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Colorado Weather Forecast | Aug. 5–11, 2021

Lots of smoke into the weekend, storm chances on Friday and Monday.

  • Very smoky this afternoon and evening.
  • Smoke clears out on Friday afternoon, at least temporarily.
  • Scattered thunderstorm activity on Friday, though main impacts are wind gusts and lightning, not precipitation.
  • Fairly dry this weekend, with only isolated storm activity on the Front Range and near the Divide.
  • Very bad smoke on Saturday, particularly in the morning, the worst of the season — maybe even raining ash.
  • Smoke will likely mix out a fair amount on Sunday — depending on fire behavior over the next 24 hours — but this improvement will probably stop or reverse as we get into Monday.
  • Another small uptick in storm potential on Monday.
  • Chances for precipitation slowly increase into the end of next week as a monsoonal weather pattern reforms.

Discussion

We’ve entered a stretch of drier weather as our monsoonal weather pattern has broken down, with flow remarkably zonal (horizontally west to east) over the western United States. Today we’re contending with the prospects of potentially the worst smoke we’ve seen all season — though this may just be an “appetizer” for what is to come this weekend.

The current HRRR model shows a remarkable amount of smoke popping up this afternoon and evening, with many areas pushing smoke concentrations of 24–29μg/km³:

Note that the title is incorrect — these units are in μg/km³, not kg/km³ — this is due to some outdated GRIB tables, and if you load this weather model up, it will use the wrong units for the smoke (MASSDEN) product on a lot of GRIB converters. (via WeatherBell)

For the record, we’ve rarely exceeded 20μg/km³ this year, and most of the worst days so far have featured smoke densities of about 10–15μg/km³.

What’s to come this weekend looks to be much worse, though with the fairly speedy zonal flow overhead, it might be a quick hit before it mixes out a bit on Sunday. We’re talking densities of 50–80μg/km³ — raining ash is not out of the question.

Friday night to Saturday at noon. (via WeatherBell)

The HRRR model — which is really one of the only weather models with robust, high resolution smoke modeling — only goes out to 48 hours. The above animation of the model’s smoke product only goes out until 12pm on Saturday, so much of the weekend is out of range. This makes medium-range smoke forecasting something you need to do the old-fashioned way, and it makes for an important point:

Online weather forecasts and apps will not show smoke in the forecast more than a couple days out, even if it’s obvious that there will be more smoke in the future.

Just because whatever website you use doesn’t show smoke in the forecast this weekend does not mean that the website doesn’t think there will be smoke — it’s just that it has no smoke data at all. Most of these sites simply bring in the HRRR’s smoke product, which again only goes out 48 hours, to add the potential for smoke to their forecast.

Smoke is much more complicated to forecast than temperature and precipitation, since it depends on air parcels moving through very specific regions of a vast 3d space, so doing it “by hand” is tough. However, we have some tricks up our sleeve to get a grasp on what may happen this weekend.

First, let’s take a look at where the strongest fires are.

(via NOAA GSL)

We have some fires in the Pacific Northwest that are producing a large amount of smoke, but the largest concern is the Dixie Fire in northern California, which exploded yesterday and is over 322,000 acres in size. Here it is right now:

Now, we’ll try to figure out where the air in Colorado on Saturday afternoon and Sunday afternoon originated. Due to zonal flow, it is of course coming somewhere from the west, but what we’re looking for is whether it may have passed into the main smoke plume from these fires and then descended to the surface in Colorado. We can really visualize this by generating a backwards trajectory with the HYSPLIT model, using GFS model data.

Here’s Saturday afternoon:

(via NOAA ARL)

This backwards trajectory indicates that the air near the surface of Colorado on Saturday afternoon has a good chance of having passed through the prominent smoke plume of the Dixie Fire — and most importantly, near the surface, where the smoke is much thicker.

There are a few scenarios of air originating from higher in the atmosphere, or up further in the Pacific Northwest, which would bring significantly less smoke into Colorado, however, a majority of solutions seem to indicate that Saturday has a good chance of being very smoky in Colorado.

What about Sunday?

(via NOAA ARL)

This looks considerably better — a vast majority of solutions have air advecting in from further north in the Pacific Northwest, and higher in the atmosphere. This should clear out some of the smoke on Sunday, but it’s hard to imagine that there still wont be some amount of smoke — what we’re looking for here is whether any fires in Oregon and Washington blow up today or tomorrow and put more smoke higher into the atmosphere. Still, conditions may very well be pretty decent, all things considered.

Now let’s do the same thing, but in reverse: where might the air directly above the Dixie Fire go over the next few days? We’ll use a forward trajectory to visualize this.

(via NOAA ARL)

This certainly raises our suspicions of a smoky weekend, at least through Saturday. While the horizontal motion of the air parcel may seem obvious (west to east, due to zonal flow), the vertical motion is really what we’re looking for — and these trajectories are showing a large number of solutions where air that originates near the surface of the fire (i.e. in the dense smoke plume) remains near the surface as it passes through Colorado. We’ve had days where dense smoke has passed harmlessly overhead, so this is important to note.

We’ll look at Monday too, just to get an idea of how smoke coverage may be trending as we progress through Sunday. A lot of uncertainty, but it indicates there’s some chance that smoke could make a return, or at least that it may stop being mixed out as effectively:

(via NOAA ARL)

So using all this data, here’s what we’re expecting for smoke this weekend:

  • Smoke starts to clear out Friday afternoon and evening, with smoke become almost non-existent around 3pm for the Western Slope and 5–8pm for the Front Range.
  • A massive amount of smoke quickly inundates the Western Slope by 7–10pm on Friday, with some of it reaching the northern Continental Divide by 9–10pm. This smoke is advected into Colorado thanks to a quick disturbance (shortwave — we’ll talk about this in a bit).
  • Incredibly dense smoke is present throughout the state by 6am on Saturday — maybe even some raining ash. It is slow to mix out during the day as the disturbance has moved past Colorado, and winds are lighter, but conditions by the evening should be notably better than the morning (though still as bad as anything we’ve seen this season).
  • Smoke likely remains bad into Saturday night, but a strong closed low that has been pushing down towards the Pacific Northwest should have started to orient winds to carry the worst of the smoke to the north of Colorado. Sunday should feature rapidly improving conditions.
  • Much like the previous shortwave, this low will pass by Colorado and could bring another hit of smoke into the state by Monday — but there’s a large amount of uncertainty about how far south this system tracks (the Euro may have it a bit too far north to really bring smoke back). At the least, conditions may not notably improve from whatever is left on Sunday.

So that’s what we’re thinking — tricky stuff! In summary, plan for the worst smoke we’ve seen all season on Saturday, with rapid improvement into Sunday. On Friday afternoon, the weekend will be entirely in range for the HRRR model, so we can see if this forecast is likely to verify, and provide you an update.

Anyways, we mentioned a passing shortwave on Friday. Here it is:

(via WeatherBell)

This will cause an uptick in storm potential that day as slightly moister air (not significantly moist — maybe 0.7–0.9" of precipitable water over Denver) advects into the state, with additional lift being provided by the shortwave. Due to its northerly track, southern Colorado will see less lift and less moisture.

The HRRR shows widespread storm coverage by 2pm on Friday:

(via WeatherBell)

The lack of significant moisture will limit the precipitation potential of these storms, with their main impacts being strong wind gusts and lightning. Models have been very consistent over the past few days in putting down swaths of 30–60mph wind gusts in the afternoon and evening:

(via WeatherBell)

The San Juans will feature the lowest thunderstorm potential on Friday, followed by the Sangres, with the Front Range most likely to see lightning activity.

Dry air immediately advects back into the state, knocking thunderstorm potential on Saturday and Sunday down to low levels — almost nonexistent for most mountains significantly west of the Divide.

We see another small uptick of thunderstorm potential on Monday as a closed low tracks across the northern United States, but this looks to be too far north to bring any significant amount of moisture.

(via WeatherBell)

The ridging between Friday’s shortwave and this low pressure system will bring in some very warm air, with Denver trying to push triple digits on Monday — most models are putting the airport at around 97 degrees.

(via ESRL)

As we progress into next week, guidance is showing a redeveloping Monsoon High over the desert southwest, but guidance isn’t pulsing moisture strongly enough to really impact Colorado significantly. However, this will increase the chances for afternoon thunderstorms each day, even if just from “basically non-existent” to “isolated.”

Here’s Monarch Pass’s probability of precipitation over the next 10 days, which shows the trend nicely:

(via ESRL)

Overall, the next 10 days looks to be a bit drier than average in Colorado…

(via WeatherBell)

…which is a change of pace after several weeks of bountiful rain.

(via WeatherBell)

As you’re likely well aware, these rains have resulted in some wild events such as the indefinite closure of I-70 in Glenwood Canyon. Some areas, like Tucson, have received record amounts of precipitation. In just the past week, Colorado’s “exceptional drought” area was knocked back to 6% from 15%.

Three months ago, 92% of the state was experiencing drought conditions (at least in the near term). As of this week, we’re now at 43%, with an improvement across all categories. This of course means that our initial long-term monsoon prediction (which we made in May and June) has been dreadfully wrong, but we’re not complaining. If it wasn’t for the Pacific Northwest burning down, we might not even be talking about smoke at all!

Stay safe this weekend!

By the way, you may have noticed Thomas writing most of the forecasts over the past couple months. Laura, one of our frequent contributors, is doing some very cool professional (i.e. actually paid) meteorological work this summer, and since these articles take quite a lot of time and effort, she’s taking a break from writing these — though still providing input and discussion in the background.

If you’re reading this — cheers! It’s tough to get these out some weeks, but we appreciate the viewership and engagement. Please, don’t hesitate to ask for questions or more personalized forecast information on our social media.

Forecast by
Thomas Horner (Twitter: @thomaschorner)

Website and app with automated, high-resolution forecasts for mountains, climbing areas, and ski resorts (+ backcountry) with elevation switcher, uncertainty visualization, and more:
https://highpointwx.com

Frequent updates, graphics, and general stoke:
Twitter / Instagram / Facebook: @highpointwx

If you enjoy our forecasts and/or website, please consider becoming a Patron to help keep the lights on. Thank you!

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Weather forecasts and articles for the recreating in the Colorado high country — mountains, ski resorts, and crags.

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Highpoint Weather Forecasting Team

Highpoint Weather Forecasting Team

The Highpoint Weather forecasting team — weather nerds who like to play outside.

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