textproduct: Sioux Falls

KEY MESSAGES

- High based sprinkles to weak storms remain possible near Highway 20 into tonight 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. The main timeframe for severe storms is between 3 pm to midnight on Sunday.

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

UPDATE

Issued at 932 PM CDT Sat May 16 2026

Convection has developed across I-80 this evening, along the effective warm front. Despite this convection, a few weak storms have developed along part of the highway-20 corridor in far northeast Nebraska. these storms will have a lightning threat with them but are not expected to be severe.

Looking further south, latest hi-res models show convection that has developed across northwest Kansas/southwest Nebraska transitioning into an MCS and pushing eastwards along the effective boundary. This MCS should keep the bulk of the strong to severe storms south of the forecast area. That said, still seeing the 850 mb front and associated moisture are on track to push northwards this evening and through the night. CAPE values could reach up to 1,000 J/kg with effective shear values on the order of 40 knots will continue the chance for strong to severe storms. Large hail to the size of a quarter (1 inch) and damaging winds to 60 mph will be the main hazards. Instability will be waning through the night so any chance for strong to severe storms should slowly wane through the overnight hours.

Attention then turns to the severe weather set up for Sunday. Some rain showers may persist from the overnight timeframe into Sunday morning. Though any shower chance will be waning as subsidence from the backside of the departing shortwave trough takes over. There continues to look to be widespread low level stratus that blankets the area tomorrow morning. Much steeper mid level lapse rates will also push into the area from the south, creating a capping inversion. While these factors could limit severe storm potential, another shortwave trough is poised to encroach on the area tomorrow afternoon. At the same time, the latest hi-res guidance is in decent agreement in low level stratus clearing out for locations south of I- 90 and east of the James River as a drier air is advected northwards within the incoming surface trough. The latest guidance does suggest that the effective warm front will push northwards despite the overnight MCS. This warm front will make up one part of the inverted trough. The other component is an incoming cold front from the west. There looks to be elevated convection Sunday morning and lasting into the afternoon hours just behind this boundary mainly west of the James River. This cold front will intersect the increasingly unstable warm sector. Convergence and surface vorticity along this boundary look to be enough forcing for ascent to overcome the cap in place. Thus, convection is expected to develop. Soundings continue to show hodographs with looping low levels in place. 0-1km shear on the order of about 20 knots and 0-3km shear with a magnitude of close to 40 knots will yield higher storm relative helicity (SRH) values and tornado potential. 0-3km SRH values look to exceed 200 m2/s2 and may exceed 300 m2/s2. Storms will quickly develop in this volatile of an environment along the cold front. Supercells look to be the main storm mode as convection develops. Large hail up to 2+ inches is expected as the mid level will be dry along with sufficient shear in the mid levels. Tornadoes are also on the table with these supercells, especially as the hodographs show nearly streamwise vorticity in the lowest 0-1km layer and a larger streamwise component up to about 2-3 km. 0-3km CAPE looks to be impressive potentially exceeding 100 J/kg if full mixing can be achieved in the warm sector. With the cold front pushing eastwards, these storms look to quickly grow upscale into a line. This will transition the main hazard from large hail to damaging winds and embedded tornadoes. 0-3km shear will be sufficient around 35 to 40 knots and oriented perpendicularly off the squall line. DCAPE values will be impressive, exceeding 1,000 J/kg as well. This squall line will quickly race eastwards on an east/northeastward trajectory, bringing the better environment northwards with it. Storms look to exit the area during the evening timeframe. The overall timeframe for severe weather looks to be between 3pm to about midnight. Locations along to just south of I-90 and east of the James River have the highest chances for seeing the initial development of severe storms after 3 pm. With the more unstable environment continuing to push northwards as the afternoon goes on, the overall parameter space looks to widen to parts of east central South Dakota, southwest Minnesota, and northwest Iowa. Any chance for strong to severe storms will come to an end by midnight though it could be a little earlier depending on how quickly the storms move eastwards. Light showers may persist through the night.

What was described above is the most likely scenario for tomorrow. The things that can alter the forecast is if morning low level cloud cover does not clear out as expected and the overnight MCS ends up pushing the effective boundary further to the south, preventing it from returning as far northwards tomorrow. Will have to see how things evolve tonight to get a better idea of how the warm sector will evolve tomorrow. Please continue to monitor the latest forecast for the most up to date information.

Strong to severe storms remain possible on Monday but this will be highly dependent on Sunday's storms. If the cold front is pushed southwards more quickly than currently anticipated, then a lower severe weather chance can be expected. If the boundary's location remains on track, the strong to severe storms will be possible over portions of northwest Iowa. Tough to say which is the most likely scenario at this time.

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 /00Z TAFS THROUGH 00Z MONDAY/

Issued at 648 PM CDT Sat May 16 2026

VFR conditions begin the TAF period. Light east/northeast winds with a few gusts up to about 25 knots persist early this evening. Winds will not only persist but strengthen through the evening hours with gusts peaking at about 30 knots overnight. Storms that have developed along I-80 early this evening will begin to push north/northeastwards. These showers and storms will persist through the night. Large hail and strong winds could accompany the strongest storms. Ceilings will be lowering throughout the night though any ceiling that drops below VFR levels will only occur in showers and storms. More widespread MVFR ceilings will push into the area tomorrow morning as winds begin to turn southeasterly. Ceilings may lower to IFR levels along and west of the James River. A few showers could continue through the morning hours as well. Ceilings look to lift back to VFR levels south of I-90 tomorrow afternoon, especially along and east of the James River. This looks to result in new thunderstorm development to end the TAF period.

FSD WATCHES/WARNINGS/ADVISORIES

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