ForecastFlood Watches in Effect Across Western CO Ahead of Significant Storm Tue-ThuWidespread precipitation is expected across Colorado, particularly western and southern Colorado, from Tuesday to Thursday. Drier Friday and Saturday, another chance for cooler weather and precip on Sunday.
- A significant storm system is bringing widespread precipitation to most of Colorado from Tuesday to Thursday.
- This will kick a potent cold front down the Front Range on Wednesday.
- Things get started Tuesday afternoon, impacting western Colorado first (the San Juans, Grand Mesa, west Elks, Flat Tops, and Park Range).
- By late Tuesday and early Wednesday, widespread precipitation should push across most of Colorado.
- Precipitation should mostly be light to moderate, with some embedded cells of heavier precipitation.
- Pockets of precipitation continue into Wednesday morning, becoming more widespread again by the afternoon.
- This should continue into Thursday morning and finally taper off Thursday night.
- Drier on Friday and Saturday. Freezes overnight at/above treeline could make for tough conditions.
- The next system should impact Colorado Saturday night / Sunday morning, with a cold front for the Front Range on Sunday and more precipitation (favoring the Divide) that day as well.
Lightning Risk Grids
It's shoulder season -- the leaves are changing, and some of the high peaks are often carrying snow and ice in the mornings. As the jet stream begins its southwards migration for winter, we're seeing better and better chances of significant storm systems impacting our region, such as what is coming Tuesday to Thursday. This is the last week with sunsets after 7pm.
Though September is frequently an excellent month to get out and do alpine objectives, it's important to be prepared to run into snow and ice above treeline. This weekend, some overnight and morning precipitation on Longs Peak resulted in treacherous, icy conditions, leading to the death of a climber. The National Park Service points a webcam at this peak 24/7 -- it's helpful to check it the night or morning before a climb.
As of now there is a significant high pressure system the southeast of Colorado which has been keeping us quite warm and on the dry side, though some moisture has been sneaking into southwestern Colorado thanks to anticyclonic flow around this system.
More and more of this moisture will work its way into southern and western Colorado tomorrow as high pressure builds further and the jet stream strengthens ahead of a closed low to our west which will lift over the region this week.
Widespread Precipitation: Tuesday to Thursday
The National Weather Service has issued Flood Watches for much of western Colorado for tomorrow. Initially, precipitation will be mostly driven by convection and thus it's likely the show wont really get started until Tuesday afternoon.
Just as this gets going, we'll see an uptick of large-scale dynamics and even better moisture fetch as the closed low to our west starts to lift eastwards:
This will cause precipitation to become more widespread by Tuesday night, even if it diminishes in strength due to better atmospheric stability. You can see that on the latest high resolution CAMs. Check out this animation (which starts 12pm Tuesday), and how heavy thunderstorm coverage in the afternoon transitions to widespread moderate rain across most of Colorado by the evening and nighttime hours:
On Wednesday morning, widespread cloud coverage should be in place across Colorado, which would likely inhibit thunderstorm development that afternoon. Regardless of lightning risk, it's quite likely we'll see pockets of light to moderate precipitation throughout the high country.
By that evening, there is disagreement in the convective environment across models, but we could see a decent round of storms push west to east across Colorado. For many areas, Wednesday PM features the best chances for heavier precipitation.
On Thursday, the jet starts to become oriented more west-to-east as the closed low continues to lift northeastwards. This puts Colorado into the right entrance region of the jet, which would result in better dynamic lift. However, the plume of deepest moisture becomes more narrow, and this will be the deciding factor for which areas on Thursday get another day of heavy, consistent rain, and which areas don't.
Right now, the Euro model is generally favoring the southern half of Colorado for the best precipitation chances on Wednesday:
This should mostly clear out by 12am Friday.
Overall, this looks like a pretty potent storm, with some impressive precipitation totals forecasted by most models and ensembles. Here's the Blend, which seems to be on the more optimistic (wetter) side of things:
The Euro ensemble is a little stingier outside of the San Juans, but still shows good chances of an impactful few days across the high country:
We've mostly mentioned rain, despite our opening discussion mentioning how common snow and ice is up high this time of year.
With this storm, we're pulling in a lot of warm, deep subtropical moisture. We'll actually see rain/snow levels go up in elevation over the next few days. Models currently have the rain/snow level at 13,500ft-14,000ft on Tuesday and Wednesday. The rain/snow level only falls again on Thursday, which means the best chances for significant snow above treeline come that night, which doesn't really coincide with decent precipitation chances.
Despite the fact that this will mostly be an all-rain storm below the very highest summits, Friday morning could be tricky as many areas above treeline should see a good freeze, which would result in very icy conditions. Low temperatures will likely continue to be below freezing in parts of the mountains each weekend night as well. Beware!
Cold Front Wednesday
The other major event with this system will be a strong cold front which is expected to impact the Front Range on Wednesday. This will result in a major temperature drop.
High temperatures will drop from the high 80s/low 90s on Tuesday to the low 50s on Wednesday afternoon.
In the northern Front Range, the leading edge of the front is expected to hit late late Tuesday (around midnight), with it reaching Denver by the early am hours of Wednesday and pushing further south down I-25 into that morning.
It's not looking like this front will provide a very deep upslope, thus despite some widespread low clouds and light precipitation on Wednesday morning, heavier precipitation will be rather limited.
This front will only have limited impacts to the west of the Divide, with the mountains experiencing a cooldown of 10-20 degrees vs. the cooldown of 25-35 degrees that eastern Colorado is expected to see.
Friday and Saturday
Friday and Saturday look quite dry, with most areas warming up a little bit. As we mentioned, temperatures in the higher elevations of the mountains will likely dip below freezing, and hard ice may be a concern.
Next System: Sunday
On Sunday, it's looking like a shortwave will slip southeastwards across the region.
This would kick another cold front down the Front Range and bring some (overnight?) precipitation impacts to northern Colorado and parts of the Divide. It's a ways out, so let's see!
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