Colorado Weather Discussion #134: Oct. 13, 2021

Snow totals, model performance analysis, tomorrow's storm, and updated expectations for snow in Denver.

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Updated Wed Oct 13, 2021 9:30pm MST | Published Wed Oct 13, 2021

By Thomas Horner

This is an update for our current valid forecast, please consult that article for any other details about the future weather forecast.

Our storm from Tuesday to Wednesday was a proper winter storm for much of Colorado, kicking off the ski season as Silverton ski patrol put down some fresh turns and Wolf Creek discussed a possible weekend opening.

The system looked decently organized on Tuesday as moist southwest flow impacted the San Juans before the system trekked through northwest Colorado and into Wyoming.

(via College of DuPage)
(via College of DuPage)

Denver Almost Got Some Snow

The center of low pressure came in a bit further south and a little slower than expected. This was enough for models to suddenly switch back to expecting snow for Denver on Wednesday night, forecasting a band of very heavy precipitation pushing off the foothills and moving east over the I-25 corridor. The potential for evaporative cooling and the arrival of slightly colder air was enough for the snow level to drop to 5,000ft. as the precipitation arrived, at least as forecasted by weather models.

The HRRR forecast from 2pm on Wednesday. (via WeatherBell)
The HRRR forecast from 2pm on Wednesday. (via WeatherBell)

Unsurprisingly, this barely ended up not panning out. The band of heavy precipitation did materialize and provided a powerful light show for the US-36 corridor (complete with hail and graupel), with snow just to the north of the Denver metro area (snow was recorded at Erie Municipal Airport). However, it was about an hour early and a little too far north, and Denver missed out on its first official snowfall.

(via Radarscope)
(via Radarscope)
(via College of DuPage)
(via College of DuPage)

There is of course a debate about whether graupel counts as snowfall or not – technically, it does, but in practice we don’t think anyone wants to count that.

Regardless, it was another exciting development in an already very exciting weather day. We had a buffet of weather highlights – tornado watches, red flag warnings (fire weather), high wind warnings, hard freeze warnings…

(via NWS Boulder)
(via NWS Boulder)

We watched snow stake cams and tornado warnings on radar. A classic October day in Colorado!

Snow Totals

Here’s the good stuff – snow totals. At the ski resorts, we saw totals of a foot or more at Wolf Creek and Silverton (our forecast called for a high end of 14”), 8” or so at Telluride, and 6-8” at Loveland, A-Basin, and Winter Park (this busted our forecast). Totals were lower at the Aspen resorts (2-3”), Vail (<2”), Copper (~4”), Breck (~5”), which was more in line with our forecast for central Colorado and Summit County.

Here are some estimated totals from SNODAS – keep in mind, there are some localized areas that are pretty incorrect in terms of snow totals (for instance, near the Eisenhower Tunnel, which got several more inches than this model thinks, or in the town of Telluride, which got less). However, it does provide a decent big picture overview of distributions across the ranges:

Model and Forecast Performance Analysis

In general, this storm met expectations – quite a decent shot of snow for the San Juans, western ranges (like the Flat Tops), and far northern Colorado. The far northern ranges (Park Range, Medicine Bow Mountains, and northern Front Range) were a hot point of debate for us. Many models had them getting pummeled with snow, but we actually had some skepticism that models were significantly overdoing wraparound moisture.

This actually was the case – for instance, Cameron Pass in northern Colorado picked up almost a foot of snow. However, that area only saw a couple inches of snow from 6pm into this morning, even though models expected that to be the most productive period due to wraparound moisture and orographics behind the passing system. Conversely, the opening phase of the storm featured very heavy convection and some lower-level dynamics in that region which led to snow accumulations of about 9” in just the first few hours as wave after wave of heavy snowfall pushed through ahead of the system, complete with embedded lightning (this was expected – but its consistency over that mountain range was not).

Neither we nor the models were expecting such prolific and consistent snow production on Tuesday afternoon for that area. In the end, this technically verified the snowfall forecast from the models, but not with the timings or characteristics that were actually forecasted! Had they not been in a good band of thundersnow early on (and a small area of convergence a bit after that), they easily could’ve ended up with 3-7” of snow, which is what we were worried about (many models were calling for 12”-18”), and this happened just to the south at several SNOTEL sites in the southewest quadrant of RMNP.

In general though, this storm outperformed model expectations for most areas. Here’s how the SNODAS map above compares to the final map of 50th Percentile snow accumulations that we posted with our forecasted:

The yellows and oranges are “less snow than predicted” and the blues are “more snow than predicted.” In general, most of the mountains saw much more snow than the 50th percentile, particularly the western San Juans, parts of Summit County (A-Basin and Loveland), and northern Colorado.

Let’s consider the reasonable range of expectations to be the 25th through 75th percentiles. Parts of the central mountains came in a bit lower than the 50th percentile, but still well above the 25th percentile – so a bit of a bust, but still in the expected range. The most significant low bust was in parts of the San Juans west of Wolf Creek. In general though, most mountain ranges were higher than the 50th percentile.

But how do totals compare to the 75th percentile, which is our guidance for the high end of expected accumulations? We would expect everywhere to be some shade of yellow or orange:

Still, we see some blues – so this storm definitely overperformed at least compared to general model sentiment (high resolution models were a bit more bullish on totals). In particular, parts of the western San Juans, Sangres, and Front Range did better than expected. In northern Colorado, the most interesting thing about this map is that westerly aspects consistently underperformed the 75th percentile while easterly aspects didn’t – this points to a model wet bias in regards to wraparound moisture (which had a westerly component) – too much snow predicted – and a dry bias or failure to attribute higher snow totals to convection in the earlier phase of the storm, which is more agnostic to the terrain (forced by instability instead of orographics). Some of this is due to artifacts in the SNODAS snow estimates, but still, we think this is a generally sound assessment.

Anyways, we now have a solid layer of snow across most of the mountains which will form our persistent weak layer that will plague backcountry recreationalists for the rest of winter. Hooray! And another storm is still on the way.

Thursday - Friday Storm

Before we amend our previous forecast and cover the system from a more technical standpoint, let’s get right to what a lot of people want to know – will it actually snow in Denver this week? Our previous forecast had just over a 50% chance of downtown Denver seeing snow. Here’s where we stand with the latest model data:

You’ll notice that basically everywhere in Colorado has solidied their snow chances…except Denver, which is now in quite the prominent donut hole. So what gives?

The main culprit is that the precipitation is coming in a few hours earlier, and the other culprit is that this system looks just a bit weaker, which goes hand-in-hand with the quicker timing. Denver will certainly see some rain – but snow? The snow level drops to 5,000ft. just after the precipitation wraps up in the city. The suburbs have less of an issue as the snow level is at about 5,5000ft.-6,000ft during the heavier precipitation. It’s going to be close, but it just may not barely be in the cards for Denver, especially with downsloping winds moving in fast after the precipitation gets going. However, we think the snow chance is a bit higher than 5%…

For the mountains, our expectations are a little lower than our original forecast, as linked at the top of the article. Not a big event (unlike yesterday), but plenty of cold air and enough forcing to make flakes fly for several hours. Snowfall rates will mostly be light to moderate, but snow-liquid ratios should be decently fluffy. We just don’t have a lot of moisture to work with, and the jet left entrance region just to our east is going to put a damper on lifting energy for much of the main phase of the storm. You can see on this animation of expected vorticity advection (which roughly corresponds to lifting energy) that dynamics are weaker than the past storm.

As we get into ski season we’ll start forecasting snowfall ranges, wind components, and snow liquid ratios for individual resorts and such, but we’re enjoying the fact that we don’t really need to do that right now. It’s a lot of work! Enjoy some maps, instead. We think totals will come in between the 50th and 75th percentile, but lets not ignore the 25th percentile.

For timings, more widespread precipitation should push into western Colorado tomorrow morning. This would reach Summit County by about noon, before a more organized band of heavier precipitation pushes east of the Divide by the late afternoon.

As you can tell from the maps, western Colorado and along I-70 are most favored for highest snow totals from this storm, which will generally feature WNW flow (makes sense). Like we said, a lack of moisture and broad, weak subsidence will make snow production a little difficult, at least at first. We’re expecting some surprises along the I-70 corridor. Unfortunately, a surprise will be Denver actually getting some accumulating snow….

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