Snowpocalypse or Fauxpocalypse? A Guide to Powerful Spring Storms on the Front Range
What causes the I-25 corridor and the foothills of Colorado to get so much snow, and do the mountains get any of it?
Header image: Water vapor imagery of Winter Storm Ulmer, the March 2019 Bomb Cyclone (source: the Weather Channel)
Summary
- An overview of how extratropical cyclones form and intensify
- The anatomy of an extratropical cyclone
- Small-scale features that can enhance snowfall production from storms like these
- Factors that would cause historic or disappointing totals in the Front Range (Fort Collins, Denver, Colorado Springs)
- A discussion on how these storms usually impact the mountains
- Some tips for getting accurate weather information from social media
The morning of Monday, March 8th 2021 was beautiful across Colorado with unseasonably warm temperatures heralding the encroaching end of prolonged winter weather. Meteorologists were aware of the potential for a snow storm later in the week or into the weekend, but unsure of what impacts it would have.
Every six hours (or even more frequently), weather models attempt to predict the weather far in the future by ingesting new meteorological data from weather stations, balloons, satellites, and other sources and use that data to perform incredibly complicated calculations. On Monday morning, weather forecasts were performing their usual routine of going over new weather model data to see how the weekend might unfold.
Suddenly, the Twitterverse and other social media circles were alight — the American GFS model was showing snow accumulations of well over three feet across the Colorado Front Range.
Other weather models very quickly joined in on predicting eye-popping totals for northeastern Colorado, and forecasting guidance converged on the high likelihood of a very impactful snow storm through the weekend. As we sit here two days before the start of the storm, those impacts remain to be seen.
Maps of incredible and often completely outlandish totals (90"+ in Estes Park?) spread like wildfire across social media for much of the week, but is there any validity to them? How is it possible that a storm could drop so much snow on northeastern Colorado?
We’ll describe the major components of a storm like this, and how they work together to produce incredible snowfall. We need all of these ingredients to come together in the right place and at the right time to have any reasonable chance of snow totals being measured in feet, and not inches. We also talk about mountain snowfall potential and how to get robust weather forecasts from social media at the end of the article.
Cyclogenesis: Colorado Gets Cyclones, Too!
You can think of a cyclone as a rotating air mass that is centered over an area of low pressure. In the northern hemisphere, winds circulate in a counterclockwise direction around the center of low atmospheric pressure. Lower pressure can often mean a more powerful cyclone. These systems can bring little more than widespread cloud cover and light precipitation… or winds over 100mph with heavy, prolonged precipitation!
The most well known types of cyclones are of the tropical variety — hurricanes and typhoons. However, cyclones can form north of the tropics too, and are called extratropical cyclones (ETCs) or mid-latitude cyclones. Tropical cyclones (such as hurricanes) can also wander outside of the tropics and become extratropical, but when we mention ETCs in this article, we are discussing cyclones that form outside of the tropics, through processes that are significantly different than what form a tropical storm.
The process that forms a cyclone is called cyclogenesis. Pressure at the surface begins to drop, and winds start to strengthen around it the area of lowest pressure, becoming counterclockwise. This, of course, doesn’t just happen randomly.
Closed Lows
A frequent culprit of ETCs around these parts are closed lows.
A closed low refers to an area of upper level (higher up in the atmosphere) low pressure that is closed off with at least one thickness or geopotential height line. A closed low can be distinguished from a trough by looking at upper air maps. These maps show wind speeds and geopotential heights at the 500mb level, which is well above the surface of the Earth:
Closed lows have earned a reputation for creating unique and unseasonable weather. These systems can manifest under ridges and during more zonal (horizontally flow aloft) conditions. Closed lows can develop during any season, but tend to peak in the US during the spring months. A major closed low that visited Colorado recently was in September of 2020. This surprise snow storm blanketed the central plains in several inches of snow, and was then followed by some of the State’s hottest October days on record.
These systems have vertically stacked mid to upper levels — think of a column rising straight up from the surface of the Earth well into the upper troposphere — and become a large-scale feature that is separate from the jet stream. As a result, both weather models and meteorologists struggle to accurately forecast closed lows.
Lee Cyclogenesis
A closed low can result in a mature ETC through a few processes, but in Colorado, this usually occurs due to lee cyclogenesis.
In general, a leeside trough forms as westerly air flow passes over a north-south oriented mountain range, creating an area of surface low pressure as it descends terrain on the leeward side of the mountains. In Colorado, that would mean westerly air passing over the Rocky Mountains and descending onto the High Plains east of the Continental Divide. This is often referred to as the Colorado Low.
As air descends the eastern slopes of the Rockies, it is forced to stretch vertically as elevation decreases. This stretching creates an area of convergence at the surface, which then helps curve the air cyclonically, increasing its relative vorticity. This concept is known as the conservation of potential vorticity.
The diagram above is as though you were standing on the surface of the Earth and looking north at the horizon. Think of the flat area to the left as the Intermountain West, the hump in the middle as the Rocky Mountains, and the flat area to the right as the High Plains. The diagram shows an air parcel moving from west to east over the mountain and stretching as it descends into the lee of the mountains.
Now, let’s look at that same process but as though you were high above the surface of the Earth, looking down at the ground:
Leeside troughs tend to follow similar paths as they traverse west to east. The air column generally moves to the north as it is forced over a topographical barrier (the mountains) and compressed, before diving back to the south as it descends and stretches.
In short, basic physical laws of conservation result in systems intensifying and undergoing cyclogenesis as they descend from the Rockies and into eastern Colorado.
Basic Anatomy of an ETC
In the situation above, a closed low has moved over the Rockies and is now undergoing rapid cyclogenesis, with surface pressure dropping and counterclockwise, cyclonic winds intensifying around it. A mature ETC over the midwest looks like this:
That’s our bomb cyclone from March 2019 — we’ll describe what a “bomb cyclone” is in a second. This storm dumped about eight inches on Denver and produced winds up to 110mph.
ETCs are often shaped like a comma. The “head” of the comma is where we can expect snowfall, and “tail” can sometimes produce severe weather outbreaks in the southeastern US. Typically, the highest snowfall amounts are found northwest of the surface occlusion (this is located under the “comma head”).
Here is the surface map from the March 2019 Bomb Cyclone. The purple frontal boundary on this map indicates the surface occlusion.
The intense snowfall under the comma head of these cyclones can be attributed to a system of low to mid level jets that fuel the storm. These jets are described as the dry air intrusion, warm conveyor belts, and cold conveyor belts.
The dry air intrusion (yellow arrow), along with the warm (red) and cold (blue) conveyor belts circulate around the low-pressure center, which is just east of the Colorado/Kansas border in the above image. The warm conveyor belts advect moisture and warm air north, along the east side of the low center. The cold conveyor belt then carries available moisture to the west, creating areas of heavy precipitation on the northwest side of the low.
This is why small changes in the movement (track) and location of these sorts of storms is so important for whether it will be a monumental snow event or disappointment. The storm only needs to track a hundred miles or two in a certain direction to, for instance, put much of the Front Range into the dry slot (hardly any snow), or conversely, put the Front Range to the northwest of the “comma head” (huge snow totals).
Weather models have a difficult time accurately predicting the track of these systems, which often bob and weave unexpectedly, and that’s a primary reason why forecasting these big storms remains highly uncertain even up to the day preceding its arrival! This also means that totals will often be huge from the Palmer Divide and north (sparing Colorado Springs and Pueblo), or the opposite (sparing Denver to Fort Collins), depending on where the center of the low tracks. Generally, Denver and Colorado Springs come away with very different totals.
Oh yeah, the above storm was a bomb cyclone — what exactly is that?
Bomb Cyclones
A bomb cyclone is a mid-latitude cyclone which undergoes a process known as explosive cyclogenesis. Explosive cyclogenesis occurs when the surface pressure at the center of the cyclone drops rapidly, generally at the rate of one millibar per hour.
This sharp decrease in pressure increases wind speed around the low, tightening the pressure gradient between the colliding warm air mass from the south and cold air mass from the north, strengthening the storm.
A bomb cyclone describes a rapidly intensifying cyclone with its central pressure, at least at Colorado’s latitude, dropping about 18mb over 24 hours. This sustained development indicates a very strong winter storm. These storms have historically produced record breaking (explosive) snowfall, and are sometimes unofficially named “Snow-zilla,” “Snowpocalypse,” and “Snowmageddon.”
For the upcoming weekend storm, we can look at the most optimistic, unrealistic weather model predictions to visualize what explosive cyclogenesis would look like.
In the above animation, surface pressure drops from 1004mb to 988mb (16mb) in less than 18 hours. That’s explosive cyclogenesis alright, though not quite a bomb cyclone. This is again something that weather models often struggle to properly forecast, so keeping an eye on the potential for rapid cyclogenesis is worth doing. In the above very unrealistic scenario, the storm would develop blizzard-like conditions for much of northeastern Colorado, with frequent winds above 40mph, powerful snowfall, and whiteout conditions.
Again, to be clear: we are NOT forecasting the upcoming weekend storm to “bomb out” into a bomb cyclone.
Blizzards
A blizzard actually has a very specific definition. “Strong, sustained snowfall” is not necessarily a blizzard, and in fact, a blizzard can occur even with fairly light snowfall. The National Weather Service defines a blizzard as a snowstorm with sustained winds or frequent wind gusts above or equal to 35mph, with visibility of a quarter mile or less. These conditions must be sustained for three hours or more for the storm to be classified as a blizzard. Note that snowfall rates are not included in the definition.
Summary
Let’s take a look at our weekend storm. It certainly looks like an ETC!
We see a center of low pressure deepening near the Oklahoma panhandle, and the signature comma shape is obvious when looking at the precipitation. The dry air intrusion is shown with the yellow arrow, and the cold/warm conveyor belts shown with the blue and red arrows, respectively.
Specific Ingredients for Enhanced Snowfall
In general, if the above forecast panned out exactly as shown, this would be a very decent snow event for the Front Range. Not only is snow falling on account of processes within the cyclone itself, winds from the northeast are upsloping into the higher terrain east of the Continental Divide and enhancing snowfall, which is why the foothills generally get considerably more snow.
However, would this be a historic event? Not exactly. There are other factors at play that need to occur to kick snowfall into an even higher gear.
Barrier Jet and Cold Air Dam
A barrier jet forms in the Front Range of Colorado when low-level winds from the east hit the foothills and turn to the south. This forms a “jet” of strong southerly winds near the surface (not high in the atmosphere, like the jet stream). Cold air, unable to cross over the higher terrain to the west, becomes trapped and often forms a “dam” or “dome” near the surface.
A barrier jet is likely with these sorts of storms. For the weekend system, we’ve seen a barrier jet forecasted consistently on models.
Many of you are winter sports enthusiasts and are probably familiar with orographic snowfall — snow produced when moist air cools as it hits a mountain range and is forced up in elevation.
A cold air dam and barrier jet act similar to how terrain would — air is forced over it which enhances snowfall. This is especially helpful for areas that are quite a ways east of the mountains! This process is called isentropic lift.
If we see this in the forecast data, we know the heavier snow will likely be more widespread instead of totals tapering off sharply east of the higher terrain as we lose the orographic component of snowfall, which can be a big factor in snow production especially towards the start and end of the storm.
TROWAL
A trough of warm air aloft (TROWAL) can often be found on the northern and western sides of a strong winter cyclone. The presence of one of these in the right spot ensures consistent and heavy snowfall. These features are generally not that large, which is yet another source of major uncertainty in forecasting these storms, as snow totals inside and outside of that area will differ quite a bit.
In the above image, moisture is being turned southwest as it wraps around the center of low pressure. The area where these moist winds are adjacent to the dry slot often features strong lift, which favors particularly heavy and prolonged precipitation.
Convection
Part of the reason why lift is stronger near the dry slot is due to instability and convection. The dry slot generally features unstable air. This can often cause severe thunderstorms to form on its eastern boundary.
Without getting into too much detail, instability (essentially, conditions that would cause air to rise uncontrollably and condense/precipitate) within the storm can form bands or cells of enhanced snowfall. A specific form of instability that is important to enhancing snow in these winter storms is called conditional symmetric instability (CSI). The resulting slantwise convection helps form a band of more intense precipitation that can greatly increase snow accumulations over a fairly small area.
In a winter storm, we sometimes see an area of CSI, which influences the development of bands of stronger snowfall.
Snow-Liquid Ratios (SLRs) and Rain
The Snow-liquid ratio refers to the ratio of liquid precipitation to actual snow totals. A common, though largely incorrect, “average” is a 10:1 SLR, which would imply that 10” of snow would accumulate for every inch of liquid precipitation that falls. In winter, Colorado often sees SLRs much higher than 10:1, especially in the mountains. “Blower pow” at resorts like Steamboat is snow that often falls with a 20:1–30:1 SLR — which means you don’t need very much moisture to precipitate to produce some nice snow totals.
Conversely, the actual amount of liquid is important from a climatological standpoint, in terms of drought reduction and forest fire danger when Colorado’s snowpack begins to melt. As the season progresses, the 10” of “blower pow” compacts, and soon only represents a tiny fraction of the overall snowpack and produces only a trickle of water once it melts.
We generally see much lower SLRs in spring. Consider the March 2003 blizzard in Denver, which dropped 30” of snow. That’s a huge amount of snow, but the actual liquid precipitation total was staggering, with some areas recording that over 6” of liquid precipitation had fallen from the storm.
This means SLRs for the March 2003 blizzard, at least in the metro area, were as low as 5:1, and this incredibly dense, heavy snow caused widespread damage, including causing roofs and structures to collapse.
These lower snow-liquid ratios are common for these big March and April snow storms. A few top historical analogs for the upcoming weekend’s storm feature metro area SLRs in the 5:1–8:1 range. With those SLRs, we would need 3–4” of liquid precipitation to fall for actual snow totals to scrape above the two foot range — and that’s a lot to ask! Guidance perhaps suggests a bit more optimistic of SLRs for the weekend storm, but again, this can be a difficult factor to forecast. For huge snow events, just a small difference in forecasted SLRs versus actual SLRs can result in snow totals many inches above or below expectations.
One thing to consider is that when, for instance, the outrageous GFS snow total maps are being shared around, these are often with a 10:1 or “Kuchera” ratio that is likely too high for this storm, so even ignoring the unreasonable precipitation totals in general, you could safely knock several inches of snow off the model’s expectations.
Sometimes, the track of the storm and environmental conditions will bring rain at the start of the event, which models have played around with for our upcoming weekend storm. This obviously has a large implication on final snow totals, and ensures SLRs will remain very low for the start of the event.
Severe Storm Activity to the Southeast
As we mentioned, severe thunderstorms can often form east of the dry slot if environmental conditions are right. For instance, in our coming weekend storm, we’ll see a cold front push into Oklahoma and a dryline setup somewhere around northwestern Texas, which could kick off some very strong storms with large hail.
These strong storms could disrupt moisture advection into the system and thus negatively impact snow production downstream in Colorado — but this isn’t usually too big of an issue.
Dry Air Entrainment
Not only is dry air bad from a moisture standpoint (we need it, for snow, of course!) it will also weaken the storm in general and halt cyclogenesis. We look for dry pockets of air entrained in the system to try to gauge the “dud” potential. The system can pick up dry air unexpectedly as it weaves and bobs along its trek eastwards, and as we’ve said, this is something the models struggle to forecast correctly.
The Mountains
It’s generally common knowledge that the mountains stay pretty high and dry during these “Front Range upslope” events. As winds travel from east to west up to the Continental Divide, they precipitate much of their moisture and then dry out as they move further west from the crest of the Divide.
Epic storms in Denver that drop two feet of snow in the west metro area often coincide with new snow reports of less than 3” at the resorts. The big exception is for ski areas to the east of the Divide, namely Eldora and Echo Mountain (and the usual backcountry spots of Hidden Valley, Moffat Tunnel East Portal, etc.), which also often report huge totals.
Not only does upsloping air from the east drop more moisture on the foothills and high terrain east of the Divide due to orographic lift, it also produces snow with higher SLRs, which further increases the totals. The amount of liquid precipitation that falls may be 1.5x higher than what fell in the city, but snow totals could be 2–3x higher than the city’s in that scenario!
The mountains stand their best chance at picking up snow before the system enters the state and as it exits. Before the storm arrives, jet dynamics and increasing moisture usually introduce large-scale lift and stronger winds which can produce snowfall. We lose the jet dynamics as the storm enters the Colorado Rockies, and winds at the surface tend to be lower than expected, which means orographically-induced upslope snowfall is also inhibited.
As the storm leaves, wraparound moisture (usually with northwest flow) can get snow going again, but this is usually quite overforecasted in weather models. We can look at historical data for these sorts of systems to see exactly how well the mountains do. These maps are messy, but look carefully and see how totals drop off sharply west of the Divide.
You can see that the resorts near the Divide (Winter Park, A-Basin, Loveland, Keystone) do stand a chance at picking up a piece of the action and reporting decent totals, but that likelihood drops sharply as you get towards Vail, Steamboat, and the Aspen resorts.
With the average track of these systems, the San Juans (Purgatory, Telluride, Silverton, Wolf Creek) and western Colorado can sometimes pick up a nice round of snow/precipitation before the storm enters the state, but they usually see very little action when the storm crosses the Rockies and exits the state.
With that in mind, if you see forecasts calling for a couple feet of snow in the mountains from these sorts of events, or even over a foot in resorts that aren’t near central / eastern Summit County — be wary! This rarely pans out, despite weather models often showing some nice totals.
Conclusion
These are some major factors that define whether the Colorado Front Range sees a snowmageddon or a fauxmageddon. Only a couple things need to go wrong for “the storm of the century” to drop single digit totals on the city and leave model gazers disappointed and forecasters blamed for their eagerness — regardless of whether they actually had any or not.
The world is changing. Weather models are more numerous in number, more accurate, and more specific. They are much easier to access. There are many more forecasters and folks interested in weather hitting social media hard with their thoughts and screenshots from weather model outputs. These sorts of storms are becoming incredibly chaotic with the sheer amount of noise surrounding the forecast and possibilities.
Use social media with caution when assessing the potential snow from these sorts of events.
- Are any of the terms we mentioned in this article used in the forecast or text accompanying a model output?
- Are ensembles mentioned?
Ensembles paint a much clearer picture about the actual potential of storms and the forecast uncertainty, and forecasters that want to come up with a realistic forecast often rely heavily on them in the days preceding a storm. - Are analogs or historical events mentioned?
- Is there any text at all, or is just a reaction accompanying a model screenshot or Skew-T plot?
- Does the publication latch onto the biggest weather impacts that were only shown in a single model run? (Six feet of snow possible in Denver!)
- Does the person have “Wx” in their Twitter handle? (Only kidding! Or…are we?)
- Is the forecaster merely passing another forecaster’s forecast along?
Use caution here. You can lose the original context, in addition to the interpretation being inaccurate.
Meteorologists have to carefully balance two things:
- Ensuring the public is adequately prepared for a huge, potentially destructive event
- That the public still trusts meteorologists even if a dud occurs
This is why you rarely see a serious discussion of potential totals for these events from reputable outlets until the final leadup to this storm.
As you can see from this article, there are so many factors at play, and enough uncertainty to swing the needle wildly in either direction. It’s not easy, but it’s often fulfilling!
Hopefully this was interesting and useful material for understanding these Front Range spring storms. For these events, you should plan for a lot of snow, but not be surprised if a near miss results in a complete dud.
Written by
Thomas Horner (@thomaschorner)
Laura Smith (@Hurricane_Laura)
Website and app with automated, high-resolution forecasts for mountains, climbing areas, and ski resorts (+ backcountry) with elevation switcher, uncertainty visualization, and more:
https://highpointwx.com
Frequent updates, graphics, and general stoke:
Twitter / Instagram / Facebook: @highpointwx
If you enjoy our forecasts and/or website, please consider becoming a Patron to help keep the lights on. Thank you!
