Colorado Weather Forecast #156: Dec. 20 - Dec. 26, 2021

Updated Sun Dec 19, 2021 7:00pm MST | Published Sun Dec 19, 2021

By Thomas Horner

7-Day Snow Planner (Through Dec. 27)

Summary

Good news everyone! The potential pattern change for the end of December that we hinted about a few weeks ago looks to be materializing as a blocking pattern sets up across the Northern Hemisphere.

We’ve also added a page dedicated to our forecast comparisons and verifications. You can see how forecasters and weather models are doing overall, in addition to the storm reports and comparisons for ongoing storms and those in the past. After last week’s snow, we continue to have the most accurate forecasts of any service and weather model that we’ve checked.

• Mild and dry through Wednesday, generally sunny.
• Windier on Tuesday and Wednesday, gusts of 50-80mph along parts of the Divide.
• Strong storm starts December 23rd, largest impacts through December 24th are in the San Juans and in the western Colorado ranges.
• Ongoing snow, with a few small breaks, likely into at least the start of January.

Forecast Discussion: Mild and Dry Through Wednesday

A lot of snow is on the horizon, but we need to get through the start of the week first. We’ll spare you the boring details.

Ridging develops and remains in place over the region through Wednesday. Temperature anomalies become quite warm, especially east of the Divide.

In Denver, this means temperatures in the low 60s are possible on Tuesday and Wednesday.

A center of high pressure off the west coast of Mexico eventually puts the jet stream directly over Colorado on Tuesday.

This will bring strong gusts to the higher ridges of the Colorado mountains, particularly the Continental Divide. There will be some downslope wind gust issues for the Front Range, but overall, these winds will help drive warmer temperatures and drier conditions.

Forecast Discussion: Active and Prolonged Pattern Starting Dec. 23

Overall Pattern

A blocking pattern is starting to emerge across North America, which means the weather pattern will be more or less consistent for at least a couple weeks.

A strong region of high pressure is emerging over the Aleutian Islands (an Aleutian High), with the associated low pressure dipole located over the west coast of North America.

Another strengthening region of high pressure is developing over the North Atlantic / Greenland, and again with a complementary area of low pressure over northern/eastern Europe. Decent blocking pattern.

The forecasted development of the Aleutian High is important – studies have found a high level of coupling of this feature with the stratosphere. This should reduce the influence of the polar vortex on the large-scale weather pattern over the northern Pacific Ocean and western United States, which means its likely these features will remain rather stationary for quite some time.

This is good news for snow lovers in Colorado (and much of the western United States), as this essentially opens the door for continuous shots of energy and moisture over our region.

As an aside, studies have found that the Aleutian High events usually kick off a Polar Vortex Disturbance down the road, which could produce an exciting, powerful winter storm across a large swath of the United States (remember the storm that hit Texas in January 2021?) later in the season.

Right now, the evidence is good that this pattern will be in place at least into the start of January (Aleutian Highs usually last 20-30 days), and that it will be productive (snow-wise) for Colorado. Right now, model ensembles are fairly consistently showing that the Colorado mountains could get several feet of snow in the two weeks following Dec. 23rd.

If we look at just a blend of weather models over the next 10 days (this only covers the first week of our active pattern), we see some nice totals in most of Colorado’s high country. And remember – this model has been fairly consistently dry this season.

Of course, you’ll note that Summit County largely misses out on the action, with minimal amounts of snow east of the Divide. This makes sense, as flow will largely be southwesterly, and the pattern will be mostly stationary – it’s unlikely we’ll be seeing any discrete systems trek across the Rockies anytime soon, which would bring upslope snow potential to the Front Range.

The only significant snow chances for the Front Range seem to be:

• Cold fronts as various disturbances in flow aloft making their way across the region. These fronts would generally be weak and dry, as the coldest air is locked up in Montana.

• Strong bands of jet-induced snow jumping the Divide. This is what brought Denver its first, and only, official snowfall earlier this month – a whole 0.3”.

So not great if you want snow on the urban corridor, or even up in Summit County, but certainly an excellent pattern for the San Juans, Elk Range (Aspen and Crested Butte), and Park Range (Steamboat), with decent scraps for Vail/Beaver Creek and diminishing returns as you look further east.

Pattern Kickoff: Storm Dec. 23-25

We’ll get things started on December 23rd as a trough drops down the west coast of the United States. This will position an atmospheric river and southwest flow over the Sierras, with only limited scraps for Colorado late Dec.22 to early Dec. 23.

As the trough deepens, the atmospheric river will become located more squarely over the desert southwest, with southwest flow delivering it almost directly to the San Juan Mountains of Colorado.

If current guidance holds, snowfall rates will start to tick up in southern Colorado (Wolf Creek, Purgatory) on the morning of the 23rd before dramatically increasing during the evening and continuing well into the morning of December 24th. Snow totals would likely be 1-2 feet by the morning of December 24th.

Conservatively, up to a foot of snow is possible in the other western ranges of Colorado (the Elks and Park Range). Jet-induced banding creates another wildcard, allowing several additional inches of snow to reach just about any mountain range of Colorado, and these are difficult to forecast the location and duration of. This means we can’t rule out even Summit County getting a nice dose of snow, especially if we can get some northwesterly flow later on the 24th or the morning of the 25th.

Some of these bands could jump the Divide and bring some brief snowfall to parts of the Front Range.

As is typical for these sorts of events, temperatures aloft are pretty warm so this is not a blower pow event. The snow level could be well above 7,000ft. Snow-liquid ratios may not get above 10:1 (or even lower) until after the initial wave of deep moisture.

A shortwave embedded in flow aloft would help turn winds more westerly or even west-northwesterly by the afternoon of the 24th, but models lack any agreement over this.

Overall –

• Morning of the 24th: A powder day is very likely in the San Juans on the morning of the 24th, fairly likely in the Aspen/Crested Butte area, somewhat likely in the Steamboat area, and more questionable elsewhere, especially Summit County.

• Morning of the 25th: Plenty of leftovers and refills in the San Juans and central mountains, with a better potential for new powder in the Park Range and towards Summit County.

December 26th Onwards

We’ll be in the active pattern, but the details are highly uncertain. Overall, we’ll be looking for disturbances aloft to move over the region and tap into moisture. Models lack agreement on the timing and impact of those potential disturbances. Regardless, it seems likely we’ll see at least some snow almost every single day through early January.

We’ll start talking some more specific snow totals for the mountains early next week. If you want a taste…

White Christmas Potential

This is the chance of snow accumulating in the 24 hours preceding 5am on December 25th:

• Denver: 25%
• Boulder: 40%
• Sterling: 5%
• Fort Morgan: 15%
• Limon: 5%
• Fort Collins: 35%
• Colorado Springs: 10%
• Pueblo: 0%
• Durango: 80%
• Grand Junction: 70%
• Vail and Summit County: 80%
• Salida: 65%
• Alamosa: 60%