DENVER — Denver just experienced its warmest September on the books, and the state’s refusal to give up on summer will likely delay our first snowfall of the season, a National Weather Service forecaster said Wednesday.
The average temperature in Denver during the month of September was a balmy 70 degrees – 5.2 degrees above normal, according to a monthly climate summary released by the NWS office in Boulder.
That number puts September 2024 as the warmest September in Denver since record-keeping began in 1872, and the fourth consecutive month with above normal temperatures and below normal precipitation.
Historical data from the NWS in Boulder shows just how much warmer the month of September has gotten over the past several years.
Out of the top 10 warmest Septembers on record, five of them have occurred within the past 9 years, according to the NWS, with all showing a temperature variation of less than 3 degrees.
It’s as this point that you’re probably wondering, “How much is climate change impacting this trend?”
“When it comes to climate change, it's hard to pinpoint one particular month, or even one particular year, that indicates the climate change aspect,” said Greg Heavener, the warning coordination meteorologist for the NWS in Boulder.
But, he added, “if you look at the culmination of several events, several years’ worth of data, the warming and the drying climate is becoming more pronounced, and that is leading to these warmer, prolonged spells, even into the fall, early winter months for us here in Colorado.”
In all, there were nine days in which the temperature reached or exceeded 90 degrees during the month of September. Those warmer than normal temperatures, coupled with drier-than-usual conditions (precipitation was slightly below normal in Denver for the month), spells trouble for the years ahead if these warming trends continue, Heavener told Denver7 Wednesday.
Without the additional amounts of moisture like rain or snow, he said, Colorado’s headwaters will be under strain and people who rely on the rivers that develop from the mountains – from recreational water use to agriculture – will be affected.
“With the warming and drying nature of our climate going back, really, the past 30 to 40 years – which is fairly pronounced – that's just going to exacerbate, so each time we get a hot, dry pattern, it kind of just begins to build upon itself,” Heavener said.
If Denver does not reach 90 degrees Wednesday — as is forecast to happen by the NWS, breaking the record for the latest 90-degree day — the Mile High would sit at 64 days where temperatures have reached 90-plus degrees.
Heavener said Denver has typically averaged between 55-58 days over the past 20 to 30 years of 90-plus degree days, with a few exceptions — such as last year, when the NWS recorded only 39 such days.
This year? “We're close to doubling what we had all of last summer, this summer,” he said.
“If we set (90 degrees) today or even on Saturday,” putting Denver at either 65 or 66 days of 90-plus degree weather for calendar year, “that puts us, I think, second or third most 90-degree days in the metro's history,” Heavener said.
And with La Niña now in full swing, going into the winter months won’t be as cool — and I just don’t mean that figuratively.
“We are expecting, unfortunately, a warm and dry winter due to the La Niña conditions… it's kind of delaying even our snowfall and our first frost and/or freeze across the region, too,” Heavener said, adding Denver could expect to see the first frost/freeze in late October — about three weeks later than usual.
But even if winter starts warmer and drier than usual, La Niña could influence Colorado’s heaviest snow months, which typically occur in March and April.
Denver7 meteorologist Lisa Hidalgo said earlier Wednesday a cold front is expected to swing in late Wednesday night into Thursday night, dropping daytime highs to the low and mid-80s.
Temperatures soar back into the upper-80s Saturday in Denver, potentially breaking another heat record. The current record for Oct. 5 is 86 degrees, last set in 1997.
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