But a bigger change is coming this weekend. An upper-level low in the Gulf of Alaska is challenging the warm upper ridge that has spread across the western United States into central Canada this week. The first pulse will bring a cold front through the north of the country on Thursday and Friday. Temperatures will drop slightly, but for the Dakotas this could be significant. Temperatures will top 25 degrees Celsius on Wednesday and Thursday, and a few 90 degrees Celsius could creep into South Dakota. However, high temperatures on Friday are expected to be in the 60s or lower 70s. There may be a few showers as the front crosses the northern Plains, but it should be mostly dry.

A much bigger change is coming with another pulse from the upper trough in the Gulf of Alaska this weekend. It will send a system from the Canadian Prairies on Saturday through the Great Lakes on Sunday, almost like a clipper system we would see in the winter. In fact, the cold front will move south across the U.S., followed by a second blast of cold air about a day later, reaching the Gulf of Mexico by Tuesday. True to a clipper system, there won’t be much precipitation, although isolated showers will move across the Great Lakes area. The cold air flowing over the warm waters of the Great Lakes is also expected to cause a few lake showers on Monday and Tuesday, like a clipper ship. It will be too warm to produce snow, which is sure to please people in the region. But that just shows how much colder this air mass is likely to be.

Models have increased the intensity of colder air in recent days and forecasts are expected to change by a few degrees in the coming days. However, behind the main front lasting this weekend into early next week, temperature drops of around 20 to 30 degrees are forecast. Below are the forecast maximum temperatures for several areas that are expected to see a significant drop as the fronts move through. The freeze is likely to occur further south than before, possibly reaching Kansas, Missouri and the Ohio Valley by early to mid next week.

The cold will last longest in the eastern states as an upper-level low forms over the eastern U.S. next week. But the ridge, which has been a persistent feature from west to central Canada, will revive next week and cause temperatures to rise again, with above-average temperatures through the middle of next week in the plains and later in the week further east.

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John Baranick can be reached at [email protected]

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