textproduct: Sioux Falls

KEY MESSAGES

- Areas of patchy fog are possible through mid morning mainly near and east of I-29.

- Thunderstorms increase in coverage through the morning and continuing through the late afternoon. An isolated stronger storm could produce ping pong ball size hail and 60-65 mph wind gusts.

- Higher risks for locally heavy rain develop by early afternoon Sunday, with potential for pockets of 2-4" of rain and localized amounts to 6". Localized flash flooding is possible into early evening.

- A shifting pattern may bring higher rain chances, slightly cooler temperatures, and a severe weather risk by Monday evening and early overnight. This risk could continue Tuesday afternoon south of I-90.

- Dry and continued above normal temperatures are favored for most of the upcoming week.

UPDATE

Issued at 316 AM CDT Sun Sep 6 2026

Convection earlier in the overnight along weak convergence near the Buffalo Ridge struggled and quickly dissipated, although better mid level convergence may develop over the next couple of hours with weak 700mb PVA and upper jet. Watching a couple areas south and west: showers across central SD and convection across NE. The area in NE is tied to a stronger wave and some LLJ, and should continue to move north through the overnight hours. Beginning to see development near the stationary boundary in northern NE. Patchy fog has developed for portions of the area, which should diminish by mid morning.

06.00z and later CAMs and other 00z deterministic guidance has slowed the progression of the southern wave and on-set of precipitation. Guidance does still eventually blend these areas together near to east of I-29, albeit a couple of hours later than previous forecast. Will have to see how things develop over the next few hours, especially between 4-7 AM.

For the rest of the day, severe risk still may be limited by the relative lack of shear. But as was seen last night/early Saturday morning, steep mid level lapse rates may be enough for some isolated occurrence of hail to the size of ping pong balls. 60-65 mph gusts also possible.

Most guidance continues to push higher QPF off to the areas east of the James River (with majority focused along and east of I-29), although guidance is still fairly spatially variable with the location of the highest amounts and think that any flash flooding issues will be localized. Given this, did not issue a Flood Watch. Deep warm cloud layer, slow storm motion, and PWATs near 1.75 to 2" are all supportive of heavy rainfall across the area. Still think most areas see 0.5-0.75" with pockets of 2-4" and more isolated areas around 6" possible.

Regardless of which convective scenario plays out, showers and storms should move southeast, tapering down this evening with possibly some lingering showers into tonight.

DISCUSSION

Issued at 218 PM CDT Sat Sep 5 2026

THIS AFTERNOON: We're slowly recovering the airmass post convection this afternoon, though continued overrunning of the convective outflow to our south has persisted in developing additional cloud cover in northeast Nebraska and northwest Iowa. Temperatures will continue to rise and it appears we'll reach into the upper 80s to lower 90s once again. Across NW Iowa temperatures have been cooler, but a late day temperature surge is still possible if clouds clear. Dew points will also recover slowly and should end up into the upper 60s to 70s by this evening. The resultant heat index values climb back to the 95 to 102 range in quite a few locations.

TONIGHT: We'll be watching modest theta-e advection kicking off additional showers and thunderstorms over the western High Plains later this afternoon and early evening. This activity will likely struggle as it tries to reach central South Dakota later this evening. Greater concerns begin to develop after midnight as the LLJ intensifies. The majority of guidance is now showing two areas of concern...one along the Minnesota river valley and Buffalo Ridge where some increased convergence and influence of mid-lvl vorticity could spark elevated storms. A second area over western/central Nebraska and central South Dakota where a potentially stronger mid- lvl area of vorticity enters the state and begins to slowly drift east northeast. Elevated convection in both of these areas would favor brief marginally severe hail and a few 40-60 mph wind gusts. The biggest uncertainty is the time of development which may range from 4am to 7am.

SUNDAY: While there isn't a lot of certainty in meteorology, my confidence is growing that we'll be dealing with some type of impactful weather during the day. Already ongoing convection at daybreak may continue to spread both east (from western complex), and southeast (from elevated Ridge convection)...an interesting artifact of being stuck under the mid-lvl ridge axis. Occasionally strong updrafts may still produce isolated severe weather through the morning as this activity meanders. Soundings by mid-day either side of the I-29 corridor show a nearly uncapped environment which could signal some potential for ongoing elevated storms to tap into the surface layer and intensify. Shear remains quite marginal, but 2000-2500 J/KG MLCAPE and decent mid-lvl lapse rate could suggest some large hail and downdraft potential. While extremely low probabilities...the low LCL and chaotic SFC wind pattern could lead to a few funnels. Gradual cold pool development and continued southeast shift in the upper vort max should push this convective cluster southeast and out of the CWA by early evening.

Another concern, which may end up being the biggest concern will be heavy rain. As evident this morning, the environment overhead remains rich for locally heavy rainfall with several areas of 4-6" of rain. A similar situation will be in place tomorrow with another deep warm cloud layer, high PWAT, and slow storm motion (AOA 10-15 knots) event. REFS/HREF 6 hr PMM through 00z all indicate localized totals of 2-4"...however as shown by last night's data...the many of the totals were closer to the ensemble max. In this case the potential shifts to higher localized values of 3 to 5"+. All this to say that these high end amounts may be quite spatially variable which makes pinning down a potential flood watch more difficult. Areas from Mitchell to Tyndall are by far the most susceptible to flooding, with 3 hr FFG in most other areas between 2.5-3.25". Will allow next shift to take a closer look and try to pin point any areas of concern as data focuses in on the potential.

SUNDAY NIGHT: Any MCS that develops Sunday afternoon should exit the area early in the evening leaving subsidence in it's wake. There still remains a 30-40 knot LLJ that will develop and may produce scattered convection into the James river valley into Monday morning.

MONDAY-TUESDAY: The mid-lvl ridge axis slides east by Monday, with deeper troughing moving into the PACNW and Northern Rockies. Lee- side troughing develops a surface boundary over the western Dakotas early on Monday with an increasing SPG bringing gusty winds to the CWA Monday afternoon. Temperatures may also warm back towards the 90s ahead of the front. While we'll have to watch return flow and boundary layer moisture along the boundary, there may be sufficient pooling of moisture to overcome lingering capping by late in the afternoon in central South Dakota. Should storms develop, then a narrow focus for severe convection may extend into the western half of the CWA into the early overnight hours. Depending on the extent of coverage of storms, the aforementioned front may dip into northern Nebraska and Iowa by Tuesday. Convection may again develop along this boundary with some increased severe weather potential focused mostly in the southern third of the CWA.

WEDNEDAY-FRIDAY: The rest of the work week favors slightly cooler temperatures and drier conditions as high pressure moves into the Plains. Temperatures will remain slightly above seasonal normals in the lower to middle 80s.

AVIATION /12Z TAFS THROUGH 12Z MONDAY/

Issued at 628 AM CDT Sun Sep 6 2026

A few aviation concerns through the period. First, patchy stratus and fog along/east of I-29 is leading to IFR and lower conditions through mid morning. Second, convection is slowly moving north out of NE into south central SD, with additional convection trying to develop near the Buffalo Ridge into northwestern IA. Showers/storms are expected to expand in coverage through the morning, continuing over the area through the afternoon. Storms may produce erratic winds and MVFR/IFR conditions. Locally heavy rain is possible with thunderstorms. Narrowed timing a bit at all sites, but still some uncertainty on how quickly storms move in (and therefore out) of the the area this evening.

Additional isolated to scattered storms possible toward daybreak Monday, but confidence is too low to include at this time.

FSD WATCHES/WARNINGS/ADVISORIES

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