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Wednesday, January 1, 2020

Why January usually isn’t a big snow month for Denver - The Denver Post

January is Denver’s coldest month of the year, and it’s the heart of winter. Despite the typical cold and time of year, though, January is also not traditionally a big snow month for Denver.

While Denver’s long-term average of 6.6 inches of January snowfall isn’t anything to sneeze at, it’s actually the least snowy month of the winter on average (December-February), and it’s only the sixth-snowiest month of the year. That 6.6-inch total falls behind November and April, and it’s only a bit ahead of October (4.2 inches).

Consider this: only one of Denver’s top-20 snowstorms on record have fallen in January. That means Denver’s seen as many top-20 snowstorms in January as the city’s seen in September, and considerably less top-20 snowstorms than both October and April.

“It just seems like (the Front Range doesn’t) get big snows in January,” said Jim Kalina, a meteorologist at the National Weather Service in Boulder. “It’s just difficult.”

It’s not just a relative lack of blockbuster individual snowstorms in Denver, either. Denver has only had three Januarys on record with more than 20 inches of snow. Compare that with 20 different Marches with at least 20 inches of snow, or 10 Aprils or 9 Novembers. It’s a big gap in higher-end months, particularly considering the deep cold that usually makes a rainy or icy mix less likely.

It’s a completely different story, though, an hour’s drive west on Interstate 70.

While January isn’t typically a month known for blockbuster snowstorms along the Front Range, it’s usually when the mountains get slammed with their biggest and most consistent snows of the season. January is Aspen’s,Steamboat Springs‘, Vail’s and Wolf Creek‘s – just to name a few mountain locations – biggest snow months of the year, on average.

But the Front Range often gets shut out from the big Pacific storms that produce some of the prolific mountain snows. Denver’s location in the lee of the Rocky Mountains – east of the prevailing westerly winds – means Denver often gets warmer and drier downsloping winds, while the mountains get consistent, steady rounds of snow, largely courtesy of moisture originating in the Pacific.

The southern jet stream is normally pointed straight at the West Coast this time of year, creating a firehose of Pacific moisture for much of the western third of the country (though which exact part of the West can often depend on El Nino’s or La Nina’s influence on the jet stream).

Again, though, that same strong jet stream often helps create a Front Range snow hole, due to downsloping winds east of the Rockies.

It’s part of the reason why Denver has had more Januarys with at least one or more days of 70-degree weather (14) than Januarys with 15 inches or more of monthly snowfall (10).

Denver and the Front Range generally get their biggest snows of the season from big areas of low pressure that draw up moisture from both the Pacific and the Gulf of Mexico. These systems often get their energy from a sharp temperature contrast between warm and cold air. That temperature contrast is usually highest during transitional seasons like spring or fall – and not in the middle of winter. That creates less overall fuel for big storms.

And when they do develop, those big low pressure systems also often tend to track south of Colorado this time of year, where the temperature contrast is highest and where the jet stream is often oriented.

“The storm track is farther south (in January), typically,” Kalina said. “We either get that northwest flow aloft, or the low pressure systems seem to track across Arizona and New Mexico. When you get into February, that track starts to lift north.”

Additionally, colder air can’t hold as much moisture as warmer air can, typically creating less favorable conditions for higher-end snow events.

The end result is that January is usually a month with a few smaller snow systems for the Front Range, with most snow events producing lighter overall totals. That’s unlike October, March or April, for example, where a much more hit-or-miss nature to those months depends on bigger snowstorms. That can skew those overall monthly average totals higher, but those higher monthly totals are often the product of just one or two big storms.

So while January might be a peak snow month for the mountains, it’s not usually a big one for Denver and the Front Range. But if you’re a Front Range snow-lover, your peak time is just around the corner.

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Why January usually isn’t a big snow month for Denver - The Denver Post
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