Get Big Pivots

Just how out of the norm is this wacky weather in Colorado in coming days?

by Allen Best

Snow during September along Colorado’s northern Front Range hasn’t been that unusual if you go back a century or more, nor is it unusual to have wide temperature swings.

But the temperature swing of this week  is predicted to be notable for its extremes. And the snow will be on the early side, too.

In the Denver area, the high temperature over the Labor Day Weekend is forecast to be 98°. By Tuesday, it will be snowing and with a low of 30° or maybe less.

Bye-bye bean harvest.

Matt Kelsch, a hydrometeorologist with the National Center for Atmospheric Research in Boulder, says September snow is not that uncommon in the Denver-Boulder area.

“If you look at records for the 20th century about once every four years there’s measurable snow in September, sometimes a big one. It’s not that unusual,” he says. In the last 20 years, though, that has happened only once.

Even in the 20th century,September snows were likely to be later in the month. Snow on Sept. 8 shows up only once in the record.

Temperature extremes have also occurred before. “In 1993, we had a high of 90 on the afternoon 12th and the morning of the 13th we had an inch of snow and 33° at 7 a.m.,” he said.

This week’s predicted temperature plunge will not be faster, it will be more extreme: from the high 90s to below along the Front Range.

West of the Continental Divide temperature swings will be as great, or greater. The Weather Channel predicts a high of 93° F in Craig followed by a swoop to 17°.  At Vail, the swing is from 80° to 24°. Granby was predicted to a high of 82° over the weekend followed by a low of 14° on Tuesday. Talk about free fall!

There’s a very real chance that we set or tie record highs on Sunday or Monday and then set or tie a record low on Wednesday, says the National Weather Service office at Grand Junction on its website. “Hang on for the ride!”

Southeastern Colorado expects to be a on a roller coaster, too. Lamar is predicted to hit 105° on Sunday and 33° on Tuesday.

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Is this greater extreme a reflection of the warming and changing climate?

Yes and no, Kelsch says. He reminds his interlocutor of the distinction between weather and climate, the latter being the long-term weather patterns. Or, as is often said, climate is what you expect and weather is what you get.

In the long term and big picture, more record highs have been accumulating than record lows. “This is a good example of how we’re seeing more of the extremes, sometimes in proximity to each other.”

What clearly stands out in an inspection of the records for the last 30 years in Boulder, where Kelsch maintains a weather observation station, is a shifted pattern in precipitation. July and August have become drier, but February, March and April have become wetter.

This year fit in with that pattern. It also fits in with trends around the world. Climate change theory forecasts longer, more intense droughts but, in places, greater spurts of precipitation.

This shift along the foot of the Front Range sets up a greater risk of wildfire. The increased late winter—and spring precipitation results in growth of more grasses, which in turn is followed by higher, drier summer.

The hot temperatures this weekend will heighten the fire risk. “The only good news is that if a fire does start, it won’t have long to live,” Kelsch observes.

From Tevas to SmartWool, air conditioners to furnaces, it’s going to be a big pivot.

You can see Matt Kelsch’s weather blog here.


Allen Best
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