textproduct: Sioux Falls

KEY MESSAGES

- High based sprinkles may develop near Highway 20 into this evening with isolated lightning strikes.

- Thunderstorms increase in coverage tonight into early Sunday. Weakening storm clusters may produce marginally severe hail and brief 60 mph wind gusts between 10pm and 5am.

- A significant severe weather episode is possible Sunday afternoon into Sunday night. Very large hail, straight line winds, and several tornadoes, possibly strong, will be possible. This risk may extend into very early Monday.

- If you have outdoor plans Sunday afternoon through Monday, the time is now to prepare and have alternative plans in place.

DISCUSSION

Issued at 101 PM CDT Sat May 16 2026

THIS AFTERNOON: We continue to see a pleasant Saturday afternoon across the region with a light to easterly wind prevailing. Temperatures are gradually rising towards the 80s, but humidity remains very low in the 15 to 20% range. Weak warm advection to our south will begin to stream elevated ACCAS overhead, perhaps with a sprinkle or two by early evening. This activity is based well aloft AOA 600 mb, with soundings showing minimal potential for deepening. Any surface based convection will remain well south of the CWA with the frontal boundary stuck north of I-80.

TONIGHT: Easterly low-lvl flow will keep the risk for convection lower through the evening hours as a gradual northward advancement of the surface front begins to our south. We'll begin to see convection form along and south of this boundary extending from western Nebraska into western Iowa late this evening. This activity will be pulled northward by a stronger shortwave lifting out of the Central Rockies into the Plains. The advancement of a low- lvl front and strong theta-e advection will bring the risk for elevated thunderstorms to the Tri-State area after 10pm with one more clusters of storms (or potentially an MCV) that form to our southwest tracking towards the area through daybreak. MUCAPE plots and soundings ahead of this convection continue to look marginal, with mid-lvl lapse rates around 7 C/KM and storms likely to be strongly elevated or marginally severe. At this point the greatest severe weather risk would be south of I-90 and perhaps more focused along the MO river and Highway 20 corridors. One thing to watch would be the potential for any MCV that develops to bring wake low conditions, or strong evaporative cooling winds northeast as convection wanes. Some high-res guidance supports pockets of 50+ mph winds moving northeast with the weakening convection.

SUNDAY-SUNDAY NIGHT:

The key takeaways for Sunday continue to be:

1. Convective debris, stratus, and an EML may be problematic and could make this forecast conditional.

2. If storms get rooted into the boundary layer, there is no shortage of spin potential into the early overnight and tornadic activity becomes an increased concern.

3. If discrete mode is short lived and storms turn linear, then duration of threat in any one area may be intense but brief.

The mesoscale picture may be quite cloudy to start Sunday. Considerable stratus will be present in the area and we'll only get a better pictures of where the cold front/inverted trough and associated warm front will be positioned through the morning. Most of the morning is likely to remain quiet, with some renewed convection possible in SW South Dakota/western Nebraska on the nose of the upper jet entering the Plains. Some CAMS do have this mostly elevated convection brushing the NW edge of the CWA by 18Z. Further east, it may take into the mid- afternoon hours for clouds to thin and the warm front to jump northward (if it does). A stubborn EML will also remain within the 600-700 MB. Guidance is still very bullish on the rapid northward advection of 2000-3000 MLCAPE northwards towards Highway 14 and west to Highway 281 by 21-00Z. The increasing CAPE will be aided by the arrival of 8.5-9 C/KM 700:500 lapse rates, and once the next wave of mid- lvl vorticity arrives, scattered convection may quickly develop along the frontal boundary stretching near Hwy 281 after 20Z. A second area of convection may form further south along the dry line intersection in eastward Nebraska and track northeast into the evening. Some uncertainty persists on how discrete storms will remain or if they will quickly turn linear as they expand and slide east/northeast into the evening and early overnight. Supercells with very large hail (2-3") and an increased tornadic risk will be possible initially, but if linear trends develop, line segments with straight line winds/embedded tornadoes may become a possibility as storms track east northeast towards I-29 and MN/IA. The degree of low- lvl helicity/shear combined with increasing 0-1km CAPE approaching 100-200 J/KG and critical LCL values along and slightly east of I-29 is particularity concerning heading into the evening and continues to support the 10-15% tornado probabilities by SPC. In fact, STP values push 5+ along portions of I-29 between SF/SC by 00Z. Most of the recent CAMs support the linear lines moving east of the CWA by midnight with lower risks into daybreak Monday.

MONDAY: There is better agreement in today's guidance pushing the effective boundary east of the CWA by Monday morning. That said, incoming shortwave energy may pull elevated instability and increased lapse rates back west of the boundary by mid-late afternoon. This could place areas of NW Iowa under the risk of elevated hailers for a period of time. For NW Iowa, this could also be the time where the most beneficial rainfall may also take place. Further west, significant cooldown is expected as temperatures struggle to climb through the 50s.

TUESDAY-FRIDAY: Quiet and cooler than normal temperatures are expected through the middle of the week as highs climb back through the 60s by Thursday. Another trough moves through the Plains by the end of the week however a lack of deep moisture prevents any significant severe weather risk. The latest AI learning guidance suggests less than 5% probabilities of any severe storm at this time.

AVIATION /18Z TAFS THROUGH 18Z SUNDAY/

Issued at 1227 PM CDT Sat May 16 2026

VFR conditions continue into the evening hours. Very high based ACCAS (AOA 14K ft AGL) will persist south of I-90 into the early evening. A random lightning strike may develop, but low probabilities.

Convection over Nebraska will organize and track northeast overnight, while severe trends will be lowering through the night, occasional stronger gusts could be possible as this activity moves northeast.

MVFR stratus will arrive behind this convection with these conditions likely to remain in place through mid-day.

FSD WATCHES/WARNINGS/ADVISORIES

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