textproduct: Sioux Falls

KEY MESSAGES

- Scattered rain and drizzle will push east of the area around midnight.

- Fog looks to develop tonight across much of the region. Visibility may drop to 2 miles or less at times along areas of elevation such as Buffalo Ridge, along rivers and water ways, and valleys. Be prepared for rapidly changing visibility conditions.

- Confidence continues to increase in widespread rainfall moving into the area Thursday late evening through Friday. Rain could be heavy at times, and minor impacts to rivers and streams are possible.

- Near normal temperatures persist this weekend and into next week. Any precipitation chances are most likely (30-70%) to remain light and non-severe through the weekend and into next week.

UPDATE

Issued at 823 PM CDT Wed Sep 16 2026

Light rain and drizzle currently located along and east of I-29 will continue to push east into the late evening hours before ending around to shortly after midnight. Winds will become light and variable overnight. Patchy dense fog has already begun to form in areas of northwest Iowa. Patchy fog is expected to expand north through much of the region. Visibility should remain above 4 miles for the most part, but areas along the Buffalo Ridge, rivers, streams, and in valleys may see visibility fall to 2 miles or less at times. No fog headlines at this time, however the trends and visibilities will be monitored throughout the night. Overnight lows will fall to the mid 50s to low 60s.

Fog should burn off by mid-morning Thursday. Clouds gradually break up and become scattered by the afternoon. Should clouds clear enough we have the potential to warm nicely at the surface into the mid-to- upper 70s. Through the day a short amplitude ridge will build over the central US, while at the same time an upper trough and closed low will deepen and dig south over the Pacific Northwest. A band of Theta-e pushes north ahead of a warm front draped through Kansas in the afternoon. This will work to weakly destabilize the environment for much of the CWA. In the evening and overnight a shortwave will work through the pattern. This will work to induce a strengthening LLJ. This jet will work to ramp up WAA in the mid-levels. All of this will work to form a band of frontogenesis in the 850-700 mb layer. This will tap into the elevated instability present and showers and thunderstorms will result. Current guidance has the strongest forcing rotate around the CWA from the western border to the northern border, keeping the heaviest rainfall over central South Dakota and along and north of I-90. However, the band of FGEN will expand south and beneficial rain will fall over most of the region Thursday night into Friday morning.

Early Friday morning the shortwave strengthens and a closed surface low ejects out onto western Nebraska. Warm air and moisture advection increase as well as instability, especially along and east of the I-29 corridor. The surface low accelerates northeast and drags an associated cold front through the region by late Friday afternoon into the evening. Showers and thunderstorms are expected to form along the front. Some of these could be strong to severe producing marginally severe hail and damaging wind gusts. Area most at risk is along and east of Highway 60 in northwest Iowa. This forecast aligns well with the current SPC Day 3 Marginal Risk for severe weather (level 1 of 5). Storms will clear out through the early part of Saturday morning. Highs for Friday will depend on how far north the warm front pushes before it is overtaken by the cold front. For now areas north of I-90 should see highs in the mid 70s, along and south of I-90 will see highs climb into the mid 80s, warmest along the southern Missouri River.

DISCUSSION

Issued at 243 PM CDT Wed Sep 16 2026

We are starting to see sprinkles to very light showers break out across parts of the area this afternoon as warm air advection (WAA) strengthens aloft. With saturation in the low levels but a dry dendritic growth zone (DGZ), sprinkles/drizzle is expected. At the same time, low level stratus not only continues to blanket most of the forecast area but is lowering in height. These conditions will persist through the rest of the afternoon and evening hours before rain chances ending after midnight. Though with lowering ceilings, sprinkles/drizzle looks to persist through the night. At the same time, with sufficient low level moisture in place, ceilings look to lower to potentially down to fog across the area. As of now, think that it is mainly across the higher elevation areas along the Buffalo Ridge in southwest Minnesota, downs through northwest Iowa, and along the Missouri River Valley. With the stratus/fog in place, temperatures will not move much, only lowering to the 50s to low 60s.

Fog along with sprinkles/drizzle will continue into the morning hours before lifting through the morning hours. This should allow for temperatures to warm to the 70s for the afternoon hours. While the afternoon hours will be quiet, things will change Thursday evening and especially for Thursday night as a shortwave trough rounds the western flank of an upper level ridge, parked over the southern CONUS. This wave will strengthen low level WAA and begin to develop showers across central South Dakota Thursday evening. The WAA will be driven by the low level jet (LLJ), tightening the temperature gradient, resulting in frontogenesis (FGEN). This band of FGEN looks to be the main driver for rain as the rain becomes more widespread as the forcing for ascent increases. There also looks to be instability above the front which looks to result in a tighter band of rain. This tighter band of rain continues to look most likely near and north of highway-14. The best forcing for ascent will push off to the northeast of the area Friday morning, resulting in slowly waning rain across the area. Could still see lighter rain persist through the rest of the morning hours. There remains some uncertainty in where this heaviest band sets up. Latest 12z deterministic guidance shows a slight wobble back southwards from the previous 00z guidance. However, the ensembles show similar placement with this band near and north of highway-14. The ensembles mean QPF remains a broad 0.5 inches to up to about 2.0 inches but their 90th percentile QPF also remains up to 2.0 to 3.0+ inches. Think this 90th percentile is representative of the highest amounts that fall within the band itself. The ensembles probabilities do support these totals as they all have a near 100% chance for 0.50 inches or more of rain. The probability for an inch or more is between 40-90% and lowers to a 20-40% chance for 2 inches or more. Have slightly trended total QPF upwards and think the overall rainfall amounts will be a broad 0.5 inch to 2.0 inches of rain across the area. Locally higher amounts up to and exceeding 3.0 inches is possible, mainly under the band itself. This may lead to minor rises in rivers and streams as well all ponding on roadways and poor drainage areas.

A small break in the action is possible for part of the afternoon hours on Friday as some breaks in the clouds occur. This will allow for high temperatures to warm to the low 70s to mid 80s, warmest south of I-90. The pockets of sunshine and diurnal heating may allow for sufficient CAPE values to return northwards, potentially resulting in strong to severe storms. This potential is highly conditional and dependent on how quickly morning showers and cloud cover can exit the area. If a severe weather threat were to materialize, it would be Friday afternoon and evening for locations south of I-90.

Saturday looks to be a dry day across the area with high temperatures near seasonable in the upper 60s to low 70s. The break from the rain will be short lived as another upper level wave looks to eject into the Plains on Sunday. This chance for rain is supported by the ensembles as they show a broad 30-70% chance for rainfall amounts to exceed a tenth of an inch. Good news is that instability is not expected, so this looks to be more stratiform rain rather than convective rain.

Medium range guidance diverges in how the upper level pattern will evolve for next week. There could be additional chances for rain but details are uncertain this far out. Temperatures on the other hand look to remain seasonable in the 60s and 70s.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1233 AM CDT Thu Sep 17 2026

Patchy dense fog is expected to expand throughout most of the region over the next few hours. Current ceilings and visibility run from MVFR down to LIFR. This is expected to remain the case through mid to late Thursday morning before fog finally lifts. Light and variable winds become easterly and slightly breezy by afternoon. Gusts of 15-20 kts are expected for areas along and west of the James River Valley.

After 18.00Z gusty winds will expand east as a surface low and warm front approach from the southwest. Precipitation will begin over western South Dakota and Nebraska and flow northeast into the CWA. Severe weather is not anticipated at this time, however very heavy rainfall is possible at times for areas along and north of I-90. Moderate to heavy rain continues through the end of the period.

FSD WATCHES/WARNINGS/ADVISORIES

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