Colorado Weather Forecast #131: Oct. 8 - Oct. 13, 2021

Storm Friday to Sunday, very windy. Stronger storm Tuesday to Wednesday, still some snow potential in Denver, though that continues to be an unlikely outcome.

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Updated Thu Oct 7, 2021 8:15pm MST | Published Thu Oct 7, 2021

By Thomas Horner

Summary

Rain and snow chances from Friday to Sunday, with strong winds on Friday and Saturday. A decent cooldown for central and western Colorado, but only a weak cold front for the Front Range later on Saturday / early Sunday. Model uncertainty is still unusably high for next week’s storm – we discuss the trends and potential outcomes and the unfolding likelihood of snow in Denver.

Friday

  • Starts off warmer, pockets of precip increase in coverage across western and northern Colorado.
  • Strong wind gusts, especially along the Divide and Sangres (50mph+).
  • Heavier, more widespread precipitation and colder air spreads across most of western and central Colorado by late morning/early afternoon.
  • Snow level still generally 11,000ft+, dropping to 10,000ft.

Saturday

  • Very windy, 50-80mph gusts possible for some high ridges.
  • Banded precipitation = pockets of high winds, low visibility, and heavy rain/snow, and no precip at all for other areas.
  • Colder air continues to advect into Colorado, and the snow level drops towards 8,000-9,000ft. by Sunday.
  • Gusty conditions in the Front Range.
  • Weak cold front for the Front Range Saturday night / early Sunday.
  • Precip decreases in coverage later in the day.

Sunday

  • Some precip might hang on in northern Colorado.
  • A final wave of lift and moisture produces some isolated showers across much of the high country in the afternoon.
  • Less windy.

Monday

  • A little warmer, especially in the Front Range.
  • Likely dry, but some precipitation could reach western Colorado ahead of the next storm.

Current Conditions

With a high of 77 degrees at Denver International Airport today, it seems that yesterday really was Denver’s last 80+ degree day of the season (barring a strong heat wave later in the month). There’s a slight chance that highs tomorrow could barely hit 80 degrees, but the latest high resolution guidance comes a few degrees short. When we look at the probablistic temperature forecast for Denver, we can clearly see our next two weather makers:

(via ESRL)
(via ESRL)

The unsettled pattern ahead comes on the heels of a two week stretch that has been overwhelmingly warmer than average, though on the precipitation front, many areas of western and southern Colorado have benefited from a series of weak storms.

(via WeatherBell)
(via WeatherBell)
(via WeatherBell)
(via WeatherBell)

The upcoming pattern could be a bit of a change of pace, which is welcome as drought has started to return to parts of Colorado that made significant progress earlier in summer.

(via WeatherBell)
(via WeatherBell)
(via WeatherBell)
(via WeatherBell)

A weak wave is clearing the state, with a large amount of moisture advecting into the region behind it as Colorado begins to feel the influence of the jet stream. There are some very isolated pockets of weak precipitation associated with this cloud cover.

(via NOAA)
(via NOAA)

The past wave produced some generally light showers, with most accumulations limited to western and southern Colorado. A dusting to an inch or two of snow fell in the higher elevations of the San Juans, with some trace amounts in some of the upper reaches of other Colorado mountain ranges. Winds should increase (especially on high ridgelines) over the next 48 hours as a jet maximum moves closer to the state. You can see it to our west-southwest:

(via WeatherBell)
(via WeatherBell)

Otherwise – pretty good showing from the fall colors this season, eh? Things will probably look pretty rough after the upcoming round of precipitation and strong winds.

Forecast Discussion: Storm Friday to Sunday

Our first order of business is a stronger wave that begins to impact the state on Friday before clearing out later on Sunday. In the upper atmosphere, we can see the system tracking across our region, with a strong jet max just over Colorado from Friday to Sunday morning.

(via WeatherBell)
(via WeatherBell)

In general, the storm should be heavier on the wind and lighter on the precipitation, unless the jet sags a bit further south. It also means that models have gone back-and-forth with precipitation impacts in Colorado, as just a slight change in track makes a large difference in the amount and duration of lifting energy that we see. The current run of the Euro model, which has the jet max a bit further south than the GFS model, has notably higher totals for northern Colorado. However, the 48hr+ duration of this event means that even weak precipitation will eventually add up by Monday.

In the mid-atmosphere, a center of low pressure should close off and track east over New Mexico by Sunday morning, while another low tracks over the Dakotas. This will should provide another shot of lift and moisture (though not much of it) into Sunday.

(via WeatherBell)
(via WeatherBell)

Looking at vorticity advection gives us an idea of when we may see the best lift, so long as it coincides with moisture:

The latest GFS has the best lift on Friday afternoon, when there will be plenty of moisture. Lift and moisture gradually decrease into Sunday, with a final bit of dynamic forcing along with a pocket of moisture towards the end of the weekend.

Winds are the big story with this storm, with gusts of 50mph+ commonplace from Friday into Saturday.

There are a lot of things going on, here are some timings:

Timings

Friday Morning

Still warmer, some pockets of light precipitation begin to work their way into Colorado from the southwest and increase in coverage. The San Juans may see more widespread precipitation later in the morning. This will be almost entirely rain except the highest elevations. Strong wind gusts (50mph) along the Divide and Sangres.

Friday Late Morning / Early Afternoon

Colder air works its way into northwest Colorado and widespread precipitation moves across much of western and northern Colorado, with a few pockets of heavier precipitation. The snow level is still 11,000-12,000ft., and wont fall below that until nightfall. Some precipitation will make it the northern Front Range.

Friday Night

Light to moderate precipitation continues in western and central Colorado, some impacts in the San Juans and northern ranges such as the Park Range. The snow level begins to drop to 10,000ft. Strong winds across most of Colorado’s high country, with gusts up to 50mph.

Saturday

As a jet max moves over Colorado, precipitation begins to organize in stronger bands. These have a good chance of impacting almost any mountain range in Colorado, the coverage is a little better in northern Colorado. The snow level continues to fall, reaching 8,000-9,000ft. by Saturday night. Under the bands, wind gusts could be 20-40mph. With the jet max overhead, the high ridges of central and southern Colorado could see 50-80mph gusts. Downslope gusts of 20-40mph could impact the Front Range.

(via WeatherBell)
(via WeatherBell)

Precipitation decreases in coverage from south to north over the course of the day.

Saturday Night into Sunday

A weak, dry cold front pushes down the Front Range. Winds are much lighter. Some light to moderate precipitation should still be going in northern Colorado, but otherwise, we may see a final wave of light to moderate precipitation Sunday afternoon.. The snow level bottoms out at around 8,000ft.

Precipitation and Snow

In general, we’re expecting a good proportion of the total precipitation to come on Friday – when snow will mostly be confined to above treeline. Saturday is a bit of a wildcard as the snow level drops steadily and heavy bands of precipitation impact small regions and leave others considerably drier – and it’s hard to predict which areas will pick up a quick 3” of snow and what areas (often just down the road, so to speak) will get nothing at all. If precipitation is strong enough, the snow level will be even lower thanks to evaporative cooling.

Sunday could offer a few more flakes especially down below treeline, but likely nothing that exciting.

Like we said, there’s still some uncertainty in the exact track, and since the jet is directly over Colorado, this is still causing decent oscillations in model expectations for precipitation by the end of the weekend.

Let’s take a look at some probablistic guidance. Overall, the QMD-adjusted products we’re using seem to be more bearish on snow totals than other weather models are at the moment. We think a lot of areas will hit the 75th percentile and then some, but with the high snow levels and banding potential on Saturday, some areas wont come even close.

In general, these maps help convey a lot of the uncertainty in the forecast, which way that uncertainty is leaning (towards the drier side in this case), and some trends across subranges and mountain ranges. Actual totals will be much less “smooth,” but this is a good baseline reference especially when considering the percentiles across specific areas.

We’ll start with the 75th, more optimistic scenario:

The 50th percentile provides some baseline expectations. Note how we’re a couple inches short of many weather models here, including the NBM deterministic itself, though models have slowly been reducing their expectations (so this guidance could be on to something):

The 25th percentile is a more “worst case” scenario, we would expect everywhere on the map to get at least this much snow:

For reference, here’s the latest Blend (NBM) deterministic. You’ll notice it’s a couple inches higher in spots then the 50th percentile above.

(via Pivotal Weather)
(via Pivotal Weather)

Perhaps the most interesting difference is in the Sangres – the 50th percentile has no snow, while the deterministic has several inches. As another reference, the Euro model also has very little snow in the Sangres (none to trace), and middling snow totals in the San Juans (0-3”), but has higher totals in northern Colorado. The GF model is the opposite (very little snow for northern Colorado), so we can see that the probablistic guidance is bringing some of that uncertainty together.

Overall, we do agree that the western and central Colorado ranges (Flat Tops, Grand Mesa, Park Range, West Elks, Elks) stand to see the best chance of decent snow totals.

Forecast Discussion: Next Week’s Storm

This is the much-hyped event that could, but still probably wont bring accumulating snow down to Denver from around Tuesday to Wednesday. We’ve seen Denver’s snow chances bump up over the past 48 hours as more and more ensemble members are predicting a strong closed low will form over the Four Corners region.

(via WeatherBell)
(via WeatherBell)

What happens next will make or break the chances for a cold upslope snow storm for the Front Range. Ideally, the low needs to track over the Colorado Rockies and into southern Colorado, where it would undergo lee cyclogenesis and set the stage to advect a bounty of moisture and cold air into northeast Colorado. There’s no agreement that this will happen, and this isn’t the likeliest scenario at this point. For instance, the Canadian model has the center of low pressure all the way up in Wyoming with its latest run.

With this much uncertainty and lead time in the forecast, it’s a fool’s game to follow the back-and-forth of model run after model run. The GFS, Euro, and Canadian models have been trading blows, with each subsequent run jumping between delivering a smackdown for Denver (6”+) or showing no snow at all. Even the ensembles and Blend have fallen prey to the intense back-and-forth. Here’s a chart of the Blend’s likelihood that Denver International Airport will see at least a trace amount of snow, as its guidance has changed over the past 72 hours:

(Thomas Horner)
(Thomas Horner)

Though the Blend hasn’t retained the optimism that it (or we) had yesterday, the trend has overall been in the positive direction (perhaps just statistical noise). One interesting point to note is that, in the latest runs, the probabilities are such that Denver either gets a decent amount of snow or none at all. This makes sense, given the overall trend in ensemble members for a closed low to at least form.

And wow, we’ve seen some impressive storms on the weather models! The latest Euro has what looks to be almost a full-on extratropical cyclone developing as the system lifts out into the Great Plains.

(via WeatherBell)
(via WeatherBell)

There’s just so much uncertainty that we can’t do anything more than continue to speculate – dream, perhaps? Not only are the models in disagreement with each other – let alone their previous run – they handle these sort of systems poorly. Off the top of our head, we can point to common inaccuracies such as a northerly bias in the track when considering lee cyclogenesis, or a failure to account for strong low level warm air advection wrapping into the NW side of the cyclone (this happened last year around this time).

Let’s take a look at some probabilities across the state – there will definitely be a decent amount of snow for parts of the mountains, perhaps even a wallop of it – but as the track changes so do the totals and favored mountain ranges.

Exciting stuff, but we still wouldn’t count on seeing any flakes in Denver. We’re keeping a close eye on things (and despite what we said, we’re definitely emotionally invested in the back-and-forth, run-to-run drama of the weather models. Follow along on Twitter for more of that.).

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