textproduct: Sioux Falls

KEY MESSAGES

- Showers and thunderstorms will continue tonight through Friday early afternoon. 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.

UPDATE

Issued at 903 PM CDT Thu Sep 17 2026

The forecast remains largely on track this period. Only minor changes to timing of precipitation ending and overall storm track. A weakly unstable environment is moving into place thanks to strong warm air and moisture advection. Aloft the right entrance region of a jet streak and positive vorticity advection add synoptic support as a short wave pushes in from Colorado and Wyoming. A strong LLJ is expected to form over the next few hours and rapidly enhance frontogenic forcing in the 850-700 mb layer. Showers and thunderstorms will continue to form in response through the overnight and into the middle of the day Friday. Severe weather is not anticipated however periods of heavy rain are expected. Soundings indicate a deeply saturated air column with tall and skinny CAPE profiles. ESAT tables indicate that the precipitable water values through early Saturday will be at the maximum for climatology, with values of 1.5 to 2 inches over western South Dakota and southeast into central Iowa. This set up will lead to wide spread accumulating rainfall across the region. The track of the short wave will be slightly crescent shaped as it progresses through the parent upper ridge. As a result, areas south of I-90 will see lower totals of 0.25 to 0.75 inches total. Areas along and north of I-90 will see the highest totals due to the strong frontogenic banding. Here 1 to 2 inches are possible, with a narrow band of 3 inches with isolated pockets of 4+ inches. Overall guidance has shifted the higher band slightly to the north with the heaviest band looking to form from roughly Lyman and Brule counties northeast into Beadle and along Highway 14 into southwest Minnesota and portions of north central Iowa. Due to current drought conditions, widespread flooding is not expected. Some rises in smaller streams and localized ponding is possible. However, under stronger storms where rainfall rates may reach 0.5 to 1.0 inches per hour, minor street flooding to brief flash flooding may occur. As always, do not drive through flooded roads. Turn around, don't drown!!

Rain tapers off by Friday afternoon. Clouds will begin to scatter out but there is uncertainty if they will scatter enough to allow for afternoon environment recovery. By late Friday afternoon the surface low pressure and attendant cold front will move through along the South Dakota and Nebraska border into northwest Iowa. Areas along and south of Highway 18 may see some strong to severe thunderstorms as the front passes through. Confidence in severe storms is low as much of the severe risk hinges on how Friday afternoon plays out. Please see the discussion for more on Friday evening's storm risks.

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 /00Z TAFS THROUGH 00Z SATURDAY/

Issued at 733 PM CDT Thu Sep 17 2026

Outside of convection, MVFR conditions currently then deteriorating to IFR for the rest of the period. Showers to drop ceilings further across the CWA over the next few hours. With the first round of showers expect light precipitation, and isolated lightning.

Temporary improvement may be seen in portions of northwest IA and around KSUX between 5z-7z. Following the periods of scattered showers, thunderstorm potential returns to the entirety of the CWA around 6z, starting in northwest IA/KSUX area and then extending northwestward. Isolated storms cause confidence in exact timing and coverage to be low. Conditions to shift back to just a rain impact after 11z for a majority of the CWA. Precipitation lingers north of I-90 for the rest of the period.

Conditions improve to lower VFR in northwest IA early Friday afternoon. Winds increase with gusts 20-25kts through the overnight and into the afternoon hours Friday. Wind direction may shift late in the period to be more northerly behind a front.

FSD WATCHES/WARNINGS/ADVISORIES

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