textproduct: Sioux Falls

KEY MESSAGES

- Dense fog remains most widespread east of Highway 81, with areas of dense fog farther west. A Dense Fog Advisory covers the entire forecast area through 10 AM. Visibility 1/4 mile or less will be prevalent through the morning commute.

- Additional showers will lift through the U.S. Highway-20 corridor in northwestern IA this afternoon/evening. Rainfall amounts mainly less than 0.25 inch are expected, but totals nearing 0.50 inch may push into far southern portions of Ida and possibly Woodbury Counties.

- Drier conditions will temporarily return on Thursday and Friday with the potential for our next rain chances returning by Saturday.

UPDATE

Issued at 220 AM CDT Wed Sep 30 2026

As anticipated by previous shifts, clearing and light winds has allowed dense fog to develop through the southwest to central portions of the forecast area early this morning. Have issued a Dense Fog Advisory mainly east of Highway 281 for now, but satellite hints at potential need for expansion farther west and will continue to monitor.

As far as our rain chances over the next 24-30 hours, 30/00Z models continue to focus band of heavier rain south of our CWA as the remnant moisture from Hurricane Polo is drawn northward ahead of a strengthening upper low swinging east-northeast out of the Four Corners area tonight. We should see some increase in mid-level clouds across our southeast (south of Yankton SD to Spencer IA), but with the 850mb front projected to remain near to south of a Sioux City to Storm Lake line, northward push of more significant moisture will be limited by a dry low level northerly flow north of the boundary. This could result in our far southeast counties being near the northern edge of a very sharp rainfall gradient, with additional rainfall potentially ranging from only a tenth or two near Sioux City to Storm Lake, to an inch or more only a county or two south. With relatively good run-to-run consistency in this southern solution over the past 24-30 hours of model runs, confidence has increased that this system no longer poses a risk of flooding in northwest Iowa, and the Flood Watch has been cancelled.

DISCUSSION

Issued at 238 PM CDT Tue Sep 29 2026

Regional radar as of 29.1930Z shows the majority of warm advective rain showers have moved into areas south of U.S. Hwy 18 across southeast SD and northwest IA. These showers will continue to move eastward into the late afternoon/early evening with most areas drying out by 00Z. That said, some CAMs show scattered convective activity re-developing along a trailing surface boundary/front late this afternoon and evening mainly along and south of a Yankton to Sioux Falls to Windom line. Area soundings show some weak instability, so will keep chance POPs/isolated thunder into the evening across far southeast SD into northwest IA. Any lingering activity will diminish overnight, although clouds will be slow to move out. Some clearing across southeast SD will allow lows to drop into the low to mid 50s, whereas temperatures will bottom out closer to 60 degrees for areas under cloud cover across northwest IA.

In addition, will need to watch for fog overnight. Given weak near surface flow and ample boundary layer moisture from rain today, could see rapid fog development if the broader cloud shield dissipates or moves far enough east.

For Wednesday, a strong Four Corners closed low will begin to eject east-northeastward, and with it will be remnant moisture from Hurricane Polo. This is a very wet system for late September with NAEFS PWATS above the 99th percentile. However, our southernmost forecast area will be on the northern periphery of the highest moisture as the surface low moves across the Central Plains and the stationary front sets up across NE/IA. 00/06Z model runs last night and now the 12Z suite this morning have trended south with the highest QPF. There is likely to be a sharp gradient in rainfall, ranging from amounts less than an inch to higher than 2-3 inches. Areas north of U.S. Hwy 18 will likely remain dry all day. Current trends suggest our forecast area will be on the lower end, but given some lingering concern about a late model shift north, will keep current Flood Watch in place and adjust as needed on later shifts. Otherwise, Wednesday will be warm and mostly cloudy with high temperatures in the low to mid 70s.

Some lingering showers are possible Thursday across northwest IA, but the majority of the area will be dry with much needed dry conditions continuing into Friday as surface high pressure moves across the region. A cold frontal passage Wednesday night will also usher in cooler air, keeping high temperatures Thurs/Fri mainly in the 60s. As long as skies remain mostly clear, overnight lows will dip into the mid to upper 40s.

Another weak wave moves through northwest flow aloft on Saturday, bringing a 20-40 percent chance of showers to areas mainly north of I-90. Synoptic 500 hPa flow begins to diverge between the major models Sunday into early next week, but the consensus now is for dry conditions with seasonable early October temperatures.

AVIATION /12Z TAFS THROUGH 12Z THURSDAY/

Issued at 645 AM CDT Wed Sep 30 2026

Widespread LIFR-VLIFR conditions, especially near and east of US Highway 81, as we start this period. Visibility is expected to improve to MVFR by 14Z-15Z with widespread VFR conditions across most areas by 18Z. Exception will be in our far southeast, KSUX to KSLB, where MVFR ceilings may linger as rain spreads north out of Nebraska and central Iowa this afternoon and evening.

Winds remain light and variable through 01/06Z, with a light northerly flow becoming more prevalent after 06Z.

FSD WATCHES/WARNINGS/ADVISORIES

SD...Dense Fog Advisory until 10 AM CDT this morning for SDZ038>040- 050-052>071. MN...Dense Fog Advisory until 10 AM CDT this morning for MNZ071-072- 080-081-089-090-097-098. IA...Dense Fog Advisory until 10 AM CDT this morning for IAZ001>003- 012>014-020>022-031-032. NE...Dense Fog Advisory until 10 AM CDT this morning for NEZ013-014.