textproduct: Sioux Falls

KEY MESSAGES

- Showers and thunderstorms will lift through the area starting tonight. While severe weather is not expected, locally heavy rainfall will be the main threat. Widespread accumulations of 1-2+ inches are expected along and north of I-90. Locally higher amounts are also possible, potentially exceeding 4+ inches. Minor impacts to rivers and streams, ponding in poor drainage areas, and urban street flooding are all possible.

- A low end severe storm threat is possible Friday afternoon and evening from about 4 pm to 9 pm, mainly across portions of northwest Iowa. Damaging winds and an isolated tornado will be the main hazards.

- Near to below average temperatures will persist through this weekend and most of next week. Additional rain chances are possible but details are uncertain.

DISCUSSION

Issued at 303 PM CDT Thu Sep 17 2026

Low level stratus persists across parts of the area this afternoon. Locations that have broken out of the stratus have warmed to the 70s while those who remained under it are only seeing temperatures up to the 60s. The broken up stratus will persist through the rest of the rest of the afternoon hours. The wave that will be responsible for tonight's heavy rain threat is currently located over Wyoming and will encounter a very moist environment, characterized by precipitable water values (PWAT) up to the max of climatology for this time of year. Quasi-geostrophic (QG) forcing for ascent will increase late this evening and night as this wave pushes into the right entrance region of an upper level jet. The low level jet will ramp up with winds as strong as 40 knots, strengthening warm air advection (WAA) in the low levels and tightening the thermal gradient. The tighter thermal gradient will result in a strengthening elevated front and thus frontogenesis (FGEN). This front will be in less stable environment characterized by moist neutral lapse rates. This will result in negative equivalent potential vorticity and thus the potential for a narrower, more intense band of rain. At the same time, weak CAPE values near to less than 500 J/kg will be advecting into the area from the south. Thus, some weak thunderstorms are possible which could result in locally heavier rainfall. The bulk of the rain looks to fall near and north of I-90 with the highest totals expected around highway- 14. A cross section analysis shows deep omega (upward motion) over the front, primarily over areas in the northwest part of the forecast area, generally northwest of Chamberlain, South Dakota to Tracy, Minnesota line. This area looks to see the highest rainfall tonight due to the greatest forcing for ascent residing in this area. The rain will be on the move out Friday morning, especially during the second half of the morning hours. While some lighter rain showers may persist through the rest of the morning and into the early afternoon timeframe, the bulk of the rain will be done falling. In terms of rainfall amounts, a broad 1-2 inches is expected along and north of I-90. Lower amounts down to 0.25 inches to 0.75 inches is expected south of the interstate. There remains the potential for a localized maximum of 4+ inches of rain northwest of the previously mentioned line as supported by the HREF local probability matched mean (LPMM), the individual hi-res members of the HREF, and the RRFS and it's ensemble counter part the REFS.

While the first half of Friday afternoon will be on the quieter side, some questions remain regarding a low end severe weather potential late Friday afternoon into the evening. Surface based instability looks to potentially exceed 2,000 J/kg with sufficient vertical shear magnitudes of about 30 knots. An inverted trough may produce enough low level convergence to potentially develop some storms. The main limiting factor is that most of the better forcing for ascent will not east/northeast of the forecast area. Think that if convection were to develop in this surface trough, it would be initiated by the vort max itself which seems to be displaced a bit north of this boundary. As of now, think convection only has a 30- 50% chance of developing due to the limited forcing for ascent. If convection were to develop, it would develop primarily over northwest Iowa between 4 pm to 9 pm before racing southeast of the area. 0-7km theta-e difference of about 15-20K along with looping low level hodographs suggest a low end damaging wind and an isolated tornado threat IF storms are able to develop.

An active pattern aloft will keep chances for rain in the forecast through the weekend and most of next week. Details remain uncertain due to increasing variance amongst medium range guidance. Temperatures look to remain near to below average for this period of time with highs in the 60s and 70s.

AVIATION /18Z TAFS THROUGH 18Z FRIDAY/

Issued at 1250 PM CDT Thu Sep 17 2026

Low level stratus continues to blanket the majority of the area early this afternoon. The stratus is sitting at MVFR/IFR/LIFR levels with a few locations showing MVFR visibilities. The stratus may break up a bit more through the rest of the daylight hours but overall should persist into this evening. Rain chances may begin to increase late this evening but rain won't reach western portions of the area until around midnight. Rain will then expand eastwards and northwards, overspreading most areas near and north of I-90. Ceilings will lower to IFR/LIFR levels across the entire area with visibilities down to MVFR/IFR levels in falling rain. Some thunderstorms are possible within the broader area of rain, which could produce locally higher rainfall rates and lower ceilings and visibilities. Rain will be pushing northeast of the area during the second half of the morning hours tomorrow to end the TAF period.

FSD WATCHES/WARNINGS/ADVISORIES

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