Colorado Weather Forecast #127: Sep. 29 - Oct. 3, 2021

A nice taste of autumn into Friday, before we warm up and dry out for the weekend.

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Updated Wed Sep 29, 2021 6:30pm MST | Published Wed Sep 29, 2021

By Thomas Horner

Summary

A nice taste of autumn into Friday, before we warm up and dry out for the weekend.

Tonight

  • Rain and snow continues in the central and northern mountains.
  • A few rounds of lighter precipitation could reach the Front Range urban corridor.
  • The snow level drops to about 10,000ft.

Thursday

  • Precipitation mostly wraps up for the mountains by the early morning.
  • Cooler, as a colder airmass advects into the state.
  • Heavy rain and snow chances for southern Colorado in the afternoon, tapering off by Friday morning.
  • Some of this precipitation will impact the central Colorado mountains.

Friday

  • Another round of precipitation for southern Colorado in the afternoon.
  • Some of this will reach the central Colorado mountains and the urban corridor, but impacts are likely confined to south of the Palmer Divide.
  • Isolated thunderstorm chances near the Divide.
  • Slightly warmer for central and northern Colorado.

Saturday

  • Isolated thunderstorm chances in the Colorado high country. Warmer, but fairly seasonal temperatures.

Sunday

  • Isolated thunderstorm chances in the Colorado high country. Warmer, but fairly seasonal temperatures.

Current Conditions

A mid-atmospheric trough is impacting Colorado, with some associated lifting energy tapping into moisture that is present over the state. A colder airmass is advecting into Colorado behind a front this morning. A nice wave of thunderstorms and snow pushed from west to east across Colorado, with a band of thunderstorms reaching the Front Range by the afternoon. Snowfall has expectedly been a bit of a mixed bag, but some areas have picked up a bit more than a dusting.

Aspen Highlands webcam at 5pm (via Aspen Highlands)
Aspen Highlands webcam at 5pm (via Aspen Highlands)

Many parts of the Front Range and Summit County saw more rain that snow, with only a dusting at the highest elevations. However, there are a few peaks in this region that we suspect got a couple inches of snow, such as James Peak. MRMS suggests 2” of accumulation as of 3pm:

(via Highpoint Weather)
(via Highpoint Weather)

Maroon Peak also picked up a couple inches of snow with this storm, which comes on top of dustings from the previous day and last week:

(via Highpoint Weather)
(via Highpoint Weather)

There has been a lull in the action over the past few hours, but another wave of rain and snow is pushing through, with the snow level continuing to drop. Precipitation chances remain fairly likely through tonight. For James Peak, the latest high resolution model guidance has another 3-4” of snow, which is corroborated by the Blend, though its range is more 1-5”.

(via Highpoint Weather)
(via Highpoint Weather)
(via Highpoint Weather)
(via Highpoint Weather)

Again, elevation is a key factor here as the snow level doesn’t look to drop much below 10,000ft. If we switch to an 11,000ft. forecast for James Peak, we see a big difference in snow totals:

(via Highpoint Weather)
(via Highpoint Weather)

On the Front Range urban corridor, a nice band of moderate to heavy precipitation moved through, with a decent amount of lightning.

(via Highpoint Weather)
(via Highpoint Weather)

Looking at MRMS’s Q3 precipitation accumulation product, we’ve seen some decent precipitation across the state from this latest wave, though parts of the Sangres and San Juans have largely missed out (which was expected).

(via Highpoint Weather)
(via Highpoint Weather)

Forecast Discussion

A somewhat weak trough has dropped into the Intermountain West, bringing cooler temperatures and lifting dynamics. Cooler, wetter weather associated with this system is impacting our region. Lift ahead of the trough axis has been driving precipitation across Colorado today:

(via Pivotal Weather)
(via Pivotal Weather)

Models expect low pressure to become cut off by the end of the week, with the closed low pressure system slowly wandering over the Four Corners before lifting eastward by the end of the weekend.

(via Pivotal Weather)
(via Pivotal Weather)

This system will largely impact southern Colorado (particularly the San Juans and Sangres) as we progress into the weekend, but will also keep the rest of Colorado a bit unsettled and cooler until the start of next week. This is a bit of a change from our previous forecast, as the closed low was not as pronounced of a feature on model runs earlier this week.

There looks to be plenty of moisture available over the state into next week, so when thinking about precipitation, we need to look for large-scale lift which is generated by various disturbances aloft. We can use a vorticity advection map to visualize some of that lifting energy:

At the start of the animation, you can see a decent amount of lifting energy pushing through the state, which has been responsible for our storm coverage today. Next, look for pink and yellow colors moving into southern Colorado on Thursday and Friday – that will drive more storm activity for that region.

You’ll notice very little vorticity advection over Colorado for the weekend, which means that despite the moist airmass present in the state, there wont be anything driving precipitation on a large scale. However, it will likely be warm and unstable enough for afternoon thunderstorms to form, which can happen even without large scale lift.

Cold Front

Colder air is working its way into Colorado behind the front this morning, and Thursday will likely be the chilliest day this week. Still, this is no arctic air mass, and we’ll rebound to seasonal temperatures by the weekend.

(via WeatherBell)
(via WeatherBell)

Rain and Snow

For the rest of today, we expect a few more rounds of precipitation that will largely impact the central and northern mountain ranges. Models are still holding on to pockets of higher snow totals:

(via Highpoint Weather)
(via Highpoint Weather)

The very latest Euro run has largely given up on northern Colorado, but continues to show nice totals in southern Colorado:

(via WeatherBell)
(via WeatherBell)

Accumulations are very hit and miss, and like we pointed out earlier, a few smaller sub-ranges look to be favored for more significant snow totals. The above run of the HRRR has final accumulations of about 6” for James Peak, which is likely a bit high, but the ~2” of accumulations so far plus a couple more inches isn’t too shabby.

The Blend is still not convinced the rain-snow level will meaningfully fall beneath 10,000ft., and this appears to be the case.

(via Highpoint Weather)
(via Highpoint Weather)

Snow will wrap up for the central and northern mountains by Thursday morning, with only some modest chances for afternoon storm activity into the weekend. The focus instead turns to southern Colorado, which looks to get hammered with rain and snow on from Thursday afternoon to Friday morning, and then again on Friday afternoon before the closed low begins to depart the region.

(via WeatherBell)
(via WeatherBell)

Models have bumped up snow expectations a bit for the higher elevations of the San Juans and Sangres, while the northern and central mountains still have a chance to pick up a few inches of snow overnight (and potentially with some afternoon storm activity through the end of the week). However, the initial upper-end expectations have not played out so far.

(via Highpoint Weather)
(via Highpoint Weather)

Let’s break down these timings with some line graphs. In northern Colorado, blended model guidance still has a few inches at play for the higher elevations (>10,000ft.):

(via Highpoint Weather)
(via Highpoint Weather)

We can see that afternoon storm potential later in the week is mostly driven by convection, and unsurprisingly, there are modest thunderstorm chances on Friday and Saturday afternoon:

(via Highpoint Weather)
(via Highpoint Weather)

This makes sense as temperatures wont be too chilly by the end of the week – eventually reaching the 50s or even low 60s in the central and northern Colorado mountain valleys.

(via Highpoint Weather)
(via Highpoint Weather)

Down on the plains, a very pleasant weekend is in store, though a batch of precipitation associated with the closed low to our south will move through on Friday afternoon, particularly impacting areas south of the Palmer Divide.

(via Highpoint Weather)
(via Highpoint Weather)

In the southern mountains, we see elevated precipitation chances from Thursday afternoon into the start of the weekend as low pressure cuts off and cyclonic (counter-clockwise) flow taps into moisture over southern Colorado.

(via Highpoint Weather)
(via Highpoint Weather)

Temperatures will also be a bit cooler than northern Colorado due to the influence of the system to the south.

Central Colorado splits the difference a bit, picking up most of tonight’s action and some of the moisture later in the week.

(via Highpoint Weather)
(via Highpoint Weather)

With all of these precipitation graphs, the upper bounds are fairly impressive but seem a little unreasonable to us. The Blend struggles a bit to recalibrate some of its products this time of year as we enter a different weather regime than the summer, but it usually gets things dialed in as we progress further into winter.

Fall Colors

Don’t forget, fall colors are peaking across the state this weekend. If you’re into that sort of thing – check them out before they’re gone!

Long Term Forecast

The long term forecast is beginning to take shape. Models have started to agree more on a warmer solution for next week, and continue to resolve the potential for a stronger, more winter-like storm the week after that (around October 13th – some snow chances for Denver are showing up in the long range ensembles). Monday and Tuesday look to be dry, with afternoon storm chances returning by the middle to end of next week.

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

The urban corridor could see a lengthy return of temperatures in the 80s, so enjoy the brief taste of autumn while it lasts.

(via WeatherBell)
(via WeatherBell)

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