Forecast: Indian Summer

We've earned this.

After a wet and chilly October and cool blustery start to this week, the forecast is looking up for this weekend. It looks like Indian Summer may pay a visit to Minnesota.

The AMS glossary defines Indian Summer this way: A period, in mid- or late autumn, of abnormally warm weather, generally clear skies, sunny but hazy days, and cool nights.

The weather maps this weekend look to fit the bill.

Indian Summer occurs in the Midwest when we get a large slow moving ridge of high pressure overhead in the upper atmosphere. As you can see from the 500 millibar (18,000 feet) chart below, significant ridging is forecast by Friday.

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Upper air forecast 6pm Friday evening Minnesota time.

At the surface, cool high pressure will begin to slide east of Minnesota Thursday. This opens the door to southwest winds which will begin to pump warm air northward into the region.

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Surface forecast 7pm Friday shows high pressure over the eastern states and a milder southerly flow in the Upper Midwest.

The southerly wind flow may persist right through Monday. That will allow plenty of time to bring milder air north. Combined with daytime sunshine, that should help boost afternoon temperatures into at least the upper 50s and low 60s in southern Minnesota Friday into the weekend. I would not be shocked to see a 70 degree reading close to the southern Minnesota border.

It is generally accepted that at least one killing frost and a period of cool weather has occurred before we can claim Indian Summer. We definitely meet those criteria this year.

The term Indian Summer has some history and regional differences attached. Again, from the Glossary of the AMS:

"In New England, at least one killing frost and preferably a substantial period of normally cool weather must precede this warm spell in order for it to be considered a true "Indian summer." It does not occur every year, and in some years there may be two or three Indian summers. The term is most often heard in the northeastern United States, but its usage extends throughout English- speaking countries. It dates back at least to 1778, but its origin is not certain; the most probable suggestions relate it to the way that the American Indians availed themselves of this extra opportunity to increase their winter stores. The comparable period in Europe is termed the Old Wives' summer, and, poetically, may be referred to as halcyon days. In England, dependent upon dates of occurrence, such a period may be called St. Martin's summer, St. Luke's summer, and formerly All-hallown summer."

It looks like we could be pushing 60 in the metro by Saturday. Our average high for Saturday in the Twin Cities is 45 degrees. That would put us a whole 15 degrees above average.

Thursday through Sunday will be a great opportunity to get those leaves up and get other fall chores put to bed. Oh yes, and don't forget to take a long stroll and bask in the sunny and milder Inidan Summer sunshine!

PH