Last week, (September 22, 2017) we welcomed fall with the autumnal equinox. But it seems Mother Nature missed the memo. From September 20-27th, Midwesterners and Northeasterners, from Minnesota to Maine, experienced an early autumn heatwave. The rare heatwave hiked daily highs into the 80s and 90s, with high humidity – conditions more characteristic of late July and August than late September. More than 600 daily high temperature records were tied or broken.
On September 23rd, Chicago reached a high of 95º F, setting a new record for the latest date in the season that a temperature of 95º F or more has been observed. (The previous record was 99º F set on September 15th, 1939). Likewise, Burlington, Vermont also observed its warmest temperature for this late in the season when thermometers climbed to 90º F on September 27th. It’s the latest 90º F temperature since the city’s records began in 1884.
Other areas around New England broke records as well:
Elsewhere, cities tied or set new records for the highest temperature observed not just in September, but in any month. At Bangor, Maine, the high of 89º F on September 24th broke the previous record of 88º F set in 1930. Temperatures in Detroit, Michigan didn’t reach an all-time high, but on September 26th the mercury hit 93º F – the city’s hottest day so far in 2017!
Even hot in Canada
Not even locations as far north as Montreal, Canada could escape the heat. According to Environment Canada (the equivalent of the NOAA National Weather Service here in the US), Montreal, which normally sees daily highs of around 17º C (62º F) this time of year, recorded highs in the upper-20s C. It was the first heatwave the city had all summer – even though it’s technically no longer summer.
Why so hot?
Observing 90-degree-temperatures in September isn’t unheard of, but observing a September heatwave – a 3-day or longer period of 90F or higher temperatures – is quite rare. Especially a heatwave in the northernmost part of the United States. The last time a similar event happened was September 2015, but that event occurred in early September over the Labor Day weekend and it wasn’t as long-lived.
So, why the historic week-long heat wave?
Blame it on a large area of high pressure over the Great Lakes and Ohio Valley, whose sinking air produced stagnant, hot weather for days on end. Believe it or not, even hurricanes Lee and Maria may have been partly to blame. Their very presence in the Atlantic created what meteorologists call a “blocking pattern.” Blocks keep weather patterns in the upper atmosphere from changing, so the ridge of high pressure already in place stays in place, and the jet stream’s southerly winds, which were steering warm air into the Midwest and Northeast, kept steering it in.
The fact that it’s been a dry September only exacerbated the warming from these weather patterns. Much of the region hasn’t seen measurable rainfall for two or more weeks; because dry ground heats up faster and more efficiently than damp ground, it didn’t take much work for air temperatures to elevate – and fast.
Has the recent warm spell left you nostalgic for summer? Or are you 100% ready for fall? Tell us in the comments below.