textproduct: Sioux Falls

KEY MESSAGES

- There is high confidence that severe thunderstorms will develop this afternoon and evening, producing large hail, destructive straight-line winds, and a few tornadoes. An Enhanced Risk (level 3 of 5) of severe storms is in place.

- A transitioning storm mode will shift hazards from large hail and tornadoes initially, to widespread wind damage as an intense line of storms tracks east of Interstate 29 this evening.

- A lower risk of severe weather exists late Monday afternoon and evening, mainly over northwest IA, before quieter weather moves in for the mid-week period.

DISCUSSION

Issued at 120 PM CDT Sun May 17 2026

The short-term convective outlook presents a highly volatile spring severe weather setup. Latest surface analysis shows a complex pattern, anchored by a strong 994 mb low over northwest KS. An inverted trough extended north of the low through central SD, while a well-defined warm front stretched northeast of the low into east central NE. A weaker low was also noted in north central NE. Visible satellite imagery revealed stable wave clouds advancing northeast over far northeast NE and extreme SE SD, ahead of the advancing warm front. Regional radar imagery showed a complex of strong to severe storms over north central NE. This storm complex is forecast by short range guidance to continue tracking east this afternoon, intensifying as interacts with the richer low- level moisture pool near and south of the warm front.

Moderate to strong instability will be in place over the southern to eastern CWA by late afternoon with MLCAPE nearing 3000 J/kg. In addition, deep layer shear will increase to around 50 kt, coincident with a mid level speed max ejecting from the deep upper trough to our west. As the system evolves over the next several hours, it appears the ingredients for a significant severe weather event will be most favored over the southern to eastern CWA. Ahead of the main convective line, some CAMs show supercell development along the northward advancing warm front. These storms would present a risk of very large hail (2"+) and tornadoes. Given the amount of low level SRH/curved hodographs present, a couple strong tornadoes are possible. The convective cluster currently entering our far southwest CWA should strengthen as it shifts east, with an initial hail and tornado threat transitioning to a damaging straight line wind threat (70+ MPH) as it grows upscale into a squall line/QLCS. Some line-embedded QLCS tornadoes are possible, especially for any line segments that become oriented north/south or northwest/southeast, given southwesterly 0-3 km shear vectors. Timing from latest high-res guidance suggests this activity exits to the east of our CWA late this evening.

Precipitation amounts will vary significantly based on convective tracks, though NBM guidance and HREF probability match mean fields project a widespread 0.50 to 1.50 inches of rainfall, particularly across northwest Iowa and southwest Minnesota. On Monday, the cold front finally pushes into the area. Instability and shear are forecast to be much lower than today, however, there will be a conditional risk of severe storms over the eastern CWA during the late afternoon to evening, if the airmass can destabilize sufficiently ahead of the front. SPC's Day 2 outlook indicates as Marginal Risk (level 1 of 5), for our MN and IA counties.

Behind the departing upper wave, quasi-zonal flow sets up for Tuesday and Wednesday, yielding quiet conditions and seasonally cool high temperatures moderating from the upper 50s Tuesday to mid-upper 60s Wednesday.

The next upper wave moves in for the Thursday to Friday period, however Gulf moisture remains cutoff ahead of this system. Evaluating the ECMWF Extreme Forecast Index (EFI) reveals no significant climatological anomalies for precipitation or wind across the upper Midwest during this period, reinforcing the the idea of a low-impact system for our area. The current forecast sticks closely to the NBM consensus, maintaining broad 20-30 percent probabilities for light rain for late Thursday into Saturday.

By late this week and especially this weekend, temperatures will embark on a steady upward trajectory, as the mid level flow backs west, then southwest. The Climate Prediction Center (CPC) 6-10 day outlook favors above-normal temperatures across the Northern Plains and Upper Midwest, which supports blended guidance high temperatures reaching well into the 80s on Sunday.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 650 PM CDT Sun May 17 2026

Showers and storms will continue to move eastward into MN through the evening hours, the forecast area should be largely clear of shower/storm activity after ~05z. Intermittent MVFR/IFR conditions in heavy rainfall as well as hail and strong winds will accompany the strongest storms. Widespread MVFR/IFR cigs will set in overnight and persist through much of Monday.

FSD WATCHES/WARNINGS/ADVISORIES

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