Update: 9:40 pm CDT Sunday
A tornado watch remains in effect until 2 am CDT Monday for southeastern portions of KELOLAND. This means that conditions will be favorable for strong to severe storms that are capable of producing tornadoes. We may also see large hail and damaging winds with any storm that fires up as well.
A rare combination of severe weather ingredients will create a very complex and potentially dangerous weather situation the next 24 hours as severe weather threats develop over the region. It is important to be informed of the latest forecasts and have a plan in place should severe weather develop in your area. This discussion is intended to give you our best insight into the chronological order of events as we see them.
Tonight: Early Evening 7-9pm
Cloud cover remains thick over the Sioux Falls area late this afternoon, but partial clearing in NW IA has allowed temperatures to climb into the mid 80s. Given the amount of instability and upper level environmental conditions, thunderstorms with large hail (golf ball size or bigger) and a conditional tornado threat may develop across southeast SD by early evening.
It appears these initial storms will be more isolated if they develop, but be alert to an approaching storm moving from southwest to northeast.
Tonight: Late Evening, after 9pm
A more organized round of storms is likely to develop later this evening after 9pm and migrate into portions of southeast SD and northwest Iowa. Keep a very close eye along an O’Neill-Yankton-Sioux Falls-Worthington axis late tonight.
We are more likely to see pockets of very strong winds (60-85 mph) with these storms as they move in clusters to the northeast. Large hail and a tornado threat may continue after dark. We would highly recommend having device nearby to alert you to a hazardous storm while you are sleeping, such as a NOAA weather radio.
Storms may still be ongoing in the early morning hours, adding to the complexity of the forecast for Monday. First, it is important to recognize the potential for a very dangerous forecast with the potential for several tornadoes in eastern SD, NW IA, and southwestern MN during the daypart forecast. The severe weather will likely develop much earlier in the day. There even exists the potential for a couple of large, long-track tornadoes in our viewing area.
The most likely scenario depends on morning thunderstorms moving north and west of the James Valley before 9am, with partial clearing expected in much of southeastern KELOLAND. While severe weather is possible in central KELOLAND near the surface low track, most of the attention will be focuses on the highly volatile atmosphere farther east, specifically east of the James River Valley. All the ingredients for severe weather will be in place as early as 11am or noon.
The main question is what mode of storms will be favored. If the storms take on more of a line or linear mode, hail and high winds may accompany a threat of a few tornadoes as the line develops in the zone bordered between Mitchell, Yankton, and Sioux Falls. This line will be move quickly to the northeast into western MN and adjacent areas of east-central and northeast SD. While this outcome would produce severe weather, it is the lesser to the two situations being considered.
The other school of thought focuses on the storms remaining discrete supercells for a longer period of time. If this second scenario develops, we will be facing a larger tornado outbreak, probably the largest in a few years for this area. As mentioned, this even opens the door to a couple of tornadoes developing and staying on the ground for many miles. These situations, while rare in KELOLAND, do occur and the Storm Prediction Center has already highlighted this possibility in the latest Day 2 outlook. We will know much more about which direction the Memorial Day’s forecast will go after tonight’s storms diminish. Either way, severe weather is expected as you should be monitoring the weather frequently through the day.
After the storms pass through, a period a very strong southwest winds with gusts to 70mph are possible in southern and southeastern SD. If you are towing a camper or driving a high profile vehicle late Monday afternoon or evening, we would advise being aware of this non-thunderstorm wind threat.
We will be updating the forecast frequently in the next 24 hours. Our KELOLAND Storm Center staff is ready and fully prepared to cover breaking weather in our Upper Midwest region.