Mountain West Discussion

[Roger Smith]

Yes it was crazy hot here and will be for several more days. Also in eastern WA, Spokane had a record high of 99F (previous 97F 1988) and on the regional roundup it was 101 at CDA and 100 at Colville WA which is just south of my location. A lot of small fires in that area too, nothing really close to my location. Quite a lot of forest fire smoke developing across Alberta now from fires in the Rockies and north central BC. Some of that smoke could appear in parts of the central U.S. soon. 

[tacoman25]

14 hours ago, Roger Smith said:

Yes it was crazy hot here and will be for several more days. Also in eastern WA, Spokane had a record high of 99F (previous 97F 1988) and on the regional roundup it was 101 at CDA and 100 at Colville WA which is just south of my location. A lot of small fires in that area too, nothing really close to my location. Quite a lot of forest fire smoke developing across Alberta now from fires in the Rockies and north central BC. Some of that smoke could appear in parts of the central U.S. soon. 

GFS says relief is coming soon. We shall see.

[smokeybandit]

And it continues. A storm 2 miles to my south, not a drop for me.

[Roger Smith]

Just looking at maps and seeing remnants of the southwestern storm moisture over n/c AZ where it's only in the mid-60s. Then I looked over at Colorado and there's one station reporting 21/15 ... is that for real or is it a Celsius conversion that's not supposed to be on the map? The station is ABH -- is that on top of a mountain or what's going on there? (data on mesoscale maps on SPC site as of past hour 19z)

[Chinook]

moonrise shot

[smokeybandit]

Oh so close to the first 30s low of the season. 40.5 degrees.

[tacoman25]

64/47 day at DEN yesterday, -13 below average day. Still haven't been warmer than 86 since 8/21.

We're a day away from the 5 year anniversary of one of the most dynamic weather events I've seen. After hitting a record 101 a few days earlier, a strong cold front pushed down the Front Range on 9/7. First it produced high winds and blowing dust, with temps falling from the low 90s in the early afternoon to the 50s a few hours later. Then came rain, turning to snow in the early am hours. High of 93 on 9/7, followed by 43 on 9/8.

DEN officially picked up an inch, but many parts of the metro area (including my house) saw 2-3". It was one of the earliest snows on record for the city.

[Mercurial]

Hit 27 this morning. (first freeze was back on July 6)

[tacoman25]

On 8/14/2025 at 6:32 PM, donsutherland1 said:

Denver reached 90° for the 40th time this year. Its warming climate has seen an explosion of years with a high number of 90° or above highs. The new regime of far more frequent 90° or above highs commenced in 2000. 2025 is on track for 50 or more such days.

An Ed Hawkins-style warming stripes visualization of annual 90° days is below.

Note: Both charts are for 1872-2024, as the 2025 total is not final.

 

One factor to keep in mind with DEN is that the airport and primary station data for the city moved locations (further north and east of the city) in the mid 1990s. But the old Stapleton airport location has continued keeping records, and what that data shows is that the new location averages about 1-2 degrees warmer for highs most summers compared to the old one.

So yeah, we do see more 90+ days than decades ago, but the "explosion" since the 1990s you reference has been also been enhanced by the airport location change.

[donsutherland1]

12 hours ago, tacoman25 said:

One factor to keep in mind with DEN is that the airport and primary station data for the city moved locations (further north and east of the city) in the mid 1990s. But the old Stapleton airport location has continued keeping records, and what that data shows is that the new location averages about 1-2 degrees warmer for highs most summers compared to the old one.

So yeah, we do see more 90+ days than decades ago, but the "explosion" since the 1990s you reference has been also been enhanced by the airport location change.

Staptleton Airport was decommissioned, demolished, and replaced by Denver Center Park. The environment is quite different from what it was when record-keeping ended in 1995 at Stapleton Airport. The environment for the new ASOS isn't comparable to what it was at Stapleton Airport.

The warming summers since 1995 isn't an artifact of Denver International Airport. It is a statewide phenomenon. 12/15 (80%) of Colorado's summers with a mean temperature of 67.0° or above and all nine of its hottest summers have occurred since 2000.  Five of the last six summers (2023 being the exception) have had a mean temperature of 67.0° or above. Statewide maximum temperatures have risen somewhat faster than statewide mean temperatures.

[tacoman25]

4 hours ago, donsutherland1 said:

Saptleton Airport was decommissioned, demolished, and replaced by Denver Center Park. The environment is quite different from what it was when record-keeping ended in 1995 at Stapleton Airport. The environment for the new ASOS isn't comparable to what it was at Stapleton Airport.

The warming summers since 1995 isn't an artifact of Denver International Airport. It is a statewide phenomenon. 12/15 (80%) of Colorado's summers with a mean temperature of 67.0° or above and all nine of its hottest summers have occurred since 2000.  Five of the last six summers (2023 being the exception) have had a mean temperature of 67.0° or above. Statewide maximum temperatures have risen somewhat faster than statewide mean temperatures.

So let me understand the logic here: the move of about 15 mi primarily east to an entirely new location for the airport records is not important and totally comparable to historical records, but the the new ASOS environment in the same general location as the old airport is not comparable?

As someone who lives here, let me explain why you're wrong. In the summer, most late mornings/early afternoons see clouds/storms build up over the mountains and then drift east over the plains. Locations that are further west get the cloud cover sooner, and therefore tend to have a bit less sunshine warming in the afternoon. When the clouds produce cooling rains, that also reaches western areas sooner.

This is the primary reason that moving the airport significantly east of its previous location has resulted in a bit higher afternoon temps at DIA, compared to the old Stapleton airport. It's also the main reason why, if you look at Boulder's long term records, the average summer highs are about 2 degrees cooler than Denver's. The Boulder airport is at almost the exact same elevation as Denver, but is located about 15 mi closer to the mountains.

Mean temps are one thing, but there's no doubt that the location change has had a small, 1-2 degree (I'm happy to provide the data) warming effect on the airport's highs, which then results in more 90+ days. We have a ton of days here that top out in the 88-92 range, so you can see how the numbers would get bumped.

No one said this makes the warming summers since the 1990s an "artifact" of the airport move. In fact, I did say we have warmed and see more 90+ days regardless.

[tacoman25]

Inverse to the warming summer effect, the airport location move has had a cooling effect in the winter, especially on mins. I'll do a more detailed post at some point about this.

[donsutherland1]

2 hours ago, tacoman25 said:

So let me understand the logic here: the move of about 15 mi primarily east to an entirely new location for the airport records is not important and totally comparable to historical records, but the the new ASOS environment in the same general location as the old airport is not comparable?

As someone who lives here, let me explain why you're wrong. In the summer, most late mornings/early afternoons see clouds/storms build up over the mountains and then drift east over the plains. Locations that are further west get the cloud cover sooner, and therefore tend to have a bit less sunshine warming in the afternoon. When the clouds produce cooling rains, that also reaches western areas sooner.

This is the primary reason that moving the airport significantly east of its previous location has resulted in a bit higher afternoon temps at DIA, compared to the old Stapleton airport. It's also the main reason why, if you look at Boulder's long term records, the average summer highs are about 2 degrees cooler than Denver's. The Boulder airport is at almost the exact same elevation as Denver, but is located about 15 mi closer to the mountains.

Mean temps are one thing, but there's no doubt that the location change has had a small, 1-2 degree (I'm happy to provide the data) warming effect on the airport's highs, which then results in more 90+ days. We have a ton of days here that top out in the 88-92 range, so you can see how the numbers would get bumped.

No one said this makes the warming summers since the 1990s an "artifact" of the airport move. In fact, I did say we have warmed and see more 90+ days regardless.

My points are as follows:

1) Denver has seen a dramatic increase in hot days. I used 90° highs as an example. The increase is more than just a result of a location change, even as location changes have an impact, otherwise there wouldn't be warming trends across the entire state.

Indeed, I've often noted that adjusted data is superior to raw data, precisely because sites move, environments change, times of observation may change, etc. for climate purposes. 

2) The broad idea of a greater frequency of hot days does not negate the existence of localized climates or microclimates. Moreover, that idea applies beyond Denver. For example, Boulder has seen half of years with 50 or more 90° or above days and four of its five years with 20 or more 95° or above highs occur since 2000. For Pueblo, 14 of its 17 years with 80 or more 90° days and 9 of its 10 years with 50 or more 95° days have occurred since 2000. In short, in Colorado's populated areas, the 2000s have seen a disproportionate share of hot days. No other period rivals the 2000s in terms of the frequency of such heat.

3. The old Stapleton Airport does not have a continuous record. The old site was demolished and replaced with Central Park. A new ASOS was installed around 2022 there.