Mar 20, 2021
Storm Forecast | Mar. 20–22, 2021
A look at expected snow totals, uncertainty, and timings.
Header image: Thomas Horner
Snow gets started this evening/tonight, but not before a brief return to springtime. Let’s take a look at the next few days of sun and snow.
Current Conditions
Yesterday, higher temperatures in the Front Range were limited by a near-surface inversion which not only kept temperatures in the 50s at best but created poor air quality.
By the afternoon, the inversion broke for most areas south of I-76/270, allowing temperatures to rise into the low 60s in some places and air quality to improve significantly.
A user of the Colorado weather Discord server captured the obvious gradient of trapped pollution from north to south:
We are in better shape today, but still not great. Gusts are increasing in the mountains as a jet maximum gets closer to the state, which will kick off the start of our next storm.
We’ve seen some gusts up to 70mph recorded in the mountains in the past hour, and a mountain wave is setting up east of the Divide which is bringing gusty conditions down to parts of the High Plains.
Storm Forecast
Most snow will accumulate between 6pm today (far northwestern Colorado) and 6pm Monday, with heavier snow reaching Summit County from the northwest by tonight. Another storm moves in quickly behind this one, so snow wont fully stop, but Monday evening is a good delineation between the significant accumulations of this storm and those of the next.
You can see two phases of this storm: an initial burst of heavy snow tonight as we enter an area of favorable lift from the jet, moisture becomes more readily available, and southwesterly winds aloft meet northwesterly winds near the surface…
…and phase two on Sunday night/Monday morning: an upslope Front Range storm caused by a 500mb low closing off south of Colorado on Monday.
The track of this low still has some uncertainty, which means our forecast for the Front Range (urban corridor, foothills, and higher terrain near the Divide) still has a bit of uncertainty, and we may update our forecast for these areas tomorrow.
Most guidance suggests a fairly modest event along the urban corridor, especially when factoring in warm temperatures and compaction. However, a 5–10" event in Denver is not entirely out of the question, even at this point, though it’s not the most likely scenario.
That range of totals is more likely as you move west and south from the city and up in elevation: snow-liquid ratios increase, rain isn’t a concern, and melting/compaction will be less pronounced, and of course, the intensity of snowfall increases due to upsloping winds. As an aside, our totals (below) for the foothills are generally for elevations at 9000ft and below — snow accumulations could be a bit higher up at 10,000ft.
Here are our expected totals through Monday evening:
In general, we need most areas that are significantly west of the Divide (like Vail, Steamboat, Aspen, and parts of the the San Juans) to pick up 2–4" of snow through tomorrow morning, or they will have a difficult time hitting their forecast range, as the second phase of the storm doesn’t favor most resorts that aren’t near the Divide (or east of it).
The HRRR currently expects 1–5" of snow to fall between this evening and tomorrow morning:
After that, many areas that aren’t near the Divide will likely struggle to pick up more than a few inches of snow through Monday evening.
What about potential upsides? We do see a chance at over a foot of snow for some places, if not as likely as skiers would prefer:
The highest chances for good powder through Monday evening are along and east of the Divide (Indian Peaks Wilderness, RMNP, Eldora, Hidden Valley), near Berthoud Pass and Jones Pass, near the 14ers south of I-70 (Greys, Torreys, Evans, Bierstadt, Guanella Pass, and A-Basin / Loveland to an extent), and in the remote parts of the Gores, Elks, and Flat Tops.
In general, we do see some models putting down 10–15" at resorts near the Divide, but as is the case with closed lows, we think this is overdone and thus most of our forecast ranges top out at 12". This could change a bit depending on the track of the closed low.
Let’s regroup tomorrow and see how we’re doing and where the closed low is tracking. You can expect a 1–5" inches of fresh snow tomorrow morning at most resorts, with higher end totals most likely in and around Summit County (maybe some totals a bit above 6" if we’re really lucky).
Here’s a summary of timings:
- A burst of moderate to heavy snow late this evening / tonight for many parts of the state
- Pockets of light and moderate snow during the day on Sunday
- Snow intensifying along and east of the Divide on Sunday evening
- Moderate to heavy snow Sunday night / Monday morning in the Front Range, lighter as you get west of the Divide
- Snow tapering off by Monday afternoon
- Along and west of the Divide, pockets of light to moderate snow due to northwest flow on Monday afternoon
Forecast by
Thomas Horner (Twitter: @thomaschorner)
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