textproduct: Sioux Falls

KEY MESSAGES

- Cool overnight temperatures may lead to valley fog by daybreak Thursday. Localized dense fog may be possible.

- Well above normal temperatures, increasing winds, and drying fuels lead to elevated to near critical fire danger Thursday and especially Friday. Harvest activity should use extreme caution.

- A significant cooldown arrives this weekend and continues into next week with highs back into the 50s.

- This cooldown will bring rain risks Saturday which will continue into Monday. Probabilities for >0.10" are near 100%, with potential for >0.50" near 70%, and probabilities for >1.0" as high as 40%.

UPDATE

Issued at 839 PM CDT Wed Oct 21 2026

Other than some morning patchy fog, the main story will revolve around increasing southerly winds. Very mild air for late October continues Thursday into Friday with southerly winds also expected to increase. Thursday should see the strongest winds in the afternoon with gusts of 20 to 25 mph likely. Friday will see breezy conditions right off the bat, with an increasing pressure gradient through the day, leading to windier conditions in the afternoon. With 35 to 45 knot winds sitting at about 900-850mb, mixing will not need to be too strong to bring 40 to 45 mph winds to the surface. The strongest winds in this layer look to be mainly near and east of highway 81, and if mixing is deep enough wind gusts around 50 mph are not out of the question.

This leads to concerns for rapid fire spread. While there will be some elevated concerns Thursday, Friday will see the highest threat for critical fire weather conditions. The only real negative is relative humidity will likely not fall below 40 percent which generally aids in suppressing rapid spread. But, with fuels nearing fully cured and afternoon winds of 35 to 50 mph, this combination will likely be enough to overcome the higher relative humidity and cause rapid fire spread if any fires should start. If harvesting and burning are not necessary on Friday, please avoid as any spark could cause a small fire which could spread rapidly.

DISCUSSION

Issued at 1235 PM CDT Wed Oct 21 2026

THIS AFTERNOON: An absolutely beautiful mid-October afternoon is unfolding as winds have remain light, skies remain clear, and temperatures are rising into the 70s. Dew points are mixing down into the middle 40s in a few areas, which could be a sign for later tonight.

TONIGHT: A bit of high lvl cirrus that crosses the CWA later this afternoon and early evening will prevent a sharp temperature drop- off initially. However nearly all guidance is indicating that we'll see a night with light to calm winds and clear skies. Given these conditions, will lower temperatures significantly in most locations, with HRRR/RAP guidance indicating potential for mid-40s. With afternoon dew points likely falling a bit further, this leaves cross- over temperatures in a favorable position to lead to fog development. The majority of this fog may be confined to sheltered river/valley areas, but given temperature falls, some may spill out into a broader area and become locally dense.

THURSDAY: Troughing sliding into the Central Rockies will help intensify the SPG throughout the Plains on Thursday. Winds will begin to increase early in the morning and by the afternoon we'll see routine gusts 20 to 30 mph at times. Slowly drying fuels and increasing wind will lead to increased fire danger. See separate discussion for further details.

FRIDAY: Late Thursday night we'll begin to see an increase in low- lvl moisture emanating from the Oklahoma Panhandle. This moisture, when combined with convergence on a 40 knot LLJ and weak dPVA, may lead to isolated to scattered high based showers after 3am Friday which will continue lifting northward Friday morning. A minor increase in elevated instability could lead to isolated lightning strikes with this activity. However, the greater threat in the afternoon and evening will be the surging high temperatures and increasing wind gusts. Latest guidance strongly supports sustained winds 20 mph in many locations, with potential for gusts of 45 mph at times. While the SE surface wind direction is a bit mis-aligned with the strongest corridor of low-lvl SWLY flow, the highest wind corridor is likely to fall along and west of a Brookings to Yankton line. This will again increase fire danger so please read separate discussion for further details.

SATURDAY-WEDNESDAY: A warm overnight is anticipated into Saturday as the SPG really doesn't weaken and promotes increased mixing through the night. By Saturday morning, a cold front will slowly slide east into Central South Dakota. Weak low-lvl convergence and moisture streaming ahead of this front could lead to high based showers into daybreak. A splitting upper trough over the Rockies will promote two distinct features, a progressive trough lifting northeast into southern Canada Saturday into Sunday, with a cutoff area of energy stalling over the Four Corners. The aforementioned frontal boundary will get nudged southeast on Saturday, bisecting the CWA. With significant cold air lagging this front, the temperature gradient is not expected to be extreme. However an increase in cloud cover, along with potential for occasional light rain will persist through Saturday and into Saturday night.

One or more weak impulses traveling northeast along the stalled front will keep clouds and occasional rain risks through the CWA Saturday night and through Sunday. QPF amounts through Sunday afternoon may be on the lighter side, with most areas seeing less than 0.25".

That earlier mentioned upper low over the Four Corners begins to eject northeast by the end of the weekend and into Monday. A strong low-lvl frontal zone is expected to develop across the CWA ahead of this system, promoting increasing rain chances. The very slow northward movement of the approaching SW trough will keep high rain chances through the CWA on Monday and will likely continue into early Tuesday. Latest NBM probabilities suggest high rain probabilities at near 100% for >0.10", potential for >0.50" near 70%- 80%, probabilities for >1.0" as high as 50% and probabilities for 2.0" totals at 20%.

The passage of this system, and then the cooler air that follows it, will promote a return of more typical mid-late October temperatures. High temperatures Sunday through Wednesday are favored to be in the upper 40s to mid 50s.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1050 PM CDT Wed Oct 21 2026

Valley fog continues to be the main concern for this forecast period with light winds and some weak moisture pooling with dew points in the 50s. The best chance for fog will be from about 10z through 14z when the visibility may drop into the IFR category in a few locations. With KSUX along the Missouri River, it will be susceptible to this development. After this southerly winds will gradually increase with gusts of 20 to 25 mph tomorrow afternoon and night.

FIRE WEATHER

Issued at 1235 PM CDT Wed Oct 21 2026

Fire weather concerns begin to increase on Thursday as sustained surface winds begin to reach the 10 to 15 mph mark. A few afternoon gusts may begin to exceed 25 mph. Relative humidity values will bottom out in the middle 40 to low 50% range.

More concerning fire weather develops on Friday. After morning sprinkles to light rain move northeast, strong southeast surface winds develop through the day with sustained winds to 20 mph and gust of 30+ mph. Greater wind focus will develop west of I-29, but especially along and west of the James River valley with occasional gusts to 45 mph are likely. While temperatures reach near 80 degrees, dew points may try to pool ahead of a weak front in central South Dakota. This may hold relative humidity values well above critical red flag warning thresholds.

That said, the combination of strong winds, drying fuels, and abnormally warm temperatures will lead to widespread high to very high GFDI values (greater west of I-29). The greater danger will be in harvest activities, with the GFDIAg values reaching well into the very high category over most of the CWA. Any agricultural activities should use caution and local partners should be aware of the increased potential for fire starts in farm fields.

FSD WATCHES/WARNINGS/ADVISORIES

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