textproduct: Sioux Falls

KEY MESSAGES

- Strong to severe storms are possible between 6 pm and 2 am, and may focus along and south of a line from Gregory to Madison and Tracy. Golf ball sized hail and wind gusts to 70 mph are the main threats, although a tornado cannot be ruled out.

- Near to above normal temperatures continue this week. Confidence is increasing in above normal temperatures and humidity for the end of this week and weekend.

- Low chances (15-30%) for rain accompanies the increasing heat and humidity by the end of week.

UPDATE

Issued at 322 AM CDT Tue Sep 1 2026

A trough of low pressure off the west coast will swing a stronger jet maximum into the central and northern Plains today into tonight. This jet energy will bring a wave into the area this afternoon which will increase southerly low level flow. Ahead of this wave this morning broad warm air advection will likely allow for isolated to scattered showers and thunderstorms but severe weather is unlikely. This increase in southerly flow will advect a deeper pool of instability across NE as seen with the 0z sounding from Omaha where roughly 4000 J/kg of sfc based instability existed. One indication of some drier air just above the surface is the Omaha's 100 mb mixed layer CAPE was around 1500 J/kg, so much less unstable and likely a better indication of storm energy potential. Just how far north this deeper pool can get as the wave ejects onto the Plains will be the big question. Most of the models creep this into far southeast SD and parts of northwest IA.

However, initial thunderstorm development will likely be more associated with the mid/upper level wave which moves into central SD from about 3-7pm. Would expect development here where the cap is weaker with slightly better mid level lapse rates as well. Initial instability likely around 1500 J/kg, so isolated severe storms will be possible by late afternoon in central SD.

Into the evening and early overnight hours the low to mid level warm front becomes more established across southeast SD into parts of northwest IA and southwest MN. This should help organize any updrafts as and shift activity east and slightly southeast towards the deeper instability. One question will be the minor to moderate shear expected, likely about 35 to 45 knots of deep layer shear. With some cold pool potential with dry air below about 700 mb and an increasing LLJ, wind may become more of a threat after sunset, which would likely be mainly near and east of highway 81. Locally heavy rain will also be possible with a freezing level around 12 kft and low level front oriented west to east with mid and upper flow running west to east as well, which could allow for a little training of convective activity.

DISCUSSION

Issued at 245 PM CDT Mon Aug 31 2026

Sunny and warm conditions persist this afternoon as temperatures have warmed to 80s. With a cold front now southeast of the area, dew points have lowered to 50s and 60s. Still seeing heat indices up to 80s to low 90s, so some humid conditions remain though not as humid as what we saw this weekend. A weak low level jet (LLJ) is expected to develop tonight which will develop showers and potentially an isolated thunderstorm across Nebraska. These showers will push east/northeastwards into a worsening environment. They could still be capable of lightning though not expected to be strong nor severe. Low temperatures will remain quite mild overnight, with lows only falling to the 60s.

The showers will push through the area during the morning to early afternoon hours. This will leave the rest of the afternoon on the dry side of things. With some cloud cover remaining aloft, highs look to remain warm but be slightly cooler than Monday's highs with temperatures in the 80s up to about 90F. Chances for severe storms begin to increase after 6 pm as a shortwave trough ejects out of the Rockies and into the Northern Plains. The previously mentioned front will begin to return northwards as a warm front due to the mass response from the wave. A sufficient warm sector will be in place ahead of the boundary with dew points in the 60s to 70s. Some capping will be in place which may limit the extent of convection early on. Latest hi-res guidance continues to show moderate variance but is beginning to show a consensus of storms developing across central and north central Nebraska and possibly south central South Dakota. These showers and storms will push into the area during the evening timeframe but the remaining uncertainty revolves around storm mode. Think a custer of storms is possible with embedded multi- cells to even supercells. As the storms push further into the forecast area, they will become more elevated due to increased capping over the area. Storm mode becomes more questionable here as storms may remain a bit more discrete to perhaps remaining in a cluster. Or if they can congeal cold pools properly, upscale growth into a line is possible. Tough to say which storm mode is correct at this time. Aside from capping, sufficient elevated instability will be in place with CAPE values up to 1,000 J/kg to 2,000 J/kg with effective shear values between 40-50 knots. Thus, damaging winds to 70 mph and golf ball size hail will be the main hazards. If a storm is able to tap into the boundary layer, then an isolated tornado will be possible. Especially if the surface warm front can push into the forecast area. The storms will push through the area through the evening and at least the first half of the overnight hours before pushing eastwards with the veering low level jet.

The daylight hours on Wednesday look to be a quiet as subsidence prevails on the backside of the departing wave. Highs will remain consistent in the 80s to even low 90s in a few locations. The break from rain will be short lived though as another shortwave trough will be pushing into the Plains Wednesday night. The ensembles support this as they show increasing probabilities up to a 20-50% chance for rainfall totals to exceed a tenth of an inch. The ensembles have the highest chances for rain east of I-29. Some elevated instability looks to be available, suggesting that thunderstorms will again be possible. Whether they will be strong to severe is uncertain at this time.

Upper level ridging will be strengthening aloft for the end of this week and weekend. High temperatures look to not only remain above average but further warm as the ridge becomes more pronounced aloft. The ensembles not only support this ridge developing, but also 850 mb temperatures being pushed to the the 99th percentile of climatology. The Euro Extreme Forecast Index (EFI) further supports this as it shows 60-80% of its members suggesting above average temperatures are possible. Humid conditions look to accompany the above average temperatures as the ensembles show a broad 50-90% chance for heat indices to exceed 90F. Chances for rain are possible (15-30% chance) during this period of time but details are highly uncertain. There are some signals that this pattern could begin to break down next week. However, medium range guidance continues to show increasing variance in this potential. Something to watch to see if these conditions will persist into next week.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1146 AM CDT Tue Sep 1 2026

VFR conditions continue early this afternoon as a broken mid-lvl cloud deck continues to move northeast. A few showers and isolated thunderstorms are possible along and west of the James River valley into mid-afternoon.

Convection risks later this evening begin to develop in NC Nebraska and SC South Dakota after 5pm. Several individual storms may begin to cluster and move east into the early overnight hours. Initially large hail and even a tornadic threat will develop, but as storms move east, risks may shift to straight line winds and brief QLCS tornadoes as they move into NW Iowa or Southern Minnesota. IN any thunderstorm brief periods of IFR to MVFR visibility/ceilings may be possible. Storm threats should move east of I-29 by midnight, with some potential for fog by daybreak Wednesday if skies clear.

FSD WATCHES/WARNINGS/ADVISORIES

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