Colorado Weather Forecast #137: Oct. 25 - Nov. 1, 2021

Warm Monday, storm Tuesday to Wednesday, warming and drying into Saturday, next storm chance Sunday to Monday.

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Updated Sun Oct 24, 2021 3:00pm MST | Published Sun Oct 24, 2021

By Thomas Horner

Summary

  • Temperatures will be quite warm on Monday ahead of our next storm.

  • This storm will bring heavy precipitation on Tuesday morning to western Colorado.

  • Flow will initially be southwesterly, which favors the San Juans and other western ranges, but temperatures aloft will be start off quite warm which could limit actual accumulations.

  • A band of decent moisture should push across much of the high country on Tuesday morning and provide a period of heavier snowfall for most mountain ranges.

  • Flow will turn northwesterly by Tuesday afternoon but moisture will be fairly scarce, so snow totals are questionable for Summit County and the Front Range.

  • Most significant snowfall should wrap up by Wednesday afternoon.

  • Gusty from Tuesday to Thursday.

  • A weak cold front will impact the lower elevations of the Front Range around noon on Tuesday (or a bit later), which could bring some rain.

  • Snow is not expected for the urban corridor.

  • Warming and drying trend through Saturday.

  • Next storm chance is Sunday (Halloween) to Monday, which could bring snow to Denver.

Retrospective and Current Conditions

An exceptional atmospheric river has started to impact the West Coast. This looks to be one of the most powerful events of its type on record for the region, with several feet of snow forecasted for the Sierra Nevadas, Pacific Northwest volcanoes, central Idaho mountains, and parts of the coastal ranges. Historic amounts of rain and snow are possible across California and the Pacific Northwest.

The primary driver of this precipitation is a powerful center of low pressure off the PNW coast which is undergoing bombogenesis – pressure is dropping very rapidly as the system develops into a full-blown cyclone.

(via NOAA)
(via NOAA)

The jet stream is wrapping around this system to the south and advecting an almost record amount of moisture into California and the Intermountain West.

(via WeatherBell)
(via WeatherBell)

A weak shortwave embedded in the flow ahead of this system passed through our region last night and tapped into a bit of moisture. Our forecast from nearly a week ago wasn’t expecting much out of this system, and despite an uptrend in model expectations in the day or two leading up to the storm, that forecast appeared to verify pretty well, with only a couple inches of snow for most mountains.

(via WeatherBell)
(via WeatherBell)

The SNODAS model seems to think that a stronger band of snow impacted the Front Range near Berthoud Pass, Rollins Pass, and Winter Park. There’s some truth to this as Winter Park reported a bit higher totals than other Front Range resorts (about 4” on their snow stake cam), and past radar data suggests a pocket of more persistent precipitation in the area, but we haven’t seen any observations suggesting those 6”+ values really occured.

Right now, the Berthoud Pass SNOTEL site is only showing an inch of snow depth – despite picking up about 4” from this storm – thanks to strong gusts along the Divide transporting the fresh snow.

(via WeatherBell)
(via WeatherBell)

Strong downsloping winds on the east side of the Divide are buffeting the Front Range urban corridor. As the air travels downslope, it compresses and warms. This will help the Front Range reach temperatures in the mid to high 60s today despite a cooler airmass working through the region. Some mountain wave clouds are also forming as another push of moisture enters the state.

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

Unfortunately (for snow lovers), this uptick in moisture isn’t going to do much besides increase cloud coverage, due to our position below the right exit region of the jet. That region of the jet is associated with subsidence, and conversely, lift in the left exit region of the jet was responsible for our snow last night. Here a sketch of the jet quandrants as of a bit earlier, with subsidence indicated by the downward reddish arrows and lift by the upwards green arrows:

(via WeatherBell)
(via WeatherBell)

All in all, this October hasn’t been too out of the ordinary. Unsurprisingly, appreciable development of the snowpack doesn’t typically occur until we get into November.

(via WeatherBell)
(via WeatherBell)

Forecast Discussion: Warm Monday

The powerful system and associated atmospheric river to our west will certainly impact our weather over the next few days. First, ridging will take hold over the High Plains, which will be amplified strength of the system off the coast of the Pacific Northwest. This ridge will help expeditiously advect warm, dry desert air into the Front Range of Colorado.

(via WeatherBell)
(via WeatherBell)

By noon on Monday, temperatures will be 10-20 degrees warmer than average across most of eastern Colorado and the urban corridor, though less of an impact will be felt in western Colorado.

(via WeatherBell)
(via WeatherBell)

Quite the pleasant late October day across the state!

Forecast Discussion: Storm Tuesday to Wednesday

Warmth will hang on into Tuesday morning – at least east of the Divide – despite a stronger wave impacting the state, which should help push a cold front down the Front Range around noon and into the early afternoon. If this cold front is delayed, many areas in the urban corridor could at least briefly see temperatures in the 70s again.

(via WeatherBell)
(via WeatherBell)

A look at upper atmospheric winds shows a narrow trough dipping across the Rockies which is responsible for our cooler temperatures and precipitation chances from Tuesday to Wednesday.

(via WeatherBell)
(via WeatherBell)

Despite the size (narrow) and speed (quick) of this feature, the strength of the jet max upstream of us should help bring lifting energy into the state by Tuesday morning, along with a good piece of the atmospheric river. Though lift continues into early Wednesday morning, we quickly lose access to the narrow plume of moisture at the leading edge of this wave and thus snow production will weaken significantly by Tuesday evening.

(via WeatherBell)
(via WeatherBell)

Storm Timings

On Tuesday morning, flow will generally be southwesterly, with snow favoring the San Juans (at least south of Red Mountain Pass), West Elks, Flat Tops, and Grand Mesa. There also looks to be a convergence zone in northern Colorado that could enhance snowfall in the Park Range and Flat Tops that morning. This will likely push east across the state through the morning, bringing a period of heavy snow for most mountains before noon.

The primary restriction for snow accumulations on Tuesday morning looks to be temperatures – likely above freezing at the 700mb level until later in the morning, especially in southwest Colorado. The Blend seems to think that snow-liquid levels wont consistently hit 10:1 until the afternoon. This may be a little conservative, but we do agree that the heavy precipitation in the first phase of the storm may not result in as high of snow accumulations as would normally be expected later in the season.

If we go off the Blend’s guidance alone we can see some decent precipitation numbers for the first phase of the storm. For a colder storm, this would amount to 6-12” of snow in the San Juans.

However, if we pull in the snow-liquid ratios from above, we see much lower totals:

Again, the Blend is likely too aggressive with the lower SLRs, but still, 4-8” before noon on Tuesday in the San Juans looks to be a more likely scenario than 6-12”. In the same vein, the snow level may be initially be higher than the lower elevations of the San Juans. Regardless, it’s not like there’s much of a base to ski anywhere, and this sort of base-building heavier snow may be more desirable than something fluffier.

By the afternoon, flow will turn northwesterly and significantly colder air will finally advect into the state. Unfortunately, this colder air mass is also quite dry and thus dynamics (both synoptic-scale lift and orographic lift) will have a hard time producing consistently heavier snowfall into Tuesday evening and night. We’ll of course see a few surprises as small pockets of better moisture embedded in flow aloft hit certain areas, but in general much of the state probably wont get more than 1-6” out of the afternoon and evening snowfall.

By Wednesday morning, snow will have mostly tapered off though upsloping winds will continue to drive a few pockets of snow throughout central and northern Colorado into early Wednesday afternoon, but this generally shouldn’t result in measurable accumulations.

Expected Snow Amounts

Here’s what the Blend thinks of snow totals. We think this is a little low, but the distribution looks good:

The upper end of probablistic guidance looks fairly similar:

Meanwhile, in California…

We’re eyeing the Park Range for the highest totals as they’ll have consistently colder air aloft, and they stand to benefit the most from convergence in the morning and northwest flow in the afternoon – we could see the Buffalo Pass SNOTEL site pick up almost a foot of snow if all goes well. Here’s what we think:

  • Park Range: 5-10” (3-7” for lower elevations like Steamboat, even less for Steamboat Springs)
  • North Front Range (Cameron Pass, Berthoud Pass, RMNP, IPW): 2-6”
  • Summit County resorts (Loveland, A-Basin, Breck, Keystone, Copper): 1-5”
  • Vail and Beaver Creek: 2-6”. Vail Pass maybe 4-8”.
  • Elk Range: 4-8”, maybe a bit lower at the lower elevation resorts (Buttermilk and Aspen Mtn).
  • San Juans, south of Red Mountain Pass and west of Wolf Creek: 5-10”.
  • San Juans, north of Red Mountain Pass: 4-8”.
  • Flat Tops: 4-9”.
  • Wolf Creek: 3-7”.
  • Sangres: T-3”.
  • Sawatch Range: 1-4”.
  • Foothills east of the Divide, 7,000ft-9,000ft.: T-1”.
  • Urban corridor: 0”.

Temperatures will be much too warm for snow in Denver or even on the Palmer Divide – by the time temperatures get below the 40s (early Wednesday morning), the entire region will be experiencing dry, downsloping winds. The Palmer Divide does have a slight chance at trace amounts of snow, but nothing would really stick.

Here’s a table of snow accumulations for various ski areas. Note that these are heavily limited by the snow-liquid ratio. We think the upside scenarios are significantly higher than advertised – though even with that in mind, not the most exciting event.

The exceedance tables are also looking particularly weak, as well:

Winds

Speaking of winds, conditions will be quite gusty in the high country on Tuesday as upper-level flow intensifies over colorado.

Conditions will remain gusty, particularly along the Divide, through Wednesday. We expect to see some wind observations of 40-80mph.

Rest of the Week

We’ll gradually warm up into next weekend as ridging becomes the norm across the west.

A bit of moisture aloft could keep some on-and-off snow showers going in the northern mountains into Thursday morning, which might result in a couple inches of snow for the Park Range and northern Front Range from Tuesday to Wednesday. Otherwise, Colorado will be dry into Saturday. High temperatures will top out on Friday and Saturday before another storm impacts us later on Halloween and into the 1st of November.

This storm looks to provide Denver’s next chance for snow.

We’ll let you know if anything changes, keep an eye out on social media!

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