The main concern over the next 12 hours is severe weather with a vigorous storm system. A deep upper level low, which is cold air aloft, is moving out of the rockies and a rapidly dropping surface low pressure is moving toward Minnesota. We look at both these features as the cold air aloft creates instability (if you want warm, moist air to rise you cool it off above). This is causing air to quickly move upward and that creates suction, literally so air is moving toward the low pressure center. That's why we have strong south winds today, not just at the surface but aloft.
At 3,000 to 4,000 feet up winds have been roaring at 60mph+ and will strengthen this evening to over 70mph. That's how we are able to pump very warm, humid air in so quickly. For Minnesota, most of this is coming in above the ground. That's right, sorry: we won't see the summer warmth areas just to our south will experience. Highs across Nebraska will be well into the 80s and increasingly humid.
That means the storms across much of Minnesota will be 'elevated.' I discussed this yesterday but basically the base of storms will be several thousand feet above the ground. They'll still be organized with some super cells (rotating updrafts) but it's harder to get tornadoes to develop when the storm base is so high. In Iowa, that will be a different story- hence the higher risk is south. With that said they're such intense shear that many storms will rapidly organize & rotate. Hail and high wind gusts will be a common issue in southern Minnesota. Semis were blown over on I-35 with hail blanketing Northfield already with what would be a relatively minor cell otherwise.
Above is plotted evening shear. Shear is either winds moving in different directions at different heights or at much different speeds so that the net affect you can imagine is wind rotating horizontally like a big Eddie. Strong thunderstorms then tilt that horizontal rotation vertical and BOOM you have a rotating updraft which organizes a storm, allowing it to grow more intense and in some cases develop an eventual tornado.
Shear along with instability and other factors combine to produce a 'significant tornado parameter' which allows us to try to discern the areas where all these things come together the most:
Moral of the story this evening: the Twin Cities has best odds of seeing some large hail and isolated high wind gusts in storms. The best chance of tornadoes is in Iowa, but because the intensity of several parameters are so high; even though southern Minnesota will see more elevated storms, they could still produce isolated tornadoes.
We'll be watching eastern SD/NE for development of the cells of concern this evening. The storm prediction center has circled that area:
From there storms will move east and northeast. That's another reason why we don't need the 'perfect set up' in southern Minnesota because these intense thunderstorms can persist on their own for a bit once they move into far southern Minnesota this evening/overnight.
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