Fronts are important when it comes to mountain recreation because they signify an abrupt change in the weather. In simple terms, a front is a boundary separating a colder airmass from a warmer airmass. However, the airmasses on either side of a front also vary in terms of pressure, wind direction, wind speed, moisture content, clouds, and precipitation.
Fronts typically produce inclement weather as they cause the air ahead of the boundary to rise, resulting in clouds and precipitation. This effect is also amplified when fronts approach a mountain range, as the rise in terrain also causes air to accelerate vertically.
Cold fronts and warm fronts are the two main types of weather fronts we'll examine in this article. However, we'll also take a look at stationary fronts and occluded fronts which can impact weather in the mountains as well.
Here are the basic definitions of each of these fronts:
- Cold Front: When a colder airmass displaces a warmer airmass.
- Warm Front: When a warmer airmass displaces a colder airmass.
- Stationary Front: When a front separating a colder airmass from a warmer airmass stalls and remains stationary for an extended period of time.
- Occluded Front: When a cold front "catches up" to a warm front.
When looking at a weather map, weather fronts can be denoted by the following symbols:
- Cold Front: Blue line with triangles pointed in the direction the front is moving.
- Warm Front: Red line with half-moons located on the side of the direction the front is moving.
- Stationary Front: Alternating blue and red line.
- Occluded Front: Purple line with alternating triangles and half-moons.
Here is an example of a weather map of the U.S. during the winter with various frontal boundaries displayed, along with forecasted precipitation type.
Next, we will dive into the details of the different types of fronts and how they can influence weather in the mountains.
Cold fronts are faster-moving than warm fronts and typically travel in an east to west, northwest to southeast, or north to south direction. However, they can occasionally move in a southwest to northeast direction or a northeast to southwest direction.
Temperatures will rise gradually in advance of a cold front, and during the cooler months, widespread moderate to heavy precipitation can also occur well ahead of a cold front when moisture is present.
Cold Frontal Precipitation in the Winter:
As a cold front approaches, organized bands of intense precipitation will often develop along and just ahead of the front along with gusty winds. This can result in near whiteout conditions during the winter and can impact both road conditions and skiing conditions in terms of visibility and lift operations (high winds can cause lift closures).
Behind a cold front, precipitation usually becomes more showery in nature but this varies in each situation. In some cases, cold fronts are relatively slow-moving and precipitation will remain widespread after the front passes, whereas when fast-moving fronts (even more so than usual) occur, the airmass can dry out quickly behind the front.
Often what happens behind a cold front in the mountains is that enough moisture lingers along with a more unstable atmosphere (due to colder air aloft) to produce scattered showers over the next 12-24 hours. If the wind direction behind the front is favorable for terrain-enhanced lift (i.e. the orographic effect), locally heavy snow showers can occur with colder air supporting better quality powder snow.
Wind and Pressure Changes with a Cold Front:
Cold fronts are also marked by changes in pressure and wind. Atmospheric pressure will decrease gradually as a cold front approaches, then will increase after the front passes.
Winds are often strong both ahead of and behind a cold front with the highest wind speeds occurring within a few hours of the frontal passage. Once a front passes, winds can remain gusty for up to an additional 24 hours due to the differences in temperature and pressure that occur over a short geographical difference when a cold front moves through.
An abrupt change in wind direction usually occurs when a cold front moves through, which can impact terrain-enhanced snowfall as well as wind loading, which in turn influences avalanche danger.
Typically, the wind direction ahead of a cold front will be out of the southwest, south or southeast, while behind a cold front, the wind direction will be out of the west, northwest, north, or northeast.
Cold Fronts During the Warmer Months
During the warmer months, cold fronts are often a trigger for thunderstorms. Warm, moist, and unstable air ahead of a cold front will become displaced upward as the front approaches, often resulting in organized lines of thunderstorms.
While we usually think of cold fronts producing precipitation, dry cold fronts can also occur when moisture is not present. These types of cold fronts can be bad news during the summer and fall if soil and vegetation conditions are already dry, because the gusty winds that occur with the front can lead to elevated fire weather concerns, and can also lead to rapidly spreading fires.
Types of Cold Fronts:
There are also several variations of cold fronts that we experience in the mountainous regions of North America. While the overall concepts are the same, these different types of cold fronts do have notable characteristics that influence weather conditions.
Pacific Cold Fronts:
These are the most common types of cold fronts during ski season (and also during the fall and spring) across Western North America. Pacific cold fronts typically occur with low-pressure systems and involve widespread moderate to heavy precipitation along and west of the Continental Divide.
Pacific cold fronts generally move from west to east or northwest to southeast. Temperature drops occur with these fronts and can result in rain changing over to snow in some areas. However, the temperature decreases that occur with Pacific fronts are less dramatic than those which occur with fronts arriving out of the north or northeast.
Along the eastern slopes of the Continental Divide, Pacific cold fronts are a different story. Due to the downsloping effect of winds blowing out of the west or northwest, Pacific cold fronts generally produce dry and windy conditions east of the Divide with only light/spotty precipitation if anything.
Pacific cold fronts can travel across the U.S. and eventually reach the East Coast, picking up moisture from other sources such as the Gulf of Mexico and the Great Lakes. Pacific fronts that move across the Central and Eastern U.S. typically do some in more of a west to east direction with less dramatic temperature drops occurring behind the fronts compared to those arriving from the north.
Canadian Cold Fronts:
These cold fronts arrive from the north, northwest, or northeast and track over the cooler continental Canadian Interior before moving southward into the U.S. Canadian cold fronts generally result in more dramatic temperature drops but also contain less moisture than Pacific cold fronts, at least across the West.
However, Canadian cold fronts can produce heavy precipitation (including snowfall) across the East once they interact with moisture from the Great Lakes and/or Gulf of Mexico.
Across the West, Canadian cold fronts usually produce lighter snowfall compared to Pacific cold fronts, but since colder air is involved, the snow that does fall tends to be lower density and more powdery in nature. When other favorable ingredients are present, such as a favorable wind direction or strong jet stream support, then significant low-density snow accumulations are possible.
Across the East, Canadian cold fronts that pass over the Great Lakes (assuming they aren't frozen over) can result in lake effect snow as cold west or northwest winds behind the front passes over the relatively warm lakes, causing moisture from the lakes to rise and condense into clouds, which then produce snowfall.
West or northwest winds then transport this moisture toward the western slopes of the Appalachians, causing additional snow showers to develop via terrain-enhanced lift. This can be a good pattern for skiers in the East.
Canadian cold fronts that penetrate into the Deep South and span the entire East Coast can also interact with moisture from the Gulf of Mexico and the Atlantic to produce heavy snow across the Appalachians.
Arctic Cold Fronts:
These are somewhat similar in nature to Canadian cold fronts, but the main difference is that arctic cold fronts originate from way up north above the Arctic Circle, and involve very cold air surging southward during the winter months. While Canadian cold fronts and Pacific cold fronts can occur at any time of year, arctic cold fronts typically only occur from November to March.
Arctic fronts can result in abrupt temperature drops, with temperatures often dropping below 0ºF across the continental interior away from the coasts. When arctic fronts arrive, frostbite becomes a very real concern when spending time outdoors.
Arctic fronts typically involve very dry and stable air arriving behind the front. However, if an arctic front manages to interact with a moisture source, then a period of heavy snowfall can occur as the front passes through. In some cases, arctic fronts can stall across an area (especially if it gets "hung up" along a mountain range), resulting in an extended period of heavy snowfall.
Backdoor Cold Front:
A backdoor cold front involves a cold front that moves in an atypical direction from northeast to southwest. These fronts typically affect the eastern slopes of the Appalachians, Rockies, and occasionally the Cascades and Sierra.
Backdoor cold fronts can produce some very interesting weather. With traditional cold fronts, the most widespread and prolonged precipitation typically occurs ahead of the cold front. However, with backdoor cold fronts, the most widespread precipitation occurs behind the cold front.
Across the West, backdoor cold fronts are essentially Canadian cold fronts that "back" their way up toward the eastern slopes of the Rockies. Northeast, east, or southeast winds that develop behind these fronts create an upslope effect in which moisture is forced to rise up the eastern slopes of the Continental Divide.
Areas east of the Divide in Alberta, Montana, Wyoming, Colorado, and New Mexico receive most of their snowfall and precipitation after backdoor cold fronts move through. Leftover Pacific moisture can produce heavy snow in Alberta, Montana, and Wyoming, while the heaviest snow east of the Divide in Colorado and New Mexico often occurs when Gulf of Mexico moisture arrives from the east.
Backdoor cold fronts are also significant along the eastern slopes of the Appalachians as cold air from the northeast gets reinforced and can support heavy snow, rain, or sleet/freezing rain when moisture from the Gulf of Mexico and/or the Atlantic overrides the colder air. This is known as cold air damming (a future weather article).
Backdoor cold fronts can also reach the eastern slopes of the BC Coast Range, Cascade Range and Sierra Nevada Range, but tend to do so less frequently compared to the Rockies. This is because cold fronts moving in a northeast to southeast direction typically lose their punch and "wash out" as they attempt to cross the Continental Divide.
However, strong cold fronts (especially arctic fronts) can sometimes push over the Continental Divide and reach the Interior Northwest and eastern side of the Cascades. Usually, the cold/arctic air becomes modified (meaning not as cold) by the time it reaches the Cascades, but cold air surging through deep gaps in the range can occasionally reach the lowlands near the West Coast, bringing the chill to Seattle and Portland.
A warm front involves a warmer airmass displacing a colder airmass. Warm fronts are slower-moving than cold fronts and typically move from south to north. They are most common along the West Coast, Pacific Northwest, and East, and less common in the Rockies where topography often causes these slow-moving fronts to fall apart.
Warm fronts cause air to rise and condense into clouds and precipitation, but not in the same way as cold fronts. When a warm front approaches, the warm air is lighter in density than the cold air ahead of the front, and as a result, the warm air is forced to rise up and over the cold air in place. This causes widespread moderate precipitation to occur.
Warm fronts can be a good or a bad thing for skiing depending on the situation. If the warmer temperatures behind the front remain below freezing, then warm fronts can lead to a prolonged snow event that adds up to deep accumulations over time.
On the other hand, warm fronts usually lead to a transition from lower density to higher density snowfall, so you may not get the blower powder you're looking for unless the air in place is already very cold. Also, warm fronts can lead to "upside down" snowfall for backcountry skiers, resulting in higher avalanche danger due to higher density snow falling on top of lower density snow.
The other obvious concern about warm fronts is when the temperatures behind the front warm up too much. This can lead to rising snow levels and a changeover from snow to rain (or sleet/freezing rain in the East).
Similar to cold fronts, atmosphere pressure decreases ahead of warm fronts and rises behind warm fronts, but the change in pressure occurs more gradually compared to cold fronts.
Also, wind speeds are lighter with warm fronts compared to cold fronts, but a change in wind direction does occur with a warm front passage, with winds typically blowing out of the south after the warm front moves through.
Stationary fronts involve frontal boundaries that stall over an area for an extended period of time. Sometimes, this happens due to the fronts simply weakening and "losing steam", but fronts can also get "stuck" when reaching a mountain range, thus stalling along the mountain range itself.
Stationary fronts can originate as either cold fronts or warm fronts, and these boundaries act as a focal point for precipitation due to winds converging along the boundary from different directions, forcing moist air along the boundary to rise.
Stationary fronts can lead to prolonged snow or rain, often resulting in significant accumulations. During the summer, stationary fronts can also act as a focal point for thunderstorm development, with storms often re-developing over the same areas.
Flash flooding can also be a concern when a stationary front is present as heavy rain showers can re-develop or stall over the same areas for an extended period of time.
Low pressure systems often involve a warm front moving across an area initially, followed by a cold front arriving at a later time. However, since cold fronts move faster than warm fronts, sometimes the cold front will "catch up" to the warm front, especially near the center of low pressure.
When this happens, the front becomes "occluded". Once this happens, the air behind the front is usually still colder than the air ahead of it, resulting in a "weaker" front in terms of a temperature drop, though heavy precipitation can still occur. This is known as "cold occlusion".
Occasionally, the air ahead of an occluded front will be colder than the air arriving behind it. This is more typical when an arctic airmass is in place ahead of an approaching frontal system. When this happens, a slight warm-up will occur when the occluded front passes, in a process known as "warm occlusion".
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