textproduct: Sioux Falls

KEY MESSAGES

- Showers and a few thunderstorms will continue into mid-late morning. Brief heavy rainfall is the greatest threat, with risks of ponding in poor drainage areas, and isolated urban street flooding.

- A low end severe storm threat is possible late this afternoon and evening from about 4 pm to 9 pm in portions of northwest Iowa. Damaging winds and perhaps an isolated tornado will be the main hazards if storms can develop.

- Near to below average temperatures will persist through this weekend and most of next week. Additional rain chances will continue into early next week, but thunder chances are low and severe weather is not expected.

UPDATE

Issued at 500 AM CDT Fri Sep 18 2026

Rain and a few embedded thunderstorms continues to move slowly east-northeast across the area early this morning, with pockets of heavier rain north through east of Sioux Falls (especially Worthington to Storm Lake) as of 445 AM CDT. Seeing some minor rises on a few area creeks/rivers, but other than some ponding in poor drainage/urban areas, do not anticipate any significant flooding issues this morning.

Main question today is how far north any clearing will develop ahead of an advancing wave/surface front, and whether this will allow for stronger thunderstorm development by late afternoon. Modest consensus in a sharp thermal boundary developing from the Missouri River Valley into northwest Iowa, though exact location is uncertain. This boundary could become a focus for a stronger storm or two as a mid-level wave passes by, though stronger mid-level forcing and associated deep layer shear appear to be focused north of I-90 which leads to lower confidence in severe potential. Nevertheless, those with outdoor plans later today into this evening will want to closely monitor the forecast and have a plan of action should storms develop.

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 /12Z TAFS THROUGH 12Z SATURDAY/

Issued at 640 AM CDT Fri Sep 18 2026

IFR-LIFR ceilings will remain prevalent through much of this forecast period with a couple of exceptions. 1: Locations from the Missouri River Valley into northwest Iowa south of Hwy 18 are most likely to see the stratus clear out this afternoon into this evening, and 2: Ceilings farther north may briefly lift into the lower MVFR range for a few hours this afternoon before quickly lowering again after sunset.

The lowest ceilings/greater potential for MVFR-IFR visibility in fog will be found along the higher elevations of southwest Minnesota after sunset tonight.

FSD WATCHES/WARNINGS/ADVISORIES

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