040 FXUS63 KFSD 182302 AFDFSDArea Forecast Discussion National Weather Service Sioux Falls SD 602 PM CDT Thu Sep 18 2025
.KEY MESSAGES...
- Scattered showers and isolated storms will continue to be possible into the weekend, with the best chances (30-60%) west of Hwy 81 and in portion of NW Iowa this afternoon/evening, and across Minnesota Friday.
- Scattered showers/isolated storms possible again Sunday, but details are uncertain.
- Near-to-slightly below normal temperatures are expected into the weekend, with a slight warm-up possible early next week.
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.DISCUSSION... Issued at 255 PM CDT Thu Sep 18 2025
LATE AFTERNOON THROUGH SATURDAY: A nearly stacked mid-upper level low will continue to slowly spin over the Dakotas and into Minnesota over the next couple of days. This will keep the weather unsettled across our area into at least early Friday evening. Will continue to see shallow showers across our western counties into this evening, but expect coverage to decrease after sunset with focus then shifting to the eastern half of the forecast area as a modest wave rotates through the southeast periphery of the main trough. This should spread showers and a few embedded thunderstorms northward across areas along/east of I-29 this evening/overnight. Limited instability/shear should keep any severe threat away from our forecast area, but nearly moist adiabatic lapse rates and decent moisture depth could lead to more efficient rainfall. Pockets of 0.50" rain or more are possible through Friday evening, mainly east through north of Sioux Falls.
As the system slides farther northeast toward the western Great Lakes later Friday night/Saturday, its influence will decrease across our forecast area. This should allow for drier conditions most areas for the first half of the weekend, with seasonable highs in the lower-mid 70s (slightly warmer than Friday for most areas).
SUNDAY-MONDAY: Broad troughiness lingers into Sunday, followed by weak upper ridging early next week. Confidence in details during this period begins to wane as model solutions show less agreement. However, we could still see occasional showers or isolated storms as waves rotate through the region. General model consensus shows some increase in instability by Sunday, though profiles show tall/skinny CAPE profiles with mid-level lapse rates below 7C/km. Shear, while still on the weaker side, does increase over what we currently have with a low (20-30%) joint probability of CAPE > 500J/kg where shear > 30kt. Given uncertainty in wave timing and weak mid-level lapse rates, severe weather risk appears low, but we may have to monitor over the next couple of days as some machine-learning outlooks do indicate a low (5%) severe risk for the Sunday/Sunday night period.
TUESDAY ONWARD: Very low confidence as models diverge further in handling of the upper pattern. If faster upper trough solutions pan out (similar to 12Z operational ECMWF/Canadian, we would trend drier for mid-late week. However, a slower evolution to the upper pattern (as seen in the 12Z operational GFS) would result in rain chances lingering into the middle of next week before drying out. Made no changes to the mid-long range NBM pops, which currently favors the faster/drier solutions. But changes may develop if models trend more toward the slower GFS solution.
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.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/... Issued at 600 PM CDT Thu Sep 18 2025
Scattered showers and isolated thunderstorms are occurring through the James River Valley and east of Interstate 29 this evening. The trend into the night will be for showers to diminish over the James River Valley, while showers to the east of I-29 will periodically continue into Thursday. While an isolated shower cannot be ruled out at KFSD and KSUX through the period, confidence is too low to include in TAFs.
MVFR/IFR ceilings currently over central SD will expand eastward overnight, affecting all TAF sites into late morning/early Friday afternoon. In addition, there will be a potential for patchy fog formation late tonight into Friday morning, with the greater probabilities residing from east central SD through southwest MN into parts of northwest IA.
Light southerly winds will slowly transition to west/northwesterly tonight into Friday.
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.FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None.
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DISCUSSION...JH AVIATION...JM
NWS FSD Office Area Forecast Discussion