Colorado Weather Discussion #152: Dec. 8 - Dec. 12, 2021

Additional details, commentary, and our official forecast for the unfolding winter storm in the Colorado high country.

Contents

This forecast is no longer valid. Please return to our list of forecast articles to find more recent posts.

Updated Wed Dec 8, 2021 5:00pm MST | Published Wed Dec 8, 2021

By Thomas Horner

7-Day Snow Planner (Through Dec. 15)

231

Days since the last measurable snowfall in Denver (At the official weather station / 2nd longest streak since record keeping began)

82%

Chance of accumulating snow in Denver by Dec. 12th (At the official weather station)

Northern Mountains

Park Range and N Front Range: Steamboat, Buffalo Pass, Rabbit Ears Pass, Bluebird, RMNP, IPW, Cameron Pass

6am New Snow

1-10"

Thu

6-21"

Fri

2-4"

Sat

0"

Sun

0"

Mon

0-T

Tue

T-2"

Wed

6am Powder Potential

20%

Thu

90%

Fri

10%

Sat

0%

Sun

0%

Mon

0%

Tue

5%

Wed

I-70 Corridor

Front Range, Gore Range, etc.: Winter Park, Berthoud Pass, Jones Pass, Loveland, A-Basin, Keystone, Breck, Copper, Vail Pass, Vail, Beaver Creek

6am New Snow

0-4"

Thu

2-15"

Fri

1-4"

Sat

0"

Sun

0"

Mon

0-T

Tue

T-2"

Wed

6am Powder Potential

5%

Thu

50%

Fri

15%

Sat

0%

Sun

0%

Mon

0%

Tue

5%

Wed

Central Mountains

Elk Mountains, Sawatch Range, Grand Mesa, West Elks: Aspen Resorts, Crested Butte, Powderhorn, Sunlight, Kebler Pass, Independence Pass, Cottonwood Pass, Monarch

6am New Snow

1-6"

Thu

7-16"

Fri

2-4"

Sat

0"

Sun

0"

Mon

0-T

Tue

1-4"

Wed

6am Powder Potential

15%

Thu

85%

Fri

20%

Sat

0%

Sun

0%

Mon

0%

Tue

10%

Wed

Southern Mountains

San Juans: Telluride, Red Mountain Pass, Silverton, Coal Bank Pass, Purgatory, Wolf Creek

6am New Snow

2-7"

Thu

10-20"

Fri

1-4"

Sat

0"

Sun

0"

Mon

0-1"

Tue

3-10"

Wed

6am Powder Potential

30%

Thu

90%

Fri

15%

Sat

0%

Sun

0%

Mon

0%

Tue

50%

Wed

Denver Metro

Denver and surrounding suburbs: Lakewood, Westminster, Aurora, Highlands Ranch

6am New Snow

0"

Thu

0-T

Fri

0-2"

Sat

0"

Sun

0"

Mon

0"

Tue

0"

Wed

6am Snow Accum. Chance

0%

Thu

40%

Fri

70%

Sat

0%

Sun

0%

Mon

0%

Tue

0%

Wed

Summary

More updates and commentary for unfolding storm, likely the most impactful of the season. Our official forecast is at the bottom in the new verification and comparison tool. Check back for updates as we start backfilling some of the other model data to compare against.

Verification: Storm Dec. 6-8

Our previous storm has wrapped up, which was about the snoozefest we thought it would be, except for the western San Juans, which saw some nice powder.

Data source: NOHRSC SNODAS
Data source: NOHRSC SNODAS

The two main forecast issues were everyone overforecasting the Park Range and underforecasting the western San Juans (some models were good about this, though).

For the Park Range, a pocket of dry air was entrained in flow aloft during the period of best dynamics. This meant they mostly missed their window for serious snow production. We had a difficult time ascertaining just how much snow fell in the upper elevations of the Park Range though. Buffalo Pass’s SNOTEL site recorded ~0.5” of SWE – which would translate to 6-8” of snow. However, the actual snow depth gauge only saw about 2” of depth increase.

Winds, compaction, and sun explain much of the difference, and in the end, we’re just going to use the depth measurement which busts almost every forecast low. Despite that, we’re guessing it’s likely that there are some nice new stashes up there.

You’ll notice that the SNODAS model thinks that the scant gains near and above treeline in northern Colorado were compacted, blown away by the wind, and hit by the sun.

Data source: NOHRSC SNODAS
Data source: NOHRSC SNODAS

It seems most of the meaningful impact was limited to the western San Juans.

Data source: NOHRSC SNODAS
Data source: NOHRSC SNODAS

Anyways, here’s an overview of model and forecaster performance. Remember, “score” is basically determined by the size of the forecast ranges (less points for larger ranges) and any serious busts (negative points). This is to help add another metric beyond just the verification percent, which would be easy to pad if forecasting huge ranges (1-12” across the board!).

Model and Forecaster Performance

This was our second storm comparing model and forecaster performance. We included OpenSnow this time around. We only sampled model runs up to Sunday night – Monday morning’s model runs (which generally trended down) were not included, so this is essentially comparing what had the best guidance as of Sunday night.

We went ahead and removed the Urban Corridor from the verification list as they did not get any snow which is what everyone was expecting. That put OpenSnow at a disadvantage as they technically did not have forecasts for the area. It was an easy pad to the verification scores for everyone else, so we removed it.

No surprise that the typically pessimistic models (NDFD and NBM) did well with this storm. The GFS and Euro did terribly, particularly the Euro, which had an outrageous wet bias.

Another point of interest is that OpenSnow’s Friday forecast verified better than their Sunday forecast, however, their Friday forecast was a bit better than our Friday forecast, but both of those forecasts outperformed the GFS and Euro, even with their Sunday runs.

For the NBM, the 50th-90th percentiles verified better than the 25th-75th percentiles. This tends to be the case in the mountains, where the probabilistic guidance just doesn’t seem to resolve the terrain quite as well as it should, though its guidance is still very useful.

Here are the standings for the two storms we’ve verified so far:

Overall Verification Rankings

  1. Thomas Horner (2 wins) • Avg. 77%
  2. CAIC WRF 2km • Avg. 56%
  3. NAM 12km • Avg. 56%
  4. NBM Deterministic • Avg. 54%
  5. NBM 25/75 • Avg. 54%
  6. GFS 20km • Avg. 53%
  7. NWS NDFD (1 loss) • Avg. 48%
  8. ECMWF (Euro) (1 loss) • Avg. 41%

Not ranked yet: OpenSnow (67%), NBM 50/90 (76%) due to only one entry so far.

Score Rankings

  1. NAM 12km • Avg. 28%
  2. Thomas Horner (1 win) • Avg. 26%
  3. NBM 25/75 • Avg. 24%
  4. NBM Deterministic • Avg. 23%
  5. GFS 20km • Avg. 23%
  6. CAIC WRF 2km • Avg. 22%
  7. NWS NDFD (1 loss) • Avg. 22%
  8. ECMWF (Euro) (1 loss) • Avg. 18%

Not ranked yet: OpenSnow (27%), NBM 50/90 (1 win, 37%) due to only one entry so far.

With the above in mind, it suggests the NAM 12km has been the highest performing model, as it is verifying well with fairly small ranges (not too much run-to-run variance). Though the CAIC WRF is also verifying well, it is evidently doing so due to large spreads in its forecast – it had the largest spread of all models for the latest storm. This impacted its score significantly.

It’s possible we’ll tweak the secret sauce for calculating scores as the season progresses – but this backfills the existing verification tables (we’ll put these all in one place eventually) so they will update automatically. Transparency is key! We’re not doing this just because it makes us look good, we’ll surely botch a few storms this season (maybe this coming one!) and it’s helpful to know how and why.

―――――――

Snow Forecast Comparison and Verification Chart

Last updated Wednesday December 8, 2021 12pm.

Forecast Start: Monday December 6, 2021 5am

Forecast End: Wednesday December 8, 2021 5am

* = No reliable observations, verification uses SNODAS estimates which may be incorrect.
"Score" accounts for the spread/range of an accurate forecast. Lower spread = higher score. For instance, if 4" falls at a certain location, the forecast for 2-7" gets less points than the forecast for 3-5". Points are deducted for very inaccurate forecasts.

Thomas Horner Weather Forecaster @ Highpoint Weather

Issued Sun Dec. 5 11pm
✔️ Verified
84%
33/39
Score
31%
62/195
🎯 Spread
3.2
average
💥 High
3
7%
❌ Low
3
7%

Note: (Fri Dec. 3 11pm) Going for tighter ranges this time around.

Central Colorado
13/45pts (28%) ✔️9/9 (100%) 💥0/9 (0%) ❌0/9 (0%)

Aspen Highlands: 2-5" Forecast Actual2" ✔️

1pts
2" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Aspen Mountain: 2-5" Forecast Actual2" ✔️

1pts
2" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Aspen Snowmass: 2-6" Forecast Actual3" ✔️

1pts
2" Dec 7 5am (Official)
1" Dec 8 5am (Official)

Buttermilk: 1-4" Forecast Actual3" ✔️

2pts
2" Dec 7 5am (Official)
1" Dec 8 5am (Official)

Cottonwood Pass*: 1-3" Forecast Actual1" ✔️

2pts
1" Dec 7 5am (Estimated)
0" Dec 8 5am (Estimated)

Crested Butte: 1-5" Forecast Actual2" ✔️

1pts
2" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Independence Pass*: 1-4" Forecast Actual2" ✔️

2pts
2" Dec 7 5am (Estimated)
0" Dec 8 5am (Estimated)

Kebler Pass*: 3-7" Forecast Actual4" ✔️

2pts
3" Dec 7 5am (Estimated)
1" Dec 8 5am (Estimated)

Monarch: 2-6" Forecast Actual2" ✔️

1pts
2" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Front Range
11/25pts (44%) ✔️5/5 (100%) 💥0/5 (0%) ❌0/5 (0%)

Cameron Pass: 1-5" Forecast Actual3" ✔️

1pts
2" Dec 9am (SNODAS)
1" Dec 8 5am (SNODAS)

East Portal*: 0-3" Forecast Actual1" ✔️

2pts
0" Dec 7 5am (Estimated)
1" Dec 8 5am (Estimated)

Eldora: 0-2" Forecast Actual0" ✔️

3pts
0" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Hidden Valley*: 0-3" Forecast Actual2" ✔️

2pts
1" Dec 7 5am (Estimated)
1" Dec 8 5am (Official)

St. Mary's Glacier*: 0-2" Forecast Actual1" ✔️

3pts
0" Dec 7 5am (Estimated)
1" Dec 8 5am (Official)

I-70 East
12/35pts (34%) ✔️7/7 (100%) 💥0/7 (0%) ❌0/7 (0%)

A-Basin: 1-4" Forecast Actual1" ✔️

2pts
1" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Berthoud Pass: 1-5" Forecast Actual2" ✔️

1pts
2" Dec 7 9am (SNODAS)
0" Dec 8 5am (SNODAS)

Echo Mountain*: 0-1" Forecast Actual1" ✔️

4pts
0" Dec 7 5am (Estimated)
1" Dec 8 5am (Estimated)

Jones Pass: 1-5" Forecast Actual1" ✔️

1pts
1" Dec 7 8am (SNOTEL)
0" Dec 8 5am (SNOTEL)

Keystone: 1-3" Forecast Actual1" ✔️

2pts
1" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Loveland S.A.: 1-5" Forecast Actual1" ✔️

1pts
1" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Winter Park: 1-5" Forecast Actual2" ✔️

1pts
2" Dec 7 5am (Official)
0" Dec 8 5am (Official)

I-70 West
11/35pts (31%) ✔️7/7 (100%) 💥0/7 (0%) ❌0/7 (0%)

Beaver Creek: 2-6" Forecast Actual2" ✔️

1pts
2" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Breckenridge S.A.: 1-4" Forecast Actual4" ✔️

2pts
4" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Copper Mountain: 1-4" Forecast Actual4" ✔️

2pts
4" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Powderhorn: 1-4" Forecast Actual2" ✔️

2pts
2" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Ski Cooper*: 1-4" Forecast Actual2" ✔️

2pts
2" Dec 7 5am (Estimated)
0" Dec 8 5am (Estimated)

Sunlight*: 2-5" Forecast Actual2" ✔️

1pts
2" Dec 7 5am (Estimated)
0" Dec 8 5am (Estimated)

Vail Pass*: 1-5" Forecast Actual N/A

Vail S.A.: 2-6" Forecast Actual4" ✔️

1pts
4" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Park Range / WY
0/20pts (0%) ✔️0/4 (0%) 💥1/4 (25%) ❌3/4 (75%)

Bluebird*: 2-6" Forecast Actual N/A

Buffalo Pass: 5-10" Forecast Actual3" ❌️

2" Dec 7 5am (SNOTEL)
1" Dec 8 5am (SNOTEL)
Note: SNOTEL recorded 0.5" of SWE change -- maybe 6"+ of snow fell, but wind may have stripped it from the station's depth gauge.

Rabbit Ears Pass: 3-7" Forecast Actual1" ❌️

1" Dec 7 5am (SNOTEL)
0" Dec 8 5am (SNOTEL)

Snowy Range*: 1-5" Forecast Actual7" 💥️

6" Dec 7 5am (Estimated)
1" Dec 8 5am (Estimated)

Steamboat S.A.: 3-7" Forecast Actual2" ❌️

1" Dec 7 5am (Official)
1" Dec 8 5am (Official)

Southern Colorado / NM
15/35pts (42%) ✔️5/7 (71%) 💥2/7 (28%) ❌0/7 (0%)

Cuchara*: 0" Forecast Actual0" ✔️

5pts
0" Dec 7 5am (Estimated)
0" Dec 8 5am (Estimated)

Purgatory: 1-4" Forecast Actual5" 💥️

3" Dec 7 5am (Official)
2" Dec 8 5am (Official)

Red Mountain Pass: 3-7" Forecast Actual9" 💥️

8" Dec 7 9am (SNOTEL)
1" Dec 8 5am (SNOTEL)

Silverton S.A.*: 3-7" Forecast Actual7" ✔️

2pts
6" Dec 8 5am (Estimated)
1" Dec 8 5am (Estimated)

Taos S.A.: 0-1" Forecast Actual0" ✔️

4pts
0" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Telluride S.A.: 3-7" Forecast Actual6" ✔️

2pts
6" Dec 7 5am (Official)
0" Dec 8 5am (Official)

Wolf Creek: 0-3" Forecast Actual2" ✔️

2pts
0" Dec 7 5am (Official)
2" Dec 8 5am (Official)