upper clouds should prevent any rapid temperature drops with fog contained to valley areas.

FSD WATCHES/WARNINGS/ADVISORIES

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