Colorado Weather Forecast | Aug. 27-Sep. 1, 2021
Isolated thunderstorms this weekend, heavier smoke returns, tropical storm impacts next week, cooler temperatures for Labor Day weekend.
Hello everyone. It’s been a busy week for us but we’re back to take a look at this weekend’s weather and some discussion about the long term forecast for the following holiday weekend.
Summary
- Friday: Fairly dry, isolated weak thunderstorms across most of Colorado. Seasonally warm, 90s expected for the Front Range. Slightly gusty in northern Colorado and along the Divide.
- Saturday: Isolated to scattered storms in southern Colorado, isolated storms near the Divide, very isolated elsewhere. Cold front for the Front Range will knock temperatures back a few degrees and push more smoke in. Smoke begins to increase significantly in western Colorado. Slightly gusty in northern Colorado and along the Divide.
- Sunday: Isolated to scattered storms near the Divide, very isolated elsewhere. Significant smoke west of the Divide, worsening through the day. Again, gusty in northern Colorado and along the Divide.
- Monday: Significant smoke for the Front Range, improving conditions for western Colorado. Isolated thunderstorms in the San Juans.
- Tuesday: Scattered thunderstorms west of the Divide. The rest of the smoke begins clearing out from south to north.
- Wednesday— Friday morning: The remnants of tropical storm Nora begin to impact the desert southwest and Colorado. Heavy precipitation possible for western and southern Colorado, likely on Thursday evening. Smoke should be gone.
- Labor Day Weekend: Cooler — snow possible at high elevations with any overnight precipitation. Possibly significantly colder for the Front Range, but lots of uncertainty. Perhaps a drying trend from Friday to Monday. Model agreement is low.
Discussion
Flow this week has been mostly zonal, with weak shortwaves traversing west to east to our north. These have been pushing smoky cold fronts down the Front Range every few days.
The shortwave currently approaching us is tapping into a pocket of moisture over Colorado and producing widespread scattered thunderstorms.
The best moisture will leave the state today, with drier air being advected in for the weekend. This drier air and subsidence behind today’s disturbance puts minimal thunderstorm potential in the forecast for Friday, though there will still be some weak, gusty cells developing over the high country.
By the weekend, it looks like we’ll see a weak pulse of surface-level monsoonal moisture from the Gulf of Mexico into Arizona and southern/eastern Colorado. Tropical storm Nora will likely make landfall on the Baja Peninsula this weekend (likely as a hurricane) which means the Gulf of California will not be providing us with our usual source of monsoonal moisture.
Another mid-atmospheric disturbance to our north immediately behind our current shortwave puts isolated afternoon thunderstorms in the forecast across Colorado for the weekend, despite the lack of moisture. Notice the pockets of pink colors in Colorado from Friday into Monday. That’s large-scale lift provided by the next shortwave.
Most prominently, you can see the signature of a major tropical storm making landfall on the Gulf Coast. More on that in a bit.
The highest thunderstorm chances this weekend are in southern Colorado (particularly Saturday) and near the Divide (particularly Sunday), but lightning will be something to think about this weekend outside of those areas as well.
Despite eeking out a few thunderstorms, Colorado will see another weekend that’s a bit on the dry side.
This comes on top of an existing precipitation deficit that has strengthened over the past couple weeks:
Luckily, moisture has favored the Western Slope, which has needed it more than the rest of Colorado.
We’ve seen a couple weak cold fronts this week push cooler temperatures and smoke down the Front Range, and our incoming shortwave puts another cold front in the forecast for Saturday, while the mountains remain warmer than average.
Though this has driven a few days of enhanced smoke this week for the urban corridor, the situation looks to deteriorate as a whole across the state as we get into the weekend.
If you watch the animation above, you can see surface-level smoke become significant west of the Divide this weekend, especially on Sunday. By Monday, the lack of any strong flow aloft (or at the surface) over our region allows smoke to settle across most of the state.
The reason we keep an eye on wildfire smoke is because of its potential toxicity. It is not common knowledge that wildfire smoke can actually be more toxic than fresh wildfire smoke, and current research suggests this sort of smoke is much more toxic than cigarette smoke. This still an emerging area of study, but initial findings have found that, after several days, wildfire smoke often breaks down into chemicals that are sometimes more toxic to humans, and these are more easily absorbed by your lungs compared to what is found in cigarette smoke. The Colorado Sun recently put out an article taking a look at some of this research.
Anyways, by Tuesday, this smoke should start to mix back out of Colorado from south to north, as the state will be between the remnants of two major tropical storms: Nora (landfall in the eastern Pacific), and TD9 (landfall in the Gulf of Mexico). TD9 is forecasted to strengthen into a hurricane and will likely be named shortly.
After impacting Baja California, the remnants of Nora will push northeast into the desert southwest and likely up into Colorado by Wednesday or Thursday. This should clear out the smoke and bring a ton of moisture into the state.
The influx of deep moisture puts heavy precipitation in the forecast from/around Wednesday to Thursday, with some of it lingering into the start of the weekend and possibly supported by a resurgence of monsoonal moisture. Depending on the speed of this storm, Wednesday, Thursday and/or Friday morning could be a washout for much of western and southern Colorado, but Friday to Sunday look like typical afternoon thunderstorm days, with the best coverage in southern Colorado.
At this point, there looks to be a slight drying trend from Friday to Labor Day across most of Colorado, but whether that means the end of the holiday weekend will be totally dry or just less stormy remains unclear. What is clear is that we’ll see a bit of a cooldown for the holiday weekend thanks to one more factor: the formation of another trough over the Pacific Northwest early next week.
We’re not sure how this will interact with Nora — it depends on timing and intensity of both systems. Nora will cause large-scale ridging in the very upper atmosphere which could limit the extent of troughing and the associated amount of cold air advection into Colorado by the weekend.
Conversely, Nora could track less east and further north, which would help advect colder air further to the south and into Colorado.
Since models can’t agree on how these systems will track and interact with each other, we can’t do much more than show you the two likely scenarios. Thus, when next weekend’s forecast begins to trend either colder or warmer as you check it over the next few days, you’ll know why it’s doing that.
If the colder solutions end up being the case, we’ll see the potential for small amounts of snow at high elevation over Labor Day weekend, which isn’t surprising for September. We would also see a much stronger cold front impact the Front Range. Initially, the urban corridor will likely cool into the 80s later next week, but the passage of the cold front (at some point that weekend) could drop high temperatures as low as the 70s or even the high 60s (if the latest Euro model is to be believed — we’re skeptical)…a taste of autumn. Again, if the warmer solutions end up being the case, the Front Range may remain lodged in the 80s. Either way, even the warmer guidance provides a respite from the 90 degree temperatures that were commonplace this month.
We’ll keep you updated.
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