textproduct: Sioux Falls

KEY MESSAGES

- 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 2 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.

- Tuesday-Thursday look to be dry, but cooler with highs in the 50s Tuesday, and 60s Wednesday and Thursday.

UPDATE

Issued at 358 AM CDT Sun May 17 2026

The forecast remains largely unchanged for today. Ongoing rain showers over northwest Iowa will continue to taper off through mid- morning. A few isolated light showers may linger into the late morning, but for the most part the rest of the morning and early afternoon will be dry. Clouds will be slow to clear out this afternoon, which may limit surface heating and therefore destabilization. This could impact when and where convection initiation occurs. However, a very strong push northward of a theta e ridge in the mid-afternoon may rapidly destabilize the lower atmosphere, overcoming the limitations of the clouds. MUCAPE increases with the theta e ridge, climbing as high as 2500-3500 J/kg. Mid-level lapse rates of 8+ Deg C/km and 0-6 km bulk shear of 40-50 kts at a roughly 45 degree angle to the cold front round out an environment ripe for supercells. One last hiccup to convection initiation is a moderately strong cap. Soundings indicate convective temperatures are reached in the early to mid afternoon, breaking the cap and resulting in explosive convective growth. At that point the strongest supercells will form, as well as the highest risk for tornadogenesis.

Along with the question of when convection will initiate is how far west? The 19.07Z HRRR and RAP indicate initiation in the early afternoon over central South Dakota. While the NAMnest, ARW, and FV3 hold back on convection until early evening over southeast South Dakota and northwest Nebraska. That being said, the most likely timing of the severe threat will be between 2 pm and 12 am CDT. All hazards are possible with these storms: hail of 2-3 inches, damaging wind gusts of 60-70 mph, and a few tornadoes.

As the storms progress east deep layer shear becomes more parallel to the front, which will result in discrete cells congealing into a line. Some bowing segments are possible. At this point the main threats will shift to damaging wind and hail, but tornadoes in embedded supercells cannot be ruled out. Storms are expected to push east of the region around or shortly after midnight. There are additional chances for thunderstorms Monday afternoon. For more details, please see the discussion below.

As always, please be weather aware, especially if you have outdoor plans. Know where your nearest shelter is and take action immediately if your area comes under a severe weather warning.

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 /06Z TAFS THROUGH 06Z MONDAY/

Issued at 1108 PM CDT Sat May 16 2026

Focus on the aviation forecast is for convection initially tonight, then low clouds with MVFR ceilings developing on Sunday, then additional thunderstorms during the afternoon hours. Isolated showers and thunderstorm are expected to the south of I-90 during the overnight hours on the north side of the much stronger area of thunderstorms well to the south in Nebraska. Generally lower confidence on these showers/thunderstorms hitting KFSD/KSUX, so have handled with PROB30. With the primary convection being farther to the south, does look to be a lull in the precipitation on Sunday morning, as the low stratus develops over the area. The focus then turns to the potential for severe thunderstorms to develop during the afternoon and continue through early evening. Confidence is lower on exact locations, especially farther northwest into less instability, so handled with PROB30, except at KSUX, where greater confidence is in place for storms evolving into a line in the late afternoon.

FSD WATCHES/WARNINGS/ADVISORIES

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