With the recent cold snap and frosts, I’ve heard a number of folks mentioning Indian Summer. In the fall it seems that almost any warm day is referred to by most people as “Indian summer.” And, while their error is certainly not of the world-shaking variety, they are, for the most part, in error. Here are criteria for an Indian summer:
- As well as being warm, the atmosphere during Indian summer is hazy or smoky, there is no wind, the barometer is standing high, and the nights are clear and chilly.
- A moving, cool, shallow polar air mass is converting into a deep, warm, stagnant anticyclone (high pressure) system, which has the effect of causing the haze and large swing in temperature between day and night.
- The time of occurrence is important: The warm days must follow a spell of cold weather or a good hard frost.
The conditions described above must occur between St. Martin’s Day (November 11) and November 20. For over 200 years, The Old Farmer’s Almanac has adhered to the saying, “If All Saints’ (November 1) brings out winter, St. Martin’s brings out Indian summer.”
Why is Indian summer called Indian summer? There are many theories. Some say it comes from the early Algonquian Native Americans, who believed that the condition was caused by a warm wind sent from the court of their southwestern god, Cautantowwit.
The most probable origin of the term, in our view, goes back to the very early settlers in New England. Each year they would welcome the arrival of a cold wintry weather in late October when they could leave their stockades unarmed. But then came a time when it would suddenly turn warm again, and the Native Americans would decide to have one more go at the settlers. “Indian summer,” the settlers called it.
El Niño & This Fall and Winter
We are in the midst of a rapidly strengthening El Niño event which will likely peak later this fall as one of the strongest El Niño events on record. So, what does this mean for the upcoming winter season?
El Niño has a reputation for bringing mild winters to much of the country, especially across the northern States. The two strongest two El Niño events on record prior to this year (1982-83 and 1997-98) were quite mild from the Pacific Northwest to the Northeast. Only the Southwestern States saw below average temperatures during those winters.
We do think that this winter will start off rather mild across most of the country. The map below with our forecast for December (and likely into early January) looks a lot like the El Niño winters of 1997-98 and 1982-83. However, we do not expect that this mild pattern will persist through the entire winter.
Later in the season, especially during February, we expect several weeks of classic winter weather over the eastern third of the country. This period will likely feature several winter storms from the south central Plains to the Northeast, with a higher than typical threat for significant snow and mixed precipitation deep into the South.