Colorado Weather Forecast | July 27-Aug. 1, 2021
Drier and hotter before a very soggy weekend. Also, a review of July’s weather.
July is rapidly coming to a close, and it has been a decent month for Colorado, especially after June brought a spell of record heat and widespread wildfires. Though we were quite cynical about how this month would shape up, the monsoon has thus far driven a favorable atmospheric setup for precipitation in the desert southwest, with big totals in Arizona, western New Mexico, and southwest Colorado.
In fact, mudslides and flash floods have become the primary weather-driven concern over the past few weeks, with mudslides repeatedly closing I-70 and occasionally blocking off easy access to areas like Lake City. That’s not to say Colorado still isn’t dealing with the threat of fires — the Morgan Creek Fire (north of Steamboat — 6,000 acres) is still largely uncontained — but explosive fire growth has been mostly restricted to the drier reaches of northern Colorado.
The latest drought monitor has reduced the short-term drought in the San Juans, and we expect to see more wins in the next drought monitor map, thanks to consistently above-average precipitation in western and southern Colorado.
Down on the I-25 corridor, we haven’t seen triple digit temperatures in a while, with high temperatures coming in near average for the month and the warmer anomalies generally limited to the high country.
We still have a chance to see triple digit temperatures before the month ends, but we’ll talk about that in a second.
Meanwhile, much of the rest of the west has been baking under several well-publicized heatwaves as wildfires continue to explode in Canada, Idaho, Montana, and Washington. This map shows just how concentrated the fire activity is in the northern Rockies:
Several large fires in California and Oregon have been moderated by cooler, moister conditions lately, but could flare up over the next few days as the heat returns. What we’re really enjoying about this map is the absence of any satellite-detected fire signatures in Arizona, Utah, and New Mexico, thanks to our active monsoon pattern.
Due to a subtropical high over the southwestern United States, smoke from the fires has been generally relegated to the northern United States, with the jet stream transporting it all the way to the eastern seaboard.
Colorado has also received several pockets of smoke from these fires as smaller atmospheric features have driven easterly winds in the region, blowing smoke back into the state and pressing it against the Front Range.
Unfortunately, after a bit of a break yesterday and today, low and mid level winds around the center of high pressure forming overhead threaten to advect another serious batch of smoke into the state over the next few days— but it’s tricky to forecast this with surety.
A large ridge is building over the western United States, with the center of high pressure dropping southeast towards Texas over the course of the week. This is creating a strong dipole pattern across North America, with associated troughing deepening near the east coast.
For the rest of the week, abnormally warm temperatures near the surface will be advected into the Front Range.
This gives Denver a chance of hitting the triple digits tomorrow and/or Friday.
You’ll notice temperatures come way down for the weekend. More on that in a bit.
The monsoon isn’t as active as it has been lately, but it’s still pumping a bit of moisture into the region. The San Juans and Western Slope continue to see a more saturated water column, which is keeping the atmosphere more unstable and driving thunderstorm activity. Elsewhere, chances for precipitation remain low until Friday.
Compare the next couple days of thunderstorm chances for Mt. Eolus (San Juans)…
…with Mt. Bierstadt (Front Range).
Again, a big change for the weekend.
Right now, flow aloft is helping transport mid-level moisture from the Gulf of Mexico into the western United States (some of which western/southern Colorado is able to tap into) and flow near the surface is advecting low-level moisture in from the Gulf of California into the desert southwest.
Meanwhile, dry, hot (and potentially smoky) air is working its way down the Front Range. This is a pretty typical monsoonal pattern — again, northern and eastern Colorado don’t actually show a very strong signal in regards to the monsoon, its impacts are mostly felt in southern and western Colorado.
By Friday, the ridge overhead elongates quite a bit and a couple of other features in / near the Pacific Ocean start to pump moisture directly into our region. At the same time, upper-level flow becomes much more meridional and vigorous and the jet stream sags down into our area. This introduces lift into the region and advects much cooler air from Canada into northeastern Colorado, all of which arrive alongside a ribbon of robust moisture that is also currently to our north.
Watch this animation carefully. At the start, you can see the signal of monsoonal moisture being reflected by cooler temperature anomalies across the southern Rockies and desert southwest, while abnormally warm air works its way down the Front Range.
As we get into the weekend, you can see a cold front pushing into into the Front Range from the northeast. This could result in a 20 degree difference in high temperatures from Friday to Saturday, depending on timing and intensity.
As this is a surface level feature, the plains, Front Range and northern / easternmost mountain ranges will be most impacted by the temperature changes. However, precipitation chances will be quite high across Colorado since we are receiving moisture from a variety of sources, though the better lift looks to be in northeastern Colorado.
And by “quite high,” we mean it. The Blend has some very decent precipitation totals by the end of the weekend, with most of the state getting in on the action.
We should pedantically point out that the Front Range’s wet weekend is not monsoonal in nature, with precipitation being driven by factors somewhat unrelated to the monsoon. The San Juans will continue to see much of their moisture this weekend driven by the monsoon, but this is certainly not the case for northern and eastern Colorado.
So should you be changing plans this weekend in response to the potential precipitation? At this point, it still looks like the mornings will be okay (maybe even downright pleasant, if you’re climbing or running at low elevations), if a little unsettled. The cold front is a bit of a wildcard, as it will likely come in many hours before models are currently expecting it to, which could get things going earlier on Saturday.
In the Front Range mountains, probability of precipitation remains escalated through the weekend, never really dipping below 20%, even in the mornings.
This is true down in the metro area and the foothills too:
However, thunderstorm chances on Saturday and Sunday mornings are not that high — precipitation in the mornings would be stratiform in nature (drizzly, foggy).
If we jump over to central Colorado we can see morning precipitation potential is lower than the eastern mountain ranges, as the cooler, moister air near the surface has trouble getting through the mountains to the northeast.
Finally, the morning precipitation potential is even lower in the San Juans, which will see more typical diurnal thunderstorm conditions that are still largely driven by an uptick in monsoonal moisture from the south.
Make no mistake: Friday, Saturday, and Sunday afternoon’s thunderstorm activity should be fierce and widespread, and heavy rains will likely squash your plans in a hurry throughout the state.
This will be true throughout the Rockies — which is great news in regards to the fires in Idaho and Montana.
This moisture looks to work its way back out of the region next week, with precipitation chances dropping on Monday and potentially continuing downwards into the week. The subtropical high associated with the monsoon looks to entrench itself over the Four Corners region again, which is where it has been frequently located over the past few weeks — take that as you will.
Keep an eye out on our social media for some updates regarding the potential smoke this week, in addition to updates to our weekend forecast as models begin to come into better agreement over timings and impacts.
Thomas Horner (Twitter: @thomaschorner)
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