November warmth: A new trend?
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Twin Cities NWS graphical forecast showing 60s Saturday.
Many are welcoming a change to warmer temperatures this weekend. There is a history of Indian Summer in early November in the past few years. Three of the past 4 Novembers have featured warm spells early in the month with temperatures reaching above 70 degrees in the Twin Cities at least one day.
While early November warm spells seem to be more common, there are several unusual and remarkable aspects to the warm spell this year.
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It is interesting to note that the sun angle and intensity at this time in November is about equal to the first week in February. Yet temperatures will be much warmer in November even with the same amount of solar energy coming in. There are some obvious reasons why this happens.
-The ground is free of snow cover right now. That means that instead of solar energy reflecting back into space on higher albedo (more reflective) snow cover, more incoming solar energy is available to heat the lower atmosphere in November than February.
-There is still plenty of warm air to the south in late fall to be moved (advected) into the Upper Midwest.
-Overall, the atmosphere at this latitude is warmer in November than in February due to the seasonal lag (residual heat) of late summer and fall.
It is also interesting to note that while it was in the upper 20s in the Twin Cities this morning, it was in the 50s in Denver at over 5,000 feet elevation. Why is this? Denver is getting downslope flow, which warms the air mass east of the Rockies. Also, some of the air mass moving into Colorado is being transported form Arizona which saw record highs in the 90s again Wednesday.
Our warm up this weekend will also feature some remarkably warm overnight minimum temperatures. It may not drop below freezing again in the metro overnight until the middle of next week. That is remarkable for November, when our average low is now running about 29 degrees in the metro this weekend. In fact, the minimum Saturday morning may be in the mid 40s. That's our average high for this time of year.
Enjoy the warm up!
PH