Updated Thu May 19, 2022 7:00pm MST | Published Thu May 19, 2022
- Impactful winter storm for northern and eastern Colorado starting tonight and into Saturday afternoon.
- Widespread double digit snow totals are possible along and east of the Divide, and in the Front Range foothills.
- The I-25 corridor will likely see meaningful snow accumulations. This could cause significant damage to trees.
- Hard freezes are possible on Friday night and Saturday night, with damage to gardens, agriculture, and swamp coolers.
- Travel to the mountains will be very difficult by Friday afternoon.
We’re on track for an impactful winter storm to end the week, with widespread precipitation and much colder temperatures progged from Friday to Saturday, at least in northern and eastern Colorado.
Weather models have more or less been 💥locked in💥 for the past 24 hours, with pretty solid agreement this morning:
The Blend has also been unwavering, with very similar totals (if a slight downward trend) over the past 24 hours:
We’ll note that the Blend has more advanced methods of calculating the snow level and snow-liquid ratio when compared to the animation of model runs that we shared earlier. This has resulted in a much tighter gradient of snowfall accumulations from lower elevations to higher elevations, with the snow forecast for Denver being dramatically cut down from what weather models are indicating.
However, we do have to wonder if the Blend is being a little too pessimistic here. There are certainly some dynamics which would drive better snow production over the I-25 corridor. We’ll go over those in a bit.
Another product that seems more sane to us is the SREF (Short Range Ensemble Forecast) – this has a pretty tight gradient across elevations and also doesn’t have as much snow to the west of the Divide, which is another thing we think the global models are overdoing:
With that in mind, here’s our generalized forecast map. The biggest question mark is over the I-25 corridor, where melting, compaction, timings, and dynamics will make a huge mess of what accumulations we actually see on the ground.
We’ve bumped high end totals down a little bit across the board, but not by much. This will be a very impactful storm and we currently see more ways that this could go right vs. wrong (bust high potential).
If we jump over to probabilistic forecasting, we’ll note the spreads are a little more locked in, with high confidence of an impactful event:
Ski areas have a pretty optimistic forecast – too optimistic?? – with even the low end 25th percentile looking pretty good.
Actual snow totals have almost never come in below the 25th percentile the entire past winter season. We do think the 75th percentile looks to be tough if impossible to hit for many ski areas.
Finally, a map of the probability of more than a foot of snow falling reveals increased confidence in the forecast – generally, areas that had a higher chance have seen probabilities go up, while areas that had a lower chance have seen probabilities go down:
So what could go wrong to ruin this forecast? Let’s take a look at some of the dynamics at play!
Dynamics – What will make or break the storm?
Let’s talk about WHY water is even supposed to fall from the sky. The two main ingredients are moisture and lift, which will have in spades from Friday to Saturday.
This factor is pretty straightforward, and one we’re not concerned about.
A nice plume of moisture will drape over Colorado on Friday and push south into Saturday. This will be plenty to work with, and precipitation total will thus mostly be defined by lift/dynamics at play as opposed moisture availability.
The moisture column looks to be quite saturated, especially at low levels. We’ll just need to watch for the potential to entrain drier air from the north earlier on. The extent of heavier precipitation to the west will also be limited by moisture availability.
As the jet drops over northern Colorado, it will introduce a source of widespread, large-scale lift. A bit of coupling and some favorable curvature introduces upper level divergence, and thus, lift.
More specifically, we’ve been eying the potential to be solidly in the right entrance region of a jet max on Friday evening. THat’s a favorable quadrant to be in.
On its own, this can drive precipitation. But at the very least, we’ll have an environment which can enhance smaller-scale dynamics.
We can verify some of this large scale lift by looking at an omega chart:
The red colors over Colorado are what we like to see. For comparison, take a look at the same chart for Sunday morning, when no precipitation is expected:
Orographic Lift: The Upslope
You know it, you love it. Air being forced upwards by terrain is certainly a lifting mechanism. When we get moist low-level winds upsloping to higher terrain, we get precipitation.
Conversely, downsloping winds have the opposite effect. That’s why our forecast favors Denver over Fort Collins, as Fort Collins is at the southern base of the Cheyenne Ridge. With a northeasterly upslope, winds will be downsloping into the city.
The “depth” of the upslope is important for delivering snow higher up the foothills and to the Continental Divide. With many storms like this, the middle elevations of the foothills get several feet of snow while the mountains get something like 6-14”.
This is due to the fact that the by the time the upslope makes it to the mountains, it is weaker and drier, having expended all of its energy over the foothills.
Look for a pretty robust, deep northeasterly upslope to develop for the Front Range around Friday evening. Will it be enough to deliver over a foot of snow to the Divide? It’s possible.
With jet streaking in close proximity to Colorado, we’ll likely see bands of heavier snow develop.
These bands tend to hang out over specific areas, resulting in two or three times as much accumulation in the same time period as areas that may be nearby but not under the band.
In this case, a jet streak is expected to drop south across Colorado, which should allow many parts of Colorado to enjoy its enhancing effects – but we’ll likely see some smaller bands pop up as well which will mess with the forecast in localized areas.
One major benefit of this sort of feature is that it brings snow to areas that may not be favored by upsloping and would otherwise be getting very little snow. In areas where upsloping is occurring, precipitation is enhanced.
We can visualize the banding potential from the jet by looking at a mid-level frontogenesis data:
This doesn’t appear to be a major factor with this system but it could come into play. For instance, the Euro has some weak convective instability over Summit County and parts of the Front Range mountains on Friday night.
This lifting mechanism tends to organize snowfall into more localized bands and cells of heavier snow. As discussed with jet-induced banding, this results in some highly variable snow totals across small areas.
You can see quite a few smaller bands of precipitation north of the primary jet-induced banding on Friday night, co-located with the above area of higher instability.
We’ll also be looking for conditional symmetric instability, which is another dynamic that can produce strong bands of heavy snow… and is really difficult to explain in layman’s terms. Weather5280 has a good write up of it here.
The passage of fronts provide a source of lift, which can drive precipitation if moisture is available.
We have a very strong cold front progged for the region tonight:
However, there’s not enough low-level moisture immediately available for this to drive precipitation, so it wont be a factor.
Isentropic Lift: The Barrier Jet
This is a fun, somewhat ephemeral dynamic that is fairly unique to the Front Range and other areas that border large mountain ranges.
A barrier jet (not to be confused with the jet stream) forms at the base of the foothills when low-level winds from the north-northeast are forced to run parallel to the terrain. The barrier jet features enhanced wind speeds and colder air.
This jet of cold air near the surface basically acts as though it were higher terrain – winds are forced upwards over it, essentially generating more lift further east of the actual terrain (more directly over the I-25 corridor).
In the above image, the green arrows are the upsloping winds. The red arrow is the barrier jet (this is actual apparent on the weather model’s wind product, with the blue splotch along the base of the foothills).
Some hatching to the east of the barrier jet indicates where we may see some enhanced lift due to winds being forced over the barrier jet (ROUGH APPROXIMATION).
If we look at a cross section, as though we were standing on the ground on I-25 and looking north, we can see the core of the barrier jet from the same weather model. It’s circled in blue:
A vertical velocity graph shows some lift to the east of this jet.
With today’s model guidance, it looks like winds will be too easterly to set up a strong barrier jet, but we’ll keep an eye on things.
Temperatures, Snow Liquid Ratios, and Melting
The snow forecast will be quite interesting below 7,000ft. thanks to warm temperatures and warm surfaces. We expect the first few inches of snow to melt and compact, with the roadways generally being just slushy. For some areas, much of the precipitation during the day on Friday may be rain.
While road impacts may be minimal, the amount of snow melting overall may not be as intense as expected. Consider the potent September 2020 storm for the Front Range. After three days of temperatures in the 100s and 90s, Denver and Boulder picked up 2-6” of snow with 0.4-1.0” liquid water equivalent. A charitable snow-liquid ratio would indicate that only the first couple inches or so melted, though this is measured on a raised surface.
Really, it comes down to snowfall rates. Heavy snowfall will overcome surface melting and start to accumulate.
Precipitation rates will also play a major factor of when rain changes over to snow at lower elevation. The heavier the precipitation, the more evaporative cooling that occurs, and the lower the air temperature. If dynamics play out right, we may see snow earlier than expected for areas underneath strong bands.
We also must consider that models are usually too slow and a bit too warm with cold fronts, so it’s quite possible we see colder than expected air move in faster than expected, which could nudge the change over to snow earlier. With the snow level at around 6,000ft. on Friday AM, it wont take much for the I-25 corridor to see flakes before night falls.
Impacts: Snapped tree limbs, frozen gardens, broken swamp coolers
Even if the roads in the Denver metro area are merely slushy, this will be a very impactful event across the region. Snow or not, the entire area is at risk of a hard freeze on both Friday and Saturday nights.
Damage to Trees
Heavy, wet snow will collected on leafed trees and cause limbs to snap. There’s a good chance for widespread damage if this storm does overproduce. This will likely result in some power outages as well.
Damage to Gardens and Agriculture
With hard freezes possible on both Friday and Saturday nights, it will be difficult to protect plants. Probabilistic temperature guidance suggests it’s possible for much of the urban corridor to dip into the high 20s on both nights. Also, if you’ve gotten your swamp cooler going already (we certainly have), you’ll need to drain it.
You can try to cover your plants. Consider putting some buckets of water alongside the plants. When the water freezes, it will release some heat.
If you have a bird nest on your property, you can protect the babies by wedging an umbrella into the branches about 12 inches or so above the nest. This should keep them safe from the heavy wet snow. I would wait until Thursday night, after the winds die down, to place an umbrella in the tree.
Travel up to the mountains by Friday afternoon will start to become quite difficult from Golden to the tunnel, even if the road surface starts off warm. Visibility will be poor and the roads will eventually start icing up, perhaps spectacularly so if there’s a good layer of liquid water on them.
Add in the fact that swarms of people from the Front Range (without snow tires) will be mobbing up to try to catch a last chance of pow at the still open resorts.
Things get better by Saturday morning, but there’s still the potential for a nasty, poorly-placed band of heavy snow across the I-70 corridor (or 285).
Travel in the metro area for the evening commute may be tough – it’s hard to say. Avoid it if you can.
We’ll see a powerful cold front push down the Front Range tonight. With a high temperature of 88 degrees at Denver International Airport today, a plunge into the 30s by Friday morning will be QUITE the change.
Heavy precipitation will push from north to south across Colorado, starting around midnight tonight, with the bulk of it reaching Denver by around noon on Friday.
We expect most snow to wrap up by Saturday afternoon, with lingering impacts further south in Colorado into that evening.
Here are some meteograms for a few areas:
And here’s the simulated radar for the next two and a half days:
Be in touch! We’ll be frantically posting to social media as this unfolds.