textproduct: Sioux Falls

KEY MESSAGES

- Areas of fog remain possible tonight. Locally dense patches could lead to visibilities as low as a half mile at times.

- Confidence continues to increase in areas of drizzle and potentially light showers developing east of I-29. Any resulting accumulations will be light.

- After a brief break late Sunday into Monday, rain chances return Monday night into Tuesday. Pockets of heavier rainfall will again be the primary concern.

UPDATE

Issued at 905 PM CDT Sat Sep 26 2026

Taking a look across the area, a broken line of showers continues to lift northeastwards across areas east of I-29. While some light accumulations will be possible, should see most of this activity lift out of our areas over the next few hours. From here, the main focus will then shift towards the fog and drizzle potential. Similar to the past few days, lower ceilings and lingering low-level moisture could lead to pockets of "insta-drizzle" mainly along and east of I-29 with lingering lift around from a weak mid-level wave. While some short-range guidance is also hinting at some additional light showers mixing in at times late tonight; my thoughts are this will just moisten the boundary layer enough to get some moderate pockets of drizzle instead.

With this in mind, started to blend in some of the GFS1hr to the forecast to swap things over the drizzle instead of light rain showers. Otherwise, with smaller dew point depression in play; areas of fog will also be possible for any areas that don't receive drizzle. Locally dense patches could lead to visibilities of a 1/2 mile or lower at times. With this in mind, we'll have to continue monitoring surface observations overnight as a dense fog advisory will likely be needed for parts of the area. However, exactly where remains the biggest question. Looking ahead, depending on how quick the stratus and fog clears Sunday morning; surface temperatures might need to be tapered back a bit since we likely won't get as much diurnal heating.

DISCUSSION

Issued at 257 PM CDT Sat Sep 26 2026

The area of steady moderate rain that impacted the area this morning is exiting off to our northeast; however, patchy drizzle continues and will continue on and off through the afternoon where clouds ceilings are lowest, which is generally from far eastern South Dakota into southwest Minnesota. Farther west, we've seen some breaks in the lowest clouds, allowing for some destabilization to occur. Up to 500-1,000 J/kg of MLCAPE has developed per latest SPC Mesoanalysis along the Missouri River Valley, and as a secondary mid level wave moves across the area, a few weak thunderstorms have already developed over northern Nebraska and are lifting northward. Severe weather is not expected given weak shear across the area, but we will likely continue to see some additional showers and isolated thunderstorms develop through the afternoon and evening. The best chance of rain (30-60%) with this activity will be over western Iowa, far northeast Nebraska, and far southeast South Dakota, stretching to far southern Minnesota by the late night hours. Tall, skinny CAPE profiles and deep warm cloud layers suggest heavy rain will be the primary threat with any thunderstorm. This could lead to a few pockets of ponding in low- lying and poor drainage areas, but flash flooding is unlikely.

Heading into tonight, the areas that saw some clearing stratus (primarily near and west of the James River) will see some fog develop, meanwhile the stratus deck elsewhere is expected to lower through the night and become fog. Locally dense fog is likely given all the near surface moisture, though coverage and exact locations where the densest fog will reside precludes a need for a Dense Fog Advisory at this time, but trends will be monitored. This fog will gradually lift through the mid morning hours, but likely become low stratus heading into much of the day on Sunday. Still the potential for some of this stratus to mix out a bit and allow for brief breaks of sun mainly near and west of the James River, but this will likely be the exception, not the norm across the area. Some instability in the low cloud layer may allow for a few sprinkles to spits of drizzle, but it's not looking as persistent as it has been the past couple of days.

Heading into early next week, a trough over the West Coast will begin to dig into the Desert Southwest. Ahead of this, an upper wave will move into the area late Monday into the day on Tuesday and bring our next round of rain and storms to the area. Latest GEFS shows around a 30-40% chance of at least an inch of rain mainly focused over northwest Iowa and adjacent areas. This is a bit higher than other guidance, but appears to be the most reasonable at this time given ensemble PWATs show anomalies nearly of +1" in place as this system moves through. The aforementioned trough over the Desert Southwest will absorb the remnants of a Pacific Ocean tropical cyclone as it ejects to the north and east during the middle to end of the next work. This may help to keep a decent amount of moisture in place over primarily northwestern Iowa into the end of the upcoming work week, though at this point it looks like the heaviest rain associated with this will stay south of the area.

AVIATION /00Z TAFS THROUGH 00Z MONDAY/

Issued at 638 PM CDT Sat Sep 26 2026

Mainly MVFR to IFR conditions and lower will continue this TAF period. Continuing to watch the showers across northwestern IA this evening. Should see this activity continue to lift northeast over the next few hours before exiting our area. Otherwise, should see light and variable winds return overnight as ceilings gradually lower. Widespread IFR to LIFR conditions are expected through mid-morning on Sunday with areas of drizzle and fog developing. Current thinking is the drizzle will be mostly confined to areas along and east of I-29 so added -DZ to KFSD and KHON through the morning hours. Lastly, light and variable winds will gradually become more easterly by Sunday afternoon.

FSD WATCHES/WARNINGS/ADVISORIES

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