Sometimes after a large thunderstorm complex has passed, an area of very cool air can drop down from the mid-levels of the atmosphere. In certain, hard to predict circumstances, this can produce a small-scale high-pressure low-pressure situation called a wake low which can cause very strong winds to blow for an hour or two. The situation defies standard National Weather Service warnings because the winds do not last long enough (three hours) for a typical high wind warning but are not part of severe thunderstorms, either, so the severe thunderstorm warning is not appropriate, either. Wake low winds are usually found in a region of cloudy, cool air. Temperatures are often in the 60s. Sometimes they are just a nuisance at around 20-40 mph but can be as strong as 50-80 mph and are capable of doing considerable damage. These wake low winds happen almost every summer somewhere over the Dakotas and Minnesota, but are relatively rare for any one location.
Meteorologist John Wheeler