Colorado Weather Forecast #155: Dec. 15 - Dec. 19, 2021

Updated Tue Dec 14, 2021 7:00pm MST | Published Tue Dec 14, 2021

By Thomas Horner

7-Day Snow Planner (Through Dec. 21)

Summary

Hello everyone. Hope you enjoyed the snow (and more recently, the sun). We try to keep on top of writing these, but we’re busy! This is merely a passion project for all of us (that we throw a decent amount of money at) and sometimes life and work get in the way.

• Storm tonight (Tuesday night) into Wednesday. Very windy, powder day in the San Juans, lower totals elsewhere.
• Weak storm Thursday evening into Friday, mostly impacting northern Colorado.
• Potentially chilly and unsettled this weekend.

Retrospective: Dec. 8-11

We’re a little late circling back to this. As you all probably know, the San Juans got hammered with 2+ feet of snow, along with the Elks (Aspen and Crested Butte) and other areas of western Colorado. Some of the top reports we collected:

• Wolf Creek: 34”
• Crested Butte: 31”
• Purgatory: 29”
• Buffalo Pass: 28”
• Aspen Highlands: 26”
• Telluride SA: 26”
• Aspen Mountain: 24”
• Powderhorn: 23”

As we were expecting, totals dropped off sharply in the eastern mountains. Here is the hall of shame:

• Eldora: 2”
• Loveland SA: 5”
• A-Basin: 6”
• Winter Park: 7”
• Berthoud Pass: 7”

Denver finally broke its streak of snowless days (tying the record of 232 days set in 1887) with an observation of a whole 0.3” of snow at the airport. This sets December 10th as the latest first snow on record for the city.

As for model and forecaster performance, here’s how it shook out.

An honorable mention is the Highpoint Weather app, whose forecast verified at 80% of the locations we checked. However, we aren’t including it on this list due to its massive forecast ranges (avg. spread of 13) – for instance, an 8-24” forecast for Aspen Mountain or 7-23” at Beaver Creek. Still, no low busts and a few high busts. It really struggled with the uncertainty, hence the large spreads at a lot of locations – was its forecast useful?

Overall, there was a strong dry bias across the models and forecasters. We had zero low busts – underpromise and overdeliver, right? In general, the San Juans and Elks blew away expectations while northern Colorado and Summit County were more of a mixed bag.

For all of the details, check out the interactive tool.

Here are the overall stats. For the average, we’ll only look at the past two storms as those were the only ones with all the models on board.

Overall Verification Rankings

1. Thomas Horner (3 wins) • Avg. 75%
2. OpenSnow • Avg. 65%
3. NAM 12km • Avg. 60%
4. CAIC WRF • Avg. 57%
5. NWS NDFD (1 Loss) • Avg. 56%
6. NBM Det. • Avg. 51%
7. NBM 50/90 • Avg. 50%
8. GFS 20km • Avg. 47%
9. ECMWF (Euro) (1 Loss) • Avg. 45%
10. NBM 25/75 (1 Loss) • Avg. 37%

Score Rankings

1. Thomas Horner (1 Win) • Avg. 25%
2. NAM 12km • Avg. 25%
3. OpenSnow • Avg. 23%
4. ECMWF (Euro) (1 Win, 1 Loss) • Avg. 22%
5. GFS 20km • Avg. 22%
6. CAIC WRF • Avg. 20%
7. NWS NDFD (1 Loss) • Avg. 18%
8. NBM 50/90 • Avg. 17%
9. NBM Det. • Avg. 16%
10. NBM 25/75 (1 Win) • Avg. 9%

Forecast Discussion: Wind and Snow, Wednesday Dec. 15

Another storm is baring down on Colorado. A shortwave will pass over Colorado on Wednesday, bringing another shot of snow to the mountains.

This is tracking fairly far south, so the jet stream will bring southwest flow across Colorado during the period of best lift and moisture. The wave will undergo lee cyclogenesis after it passes over the Rockies, turning into a strong storm that will bring powerful winds and heavy rain and snow to parts of the midwest. Unfortunately, its track is too far north to bring much snow to the Front Range east of the Continental Divide.

Strong Winds!

There is a tight pressure gradient associated with this storm. Outside of western Colorado and the San Juans, the most impactful feature of this system are likely to be winds.

Here’s what the HRRR model thinks the strongest wind gusts will be on Wednesday:

This model is showing 100+mph gusts west of I-25 – metro area included. This means Broomfield, Westminster, Boulder, Arvada, Golden, and Morrison in addition to many locations in the foothills. The strongest gusts are forecasted for the high Sangres (near Culebra Peak), with the HRRR putting down a 135mph wind gust.

Other weather models agree. Our blended forecast looks like this:

The more realistic forecast in the metro area is for 40-80mph gusts from the morning to the late afternoon, but 100mph is possible near the base of the foothills (or in the foothills). If you live near, or west of I-25, make preparations for destruction and loss of outdoor property!

Here’s how Wednesday is displayed in the new version of our app. You can also see that an inch of snow is possible in the Denver metro area on Wednesday morning, though unlikely.

Snow

What about snow? Well, it gets weird. This storm is moving fast, and winds are intense. We’ll likely see a powerful front push through Colorado from west to east early this morning, with some very strong gusts and almost no visibility.

This front will be moving at around 30mph and step down off the foothills around 6-8am, bringing a quick burst of heavy precipitation (likely rain) to the I-25 corridor.

At same time increasingly deep upslope flow is starting to kick snow off in the San Juans. As winds increase overnight, precipitation will get less consistent and start to organize into bands of heavy snow, delivering impressive snowfall rates to some areas while leaving others just 10 miles away fairly dry.

Wolf Creek looks to be favored over the western San Juans, with well over a foot of snow possible across most of the southern side of that mountain range by noon on Wednesday.

Some of these bands of snow will kick north and east early Wednesday AM, providing chances for a few inches of snow for Monarch, Crested Butte, Independence Pass, Breckenridge, Copper, and possibly up into the eastern I-70 resorts (Loveland etc.)

Otherwise, the first real taste of snow will be that powerful front moving west to east through Colorado from 3am - 9am. This could bring snowfall rates up to 3”/hr to most of Colorado’s mountains. This feature is tricky to forecast as it will be moving fast and the actual spots of heaviest precipitation along the front are difficult to pinpoint. However, we would expect a few areas to open at 5am with up to 4” of snow just from this impressive band of snow.

Otherwise, snow becomes more westerly by the late morning, which should bring several inches of snow to the Vail area, Copper area, Monarch area, Park Range (Steamboat), Elks (Aspen/Crested Butte), and San Juans, with lower totals to the east. Snow will wrap up by Wednesday night with drier air and subsidence taking hold over the state.

Let’s take a look at snow distributions and potential totals. We think the Blend is reasonable with the overall distribution and snow potential.

A legit powder day in the San Juans tomorrow, and likely for the Elks and Park Range. The potential is there for Vail and western Summit County, but it’s possible for only a few inches. The eastern resorts wont see much.

If everything goes right?

We think that a couple feet at Wolf Creek isn’t out of the question, but our forecast is 10-20”. The Vail area may be underdone a little bit, with up to 6-7” perhaps. Our official forecast will come in a bit.

If everything goes wrong?

Still a good event for the San Juans, but only a few inches elsewhere.

Regardless, outside of the San Juans, most snow isn’t coming until after the lifts start spinning. It’s quite possible that many lifts wont even spin, thanks to the strong winds into the afternoon!

Here’s our official forecast:

Forecast Discussion: Storm Thursday Dec. 16 PM to Friday Dec. 17 AM

A weak wave traverses over our region on Friday.

Lift ahead of the wave, along with some shallow moisture, bring snow from about 5pm Thurday to 5am Friday, though light snow will persist over the northern Colorado mountains (especially the Park Range) into the afternoon. This is mostly a 1-5” event for most mountains, with even lower totals in the San Juans. In the Park Range (Steamboat and Buffalo Pass), this would be more of a 4-8” event.

Very little snow is forecasted east of the Divide.

Forecast Discussion: The Weekend

A backdoor cold front brings more wintry temperatures to Colorado, including for the Front Range, with a relatively chilly weekend forecasted, though there’s enough uncertainty with the storm track where temperatures could be a bit more mild.

An incredibly weak wave could make Sunday relatively unsettled in northern Colorado, though only light snow is forecasted in the best case scenarios. We’ll see if this wave evolves any, but chances of >1” of snow are much less than 25%.