The first snowstorm of the winter season is headed our way! We discuss when the cold air will arrive and how much snow the Front Range can expect Saturday night into Sunday.
General set-up
A quick-moving storm system is staged to bring our region’s first widespread snowfall of the season (for the lower elevations) and has shown remarkable consistency in the models for many days now. The timing and snow amounts won’t be much different than what we discussed in our forecast issued back on Tuesday. Things are coming together quite nicely. Finally a proper autumn snow event! In the 500 mb height anomaly animation below, you can see the main storm system dropping out of Canada and into Colorado on Saturday. A weak cut-off low will be merging with the main system near the Four Corners as well.
GFS 500 mb height anomaly forecast animation for Saturday into Sunday.
This merger isn’t only going to happen at 500 mb. If we look higher in the atmosphere at 250 mb (the level of the jet stream), we see that a similar union is also taking place. The subtropical jet stream is meeting head-on with the polar jet stream right over Colorado. This set-up will bring together moisture from the south and cold air from the north, with the end result being a wintry mess for nearly all of eastern Colorado.
GFS 250 mb wind forecast for Saturday night. The polar and subtropical jets will meet over Colorado.
Timeline of the storm
After sunshine and highs in the upper 50’s to lower 60’s early Saturday, the surface cold front will arrive into the Metro area from the northeast late Saturday afternoon, between 3:00PM and 6:00PM. Temperatures then quickly fall into the 30’s and eventually the middle 20’s overnight into Sunday morning. Sunday’s high temperature will most certainly be at 12:01 AM. A high temperature in the 20’s so early in the season is nearly 40 degrees below normal and will surely make the record-books for some Front Range cities.
NAM 800 mb temperature map for Saturday night at midnight. The cold air will be in place.
After the frontal passage, it will take a little while for things to saturate and precipitation to begin reaching the ground. We expect that the Plains will see a brief period of light rain (a couple hours or less) before fully changing to snow quickly Saturday evening. More than likely, snow will be falling before 9:00 PM in most areas.
The upslope for this event will be fairly good. We’ve seen better, but we have certainly seen worse. There is decent model agreement overall that upslope will remain intact from Saturday evening into Sunday afternoon, topping out at about the 700 mb level. The direction looks good as well, efficient from the northeast through the duration of the storm. More times than not, we battle a northerly direction which can downslope the northern Metro area from the Cheyenne Ridge. This does not appear to be the case this weekend. With this upslope in place, widespread snow will continue into Sunday afternoon.
NAM time-height forecast of temperature (red lines), winds (barbs), and relative humidity (blue contours). The upslope for the storm is boxed in pink.
The passing polar jet stream will also be playing into the snow potential from this storm. Northern Colorado will be in the right entrance region much of Saturday night. This means that a few bands of heavy jet-forced snow will be inter-twined within the generally light upslope-forced snow. It’s difficult to say exactly where these bands will develop, but our best guess right now is somewhere from Denver northward to Fort Collins. Snowfall rates in these bands will be 1 to 2″ per hour at times.
The overhead jet stream will spawn a few heavy bands of snow during the event across northern Colorado.
Snow amounts
The GFS and European ensembles are presenting good agreement for around 0.5″ of liquid for the Metro area, with up to 1″ in the Foothills. 90% or more of this will be snow.
Our snowfall probability charts tell a convincing story for the Front Range for this event. Across the lower elevations, there is a near certainty of seeing at least 2″ and fairly high chances of seeing at least 4″. Percentages drop down from there sharply, especially for areas east of Interstate 25.
Our snowfall forecast map for the region is below. Generally we expect 3-6″ in most of the Denver Metro area, with those upslope favored spots like the city of Boulder possibly seeing a little more. The biggest snow totals will be between 7000 and 9000 feet elevation in Boulder and Larimer Counties where we are predicting 5 to 10″. Localized amounts of 12″ may be possible in this region as well. The ground is warm which may limit accumulation slightly in the early part of the event. However, with temperatures dropping well into the 20’s as the storm progresses (teens in the Foothills), this isn’t a major concern. We can’t stress enough that our confidence for this snow event is quite high.
Travel throughout the region, especially in the Foothills, will become treacherous after midnight Saturday night and through Sunday morning. We would advise against any major ventures unless necessary. We all know everyone forgets how to drive in the snow here, even if our “snow free” season is only a few months.
That’s all for now. We’ll pass along any updates to this forecast if needed.
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