Colorado Weather Discussion #132: Oct. 9, 2021

Ongoing storm this weekend, 100mph+ wind observations, more updates on next week's snow.

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Updated Sat Oct 9, 2021 2:00pm MST | Published Sat Oct 9, 2021

By Thomas Horner

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

Current Conditions and Retrospective

A wave of precipitation pushed through western and central Colorado yesterday, with snow falling at high elevations (the cold air hasn’t fully arrived yet). This has been in line with expectations. Most mountain ranges above 11,000-12,000ft. have picked up a dusting to a couple inches of snow, with a few pockets of a bit higher accumulations.

(via Pivotal Weather)
(via Pivotal Weather)

An interesting note is that doppler radar indicated a large amount of precipitation over the urban corridor yesterday.

(via WeatherBell)
(via WeatherBell)

This was merely virga, which is precipitation that evaporates before reaching the ground. Because the Denver radar is down for maintenance, these returns were amplified by further away radar sites seeing higher into the atmosphere, where the precipitation had not yet evaporated. Dewpoints were below freezing across much of the metro area, so it makes sense that precipitation struggled to reach the surface. A little further west, such as in Golden and Boulder, dewpoints were a bit higher and a few drops were hitting the ground.

As expected, precipitation has now organized into strong bands thanks to the jet stream overhead.

(via WeatherBell)
(via WeatherBell)

One of these bands have reached Summit County, where it’s now snowing fairly hard. The snow level has dropped to the valley, thanks to the heavy precipitation (evaporative cooling) and growing influence of a colder air mass today.

(via Keystone Resort)
(via Keystone Resort)

Despite a few inches of snow at higher altitude ski resorts, the main story with this storm are the winds, especially along the Sangres in southern Colorado. Here are some notable observations over the past 24 hours:

The RTMA shows the bigger picture across Colorado over that time period:

The system is lifting out of the area today, and winds are dying down, though the Front Range still has an afternoon/evening of gusty conditions to contend with.

The coldest air wont arrive in the state until later today, after most of the precipitation has wrapped up, though we’re still expecting a weak wave of precipitation across the mountains tomorrow afternoon.

(via WeatherBell)
(via WeatherBell)

It looks like snow totals will mostly end up near our 50th percentile snow forecast maps we shared in the current forecast, with a few areas near or slightly exceeding the 75th percentile.

Next Week’s Storm

Models continue to struggle with the path of next week’s (Tuesday to Wednesday) storm, though the consensus is slowly growing. We’ve seen the Blend start to settle into a smaller range of expectations for snow accumulations in Denver:

The last few runs of the Euro, Canadian, and GFS models have had the systen tracking a little too far north to produce a good upslope snow storm for the Front Range. There are multiple problems with this, namely that the storm track might not even bring much precipitation to the Front Range at all – rain or snow. There’s a chance of a changeover to snow on Tuesday night (will likely be a little too warm – the Blend has the snow level at about 5,500ft.), but obviously that would be moot if precipitation isn’t falling in the first place.

However, models are resolving a wave immediately behind this center of low pressure that looks to drop further south and could actually get a few flakes flying in the metro area on Thursday night. If we look at both those systems (Tuesday - Wednesday and Thursday - Friday) we see where that ~30% chance of snow in Denver is coming from.

Despite the hype, there’s really no guarantee this would be a blockbuster storm for the Colorado mountains, at least from Tuesday-Wednesday, though Thursday-Friday may be a different story. The best chances for serious snow accumulations are well north into Wyoming, and a probability map for >6” of snow accumulations isn’t that exciting, though it still looks like this will be the strongest storm we’ve seen so far this season.

At the very least, the snow level will definitely drop far enough for snow to fall across all elevations in Colorado’s high country, including much of the foothills. For most of Colorado’s mountains, the best lift is in place from Tuesday morning to Tuesday night, so there’s just not a big window for snow production, not considering the fact that there isn’t a plethora of moisture available during that time either. So, a decent, winter-esque storm, relative to what we’ve seen over the past couple weeks, but likely not anything that’s going to kick off the ski season. If we look at the latest probablistic guidance for Berthoud Pass, it’s hard to be too excited:

This will be the start of our classic persistent weak snowpack layer which will plague backcountry travellers through the winter.

At the very least, lows will drop into the teens in the mountains, with highs below freezing for a few days, which is a change of pace.

We’ll put out another forecast tomorrow as model certainty continues to improve.

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