textproduct: Sioux Falls

KEY MESSAGES

- Patchy valley fog is again possible tonight, mainly across parts of northwest Iowa. Any fog that does develop should burn off by mid morning.

- Dry conditions and seasonably warm temperatures continue into mid-afternoon Monday. Breezy south winds will be gusting 25-35 mph with some 40 mph gusts possible west of the James River Valley.

- Shower and storm chances return Monday night and continue into Wednesday. An isolated strong to severe storm is possible in portions of southeast South Dakota Monday evening, if storms can develop.

UPDATE

Issued at 850 PM CDT Sun Aug 2 2026

The rest of this evening will be pleasant and dry, but on the breezy side with south winds gusting 20-25 mph. Lows tonight will fall to the 60s. Some valley fog is possible, mainly over northwest Iowa where winds will be lightest. However, confidence that winds will become calm enough for fog is low. Upper level clouds will flow in from the west ahead of an incoming cold front. As the cold front approaches mid-morning winds will begin to increase and spread east. Region wide gusts of 25-35 mph are expected with the strongest winds along and west of the James River Valley in the afternoon. Here occasional gusts to 40 mph are possible. Winds decrease through the evening Monday but remain breezy through the overnight.

Monday during the day conditions will be dry and warm with highs in the 80s and 90s. Skies will be partly cloudy and hazy from wildfire smoke. Monday afternoon the cold front and surface low pressure system will be positioned over central South Dakota. As this progresses east scattered showers and isolated thunderstorms are possible through the overnight. Strength of storms is in question due to a well capped environment. However, should one lift off the 850-700 mb elevated front it could become strong to severe. Primary threats are wind gusts to 65 mph and possibly marginally severe hail to the size of a quarter. As the main upper wave pivots over the region showers and thunderstorms continue through the day Tuesday. These look to remain mostly south of I-90 and over northwest Iowa.

Tuesday evening has renewed chances for showers and thunderstorms as another shortwave pivots through the region. Currently the best dynamics are located to the southeast of the region over Iowa. Therefore severe storms risks for our CWA Tuesday evening are low. A third, slightly deeper shortwave pivots through the region Wednesday evening. Once again the best dynamics are to the south and east of our CWA, and confidence in severe weather is low.

Persistent cloud cover and cold air advection on northerly winds behind the front will keep high temperatures for both Tuesday and Wednesday on the cool side of climatology, in the 70s.

DISCUSSION

Issued at 151 PM CDT Sun Aug 2 2026

Clear skies persist across the area this afternoon as high temperatures have warmed to the 80s. Winds remain more on the breezy side, mainly for locations along and west of the James River where gusts up to 20-30 mph are being observed. Winds will weaken some through the evening hours but remain elevated overnight as an upper level wave begins to eject into the Northern Plains, strengthening the low level jet (LLJ). The LLJ will also keep low temperatures on the warm side with lows only falling to the 60s to low 70s overnight. Patchy valley fog is possible again tonight though northwest Iowa will have the best potential for fog as winds will be weakest there.

Monday will be a warmer day as high temperatures warm to the 80s and 90s. With will remain breezy with gusts up to 35 mph expected during the afternoon time frame. A cold front will be pushing into the area from the west by the late afternoon and early evening timeframe. This front may have the potential to develop showers and thunderstorms. However, steep mid level lapse rates will yield a stout capping inversion. There also looks to be mid to high level clouds situated above the front throughout the day on Monday that could result in slightly cooler temperatures. That said, both the HREF and REFS have decreased their probabilities for a strong cap to remain in place. They now show a 40-70% chance for convective inhibition (CIN) to remain at -50 J/kg or less. This suggests that the cap could be weak enough for one or two storms to develop along the cold front as it pushes into the Missouri River Valley. This potential is still uncertain as the latest 12z hi-res guidance shows some variance in storms developing along the front. This may be a scenario where storms develop in the evening along the elevated portion of the cold front where CIN will be less. Should storms develop, they continue to look to be just ahead of the stronger vertical shear. Still not out of the question for these isolated storms to be strong to severe. Effective shear values look to be on the order of about 40 knots while min to max theta-e difference values will be down to about -15K. Thus, large hail up to the size of half dollars and damaging winds to 60 mph will be possible if the isolated storms do develop. Chances for strong to severe storms will be waning into the overnight hours but rain showers look to persist through the night along the elevated front.

The surface front will push through the forecast area during the overnight hours. However, the elevated front will remain over mainly northwest Iowa for the day. This is where the bulk of the rain is expected to fall. The rain and cloud cover will keep high temperatures on the cool side with highs only reaching up to the 70s. The rain will persist through the entirety of Tuesday and through Wednesday morning before pushing southeast of the area. The ensembles continue to show moderate to high probabilities between a 50-80% chance for rainfall amounts to exceed a tenth of an inch. Their probabilities decrease to 0-40% and are concentrated along and south of highway-20. Thus, light rainfall amounts of a tenth of an inch to possibly exceeding a quarter of an inch is most likely at this time.

With lingering cloud cover around, Wednesday will be another cool day with highs only warming to the 70s. Broad troughing will will reside over the Northern Plains for the back half of the week and next weekend. Temperatures look to warm back to near seasonable in the 80s to potentially 90s each day. There could potentially be more chances for rain but these chances are uncertain for now.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 614 PM CDT Sun Aug 2 2026

VFR conditions are expected through this period. Southerly winds gusting 17-27 kts will expand east into Minnesota and Iowa through the overnight. Shortly after sunrise winds will increase ahead of an approaching cold front. Southerly gusts of 20-30 kts will be widespread across the region. The highest gusts are expected along and west of the James River Valley where they could reach up to 37 kts. Winds begin to decrease near the end of the period. At the same time the cold front should be entering central South Dakota turning winds to the north.

Directional LLWS is expected for areas along and west of the James River beginning around midnight and continuing until shortly after daybreak Sunday. Sporadic shear is also possible as the cold front passes. However, confidence was too low to include at any of the TAF sites at this time.

FSD WATCHES/WARNINGS/ADVISORIES

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