textproduct: Sioux Falls

KEY MESSAGES

- Patchy fog, possibly locally dense, continues through mid morning. Visibility may drop below a mile at times.

- Low chances (less than 20%) for showers near and north of US Highway 14 this afternoon and evening. Low chances for thunderstorms in the late afternoon and into tonight along and south of the Missouri River. Severe weather risk is low.

- Slightly cooler weather for the weekend. Saturday should remain dry. Rain chances (40-70%) return to western portions of the area by Sunday morning, expanding east through Sunday night.

- Additional chances for rain, and potentially thunderstorms (10-20%), are possible through the first half of next week. Details are uncertain, so continue to check the latest forecast for the most up to date information.

UPDATE

Issued at 321 AM CDT Fri Aug 21 2026

Light winds, clear skies, and temperatures/dew points in the 60s are leading to patchy fog development once again early this morning. A few CAMs and ensembles are showing the potential for dense fog, so will keep an eye on potential for headlines if this develops and becomes widespread.

Most of today should be dry with temperatures warming into the 80s. Showers/sprinkles are possible later today north of US Hwy 14 with a weak wave, although chances are low (less than 15%) due to dry air. Further south to the MO River Valley into NE, a stronger wave is progged to move through this afternoon and this evening, which could lead to some isolated storms. Will also have to watch for where convection this afternoon/evening in western SD tracks tonight as this may also graze the MO River Valley. Although we do have some instability, shear, and modest mid level lapse rates, soundings show a fairly robust cap which should limit convection. Additionally, better dynamics and boundary lie across NE, further reducing convective potential and minimizing potential severe risk. As has been the trend with this continued northwesterly flow regime, CAMs are showing little agreement in convection today/tonight.

Pleasant day expected Saturday, with rain chances returning to the area Sunday into Monday.

DISCUSSION

Issued at 210 PM CDT Thu Aug 20 2026

A light shower or two persists across southwest Minnesota this afternoon. These showers remain ahead of a cold front that is just pushing into the forecast area. This front will serve as the focal point for potential severe storm development later this afternoon. The main shortwave is just north and east of the forecast area. However, the upper level jet will be nosing its exit region into the southwestern half of the forecast area. This presents a bit of a displacement in where the better shear and forcing will lie. Some capping remains across the area but the weakest amount of capping looks to be just along the front itself. Think that 1 to 4 storms is possible this afternoon and evening IF storms are able to develop. IF storms develop, more than sufficient instability will be in place with CAPE values up to 2,000 J/kg to 2,500 J/kg as moisture pools ahead of the front. With part of the upper level jet overhead, vertical shear will also be sufficient with deep layer shear values between 40 to 50 knots. DCAPE values will be highest along and just behind the front with magnitudes up to 1,200 J/kg. Thus, Large hail to 2+ inches and damaging winds to 50-70 mph will be the main hazards. A low 2% chance exists for a tornado as the low level jet begins to strengthen this evening, slightly expanding low level hodographs and increasing 0-3km storm relative helicity (SRH) values up to 100+ m2/s2. Any chance for severe storms for be waning through the evening hours as nocturnal stabilization increases capping. The previously mentioned cold front will stall out over the forecast area tonight, leaving muggy conditions in place. With light winds expected, patchy fog is once again possible.

Highs will remain in the 80s across the area on Friday. The previously mentioned cold front will push through the forecast area and end up right along the Missouri River. This could result in a few storms developing on this boundary before quickly pushing off to the south and east. However, latest LREF probabilities remain low (20-30%) for a tenth of an inch of rain or more. The HREF shows little to no probabilities for any precipitation. This is in line with the latest hi-res guidance as the bulk of this guidance shows storms developing south of the surface front in Nebraska. Will keep an eye on this potential in case storms trend back north, but for now, it looks most likely that the forecast area will remain dry tomorrow. Low temperatures will fall to the upper 50s and low 60s overnight.

Saturday will be a dry day as high pressure passes through the forecast area. High temperatures will be a little cooler, only warming to the upper 70s to low to mid 80s. The dry conditions will be brief as the next round of rain will return on Sunday as another shortwave trough will push into the Northern Plains. Latest ensembles show a 40-70% chance for rainfall totals to exceed a tenth of an inch through the day. Latest machine learning (ML) guidance keeps the highest chances for severe storms to the west of the forecast area. This may change over the next few days so will continue to monitor the potential for severe storms on Sunday.

High temperatures will remain near seasonable in the upper 70s and 80s for the first half of next week. Active zonal to northwest flow will persist aloft as well, contributing to additional risks for showers and storms. ML guidance does shows a small increase in potential severe storm risk, though the probabilities only 5 to 10%. Something to keep an eye on going forwards as weak shortwaves within the active flow can deliver additional chances for potential severe storms.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/

Issued at 619 AM CDT Fri Aug 21 2026

Patchy and locally dense fog with visibility below a mile at times will continue into mid morning. Otherwise, VFR conditions prevail through the period. Low (less than 20%) chances for any precipitation, with the areas of focus near US Hwy 14 and along/south of the MO River Valley this afternoon into tonight. Light and variable winds again tonight may lead to patchy fog early Saturday morning.

FSD WATCHES/WARNINGS/ADVISORIES

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