Meteorologists who forecast in mountainous regions of North America often refer to areas "West of the Divide" or "East of the Divide" in terms of precipitation and temperatures.

The Continental Divide is not only a hydrological divide that separates which rivers flow toward the Atlantic and which flow toward the Pacific, but it also acts as a divide in terms of climate zones and weather patterns.

Whether you are skiing in the winter or hiking in the summer, your location relative to the Continental Divide can make a big difference in terms of conditions. 

What is a "Continental Divide"?

A continental divide is a naturally occurring terrain boundary that separates a continent's river systems. Continental Divides span long distances and often run along the crests of major mountain ranges, while also occurring over elevated basins in less mountainous terrain.  

Water originating from precipitation runoff, streams, and rivers on one side of a divide will drain toward a specific ocean basin, while water on the other side of a divide will drain toward an ocean basin on the opposite side of a continent.

Continental divides are located on every continent, and many continents have multiple continental divides depending on how many oceans surround the continent, and also based on the location of major mountain ranges or other terrain features. 

The Continental Divide of North America

North America's Continental Divide runs along the crest of the Rocky Mountains from Mexico to Canada.

From south to north through the U.S., the divide runs through Western New Mexico and Central Colorado, before turning northwest across Wyoming. The divide hugs the Southern Montana/Idaho border briefly, before turning east and then north into Northern Montana. 

In Canada, the divide runs along the British Columbia/Alberta border for a significant distance, before eventually cutting northwest through Northern BC and into the Yukon and Alaska. 

One interesting note about the Continental Divide is the amount of mountainous terrain that exists west of the Divide versus east of the Divide.

Many major mountain ranges from the Wasatch to the Tetons to the Sierra to the Cascades are located west of the Divide. There are some major mountain ranges located east of the Divide as well, but they are much fewer in number compared to west of the Divide. 

In some locations along the Divide, there is a major drop-off from high elevation peaks to the Great Plains to the east over a short distance. 

A "Dividing Line" Between Moisture and Temperature

The Continental Divide often acts as a "separator" of weather airmasses in terms of moisture and temperature. One side of the divide can experience a wetter pattern with cooler temperatures, while simultaneously, the other side of the divide may be warmer and drier.

These patterns typically depend on the location of high and low pressure systems, wind direction, and stability in the atmosphere (the "ease" at which air can rise vertically). 

Winter/Cool Season Weather Influences – West vs. East of the Divide

In the winter time, atmospheric stability tends to be higher due to colder temperatures, but frontal systems with strong winds, strong atmospheric dynamics, and abundant moisture in the low to mid levels of the atmosphere are common.

In such patterns, there tends to be a significant orographic influence in mountainous regions, in which moist air is forced to rise, expand, and condense into clouds and precipitation when it encounters the windward side of a mountain range.

Whereas, moist air is forced to move downslope on the leeward side of a mountain range, which has a warming and drying effect known as the rain shadow effect.

Orographics occur on a highly localized scale with individual mountain ranges. But there is also a broader scale effect that occurs along the Continental Divide. 

Winds Blowing from the West

When winds blow from the southwest to west, areas along and west of the Continental Divide are favored for snow or rain, whereas areas east of the Divide are in the rain shadow with these winds and experience drier conditions and milder temperatures.

Climatologically speaking, winds with a westerly component are more common. Therefore, areas west of the Continental Divide receiver heavier and more frequent snowfall in the winter time. This is why a high percentage of major ski resorts are located near or west of the Divide. 

Winds Blowing from the East

The effect is reversed when winds blow from the east or northeast (or even due north across the Northern Rockies). In these instances, the east side of the Continental Divide will see heavier precipitation with drier conditions west of the Divide.

While less common, easterly upslope events sometimes result in major snow storms because there is less terrain to "block" moisture intrusions from the east. The same is true for cold air intrusions east of the Divide, where low temperature extremes are more common.

Summer/Warm Season Weather Influences – West vs. East of the Divide

During the warmer months, the atmosphere tends to be less stable due to the effects of the sun heating up the surface and lower atmosphere.

This makes it easier for moist air parcels to "rise on their own", and this is a contributing factor to convective precipitation – a fancy term for "pop-up" or "hit-or-miss" showers and thunderstorms that can lead to highly variable rainfall amounts over short distances.

During convective precipitation patterns, orographic lift plays a role in the development of showers and thunderstorms, but the rain shadow effect is less pronounced as showers and thunderstorms with strong updrafts can easily track over the leeward slopes of mountain ranges and deposit rainfall.

In fact, localized upslope winds that occur with solar radiation and daytime heating can sometimes favor the leeward slopes of mountain ranges for thunderstorm development.

Despite the limited orographic influence, the Continental Divide still plays a major role as a moisture boundary. Moisture is a fuel source for convective showers and thunderstorms, with higher moisture levels increasing the likelihood of showers/storms as well as potential rainfall amounts.

Depending on the location of high and low pressure systems, wind direction, and moisture transport from "source" regions (such as oceans or the North American Monsoon circulation), areas east of the Divide may see higher moisture levels that can fuel showers and thunderstorms compared to areas west of the Divide, or vice versa.

Weather Patterns that Favor One Side of the Divide Versus the Other

The location of ridges and troughs in the upper atmosphere have major influences on precipitation and temperature differences between areas west of the Divide and east of the Divide.

Generally speaking, a trough is a dip in the jet stream that favors cooler and more active weather and a ridge is a rise in the jet stream that favors warmer and more benign weather underneath the ridge.

Let's take a look at four key upper atmospheric patterns and how they impact one side of the divide versus the other...

1) Western Trough and Central Ridge

This is a classic pattern that favors snowfall at ski areas west of the Divide during the winter months. Storms originating in the Pacific Ocean move into the West and bring heavy precipitation, while the rain shadow effect results in limited precipitation east of the Divide.

Wind direction is typically out of the west to southwest, or in some cases the northwest on the backside of the trough. 

Sometimes, troughs remain stationary for several days at a time near the West Coast or over the Interior West with multiple storms tracking across the area. In more progressive patterns, large troughs may quickly track across the Rockies and deposit brief and light precipitation east of the Divide depending on wind directions with the trough passages.

Deep or persistent low pressure troughs can also open the door to cold air and below average temperatures west of the Divide, meaning better snow quality (dry and powdery) for skiers.

These patterns are most common in the fall, winter, and spring, but can occasionally occur during the summer too, in which case it can lead to upticks in monsoonal moisture and thunderstorms over the Rockies.

2) Zonal Flow with a Strong Jet Stream

A zonal flow refers to a pattern in which there is a strong west to east flow with less pronounced ridges or troughs. These are most common during early to mid winter (November to February) when the jet stream tends to be strongest.

Shortwaves tend to track right along the jet stream in these patterns and can bring heavy snow or rain along with gusty winds to areas west of the Divide. Temperatures can be milder in this pattern, though, as the west to east flow from the Pacific prevents significant cold air surges from the north. 

East of the Divide, these patterns are conducive to downslope Chinook wind events, which lead to mild temperatures and very strong wind gusts near the foot of the Rockies. 

3) Western Ridge and Central Trough (Northwest Flow)

This pattern is the reverse of #1 and favors warmer and drier conditions west of the Continental Divide (especially near the West Coast) while areas east of the Continental Divide are more likely to experience cooler and wetter conditions.

This is often referred to as a northwest flow pattern, because winds in the upper atmosphere blow from northwest to southeast, following along the ridge/trough interface.

However, in the lower levels of the atmosphere, this pattern supports frequent backdoor cold fronts originating in Northern Canada that slide down the Great Plains, and back up into the eastern slopes of the Continental Divide.

Winds in the lower levels of the atmosphere switch to easterly (blowing from the east to northeast) in these scenarios, resulting in precipitation east of the Divide.

In the Northern Rockies (Montana especially), during instances when winds blow from the northwest in the low levels, this can also be favorable due to the change in trajectory of the Continental Divide.

This type of pattern is favorable for snowfall at ski areas located east of the Continental Divide, and snow quality is often dry and powdery since this pattern also supports cold air intrusions from the north.

Northwest flow patterns can also open the door to arctic air intrusions during the winter months, which can result in temperatures plummeting well below 0ºF east of the Divide. Often times, modified arctic air will spill over into areas west of the Divide in these patterns, too. 

Sometimes, northwest flow patterns can bring snow to areas west of the Continental Divide. If the ridge is centered far enough west, it can open the door for disturbances to move into BC, Washington, Idaho, Western Wyoming, Utah, and Western/Central Colorado. In fact, Western/Central Colorado is often best positioned for northwest flow snow events compared to other areas west of the Divide. 

This pattern can happen at any time of year and is not uncommon in the summer months. When this pattern occurs in the summer, rich moisture originating from the Gulf of Mexico and Great Plains will travel from east to west toward the eastern slopes of the Divide, resulting in heavy rain producing thunderstorms and occasionally severe weather. 

4) Closed Low or Cut-Off Low

The last type of pattern is one of the trickiest to forecast, but has the highest upside potential to bring heavy snow or rain to areas east of the Divide.

Closed lows and cut-off lows are unlike typical open wave troughs which do not have a closed circulation, and feature southwest winds on the leading edge of the trough and northwest winds on the back side of the trough. 

Instead, closed lows and cut-off lows feature a low pressure center in which winds rotate counterclockwise (or cyclonically) and completely circumnavigate around a low pressure center. This results in winds with an easterly component on the east side of these lows, that can favor the east side of the Continental Divide.

Closed lows and cut-off lows are similar, but there are some slight differences.

Closed lows are part of a trough and move with the jet stream. But unlike an open wave trough, a low pressure with a closed circulation develops (winds rotating and circumnavigating the low).

Cut-off lows also have a closed circulation around a low pressure center. However, these lows break away from the jet stream and can hang around an area with slow/erratic movement. Often these lows "undercut" high pressure ridges. 

Learn More → Cut-off Lows, Explained

Closed lows and cutoff lows can develop at any time of year. However, they are most common in the spring and fall when more amplified ridges and troughs (higher peaks and dips) tend to arise due to sharp contrasts in temperatures during the changing seasons.

These "amplified" patterns can cause troughs to slow down, with closed circulations developing, or in the case of closed lows, areas of low pressure breaking away from the main flow and diving south. 

East of these low pressure centers, winds will take on an easterly component (blowing from the southeast, east, or northeast) which can load up the eastern slopes of the Continental Divide with moisture, resulting in heavy precipitation.

Heavy snow can occur on the east side of the Divide in these patterns in the spring and fall months, often adding up to feet of snow. Snowfall is usually on the heavier and wetter side in these patterns.

In the late spring and summer months, these patterns also bring abundant moisture to areas east of the Divide and can result in intense, heavy rainfall producing thunderstorms.

Areas west of the Divide can see significant moisture in these patterns as well, but it's highly dependent on the precise location of a low pressure center and resulting wind directions which could be favorable or unfavorable for a given mountain range. 

Continental Divide by State/Province

Next, let's take a look at statewide geography to point out which mountain ranges and locations are east versus west of the Divide.

New Mexico

The Continental Divide runs along far western portions of New Mexico, from the Black Mountains into the southern portion of the San Juan Range as it reaches the Colorado border.

The highest mountain ranges in the state, such as the Sangre de Christo and Sacramento Ranges, are located east of the Divide. Also, nearly all New Mexico ski areas are located east of the Divide. 

Image courtesy of the Center for Land Use Interpretation

Colorado

The Continental Divide passes over the San Juan Range, Sawatch Range, Front Range, and Park Range. Most ski areas and major peaks are located along or west of the Divide where the heaviest snowfall occurs, but there are some exceptions.

The Sangre de Christo Range and Pikes Peak are located east of the Divide, and several other major peaks in the Front Range are located just east of the Divide as well. Eldora Ski Area is also located east of the Divide. 

Image courtesy of the Center for Land Use Interpretation

Wyoming

The Continental Divide passes through the Sierra Madre Range, before splitting around the Great Divide Basin (a closed basin, where water doesn't flow toward any ocean and much of the water evaporates). 

The Divide then passes along the crest of the Wind River Range and Southern Absaroka Range, before cutting across the southern third of Yellowstone.

Mountain ranges west of the Divide include the Teton, Gros Ventre, and Salt River Ranges. Mountain ranges east of the Divide include the Beartooth Range, much of the Absaroka Range, Bighorn Range, and Snowy Range.

Image courtesy of the Center for Land Use Interpretation

Montana and Idaho

The Continental Divide takes on a more variable path through this region. The Divide is located along the Idaho/Montana border from Yellowstone to Lost Trail Pass, before swinging eastward toward Butte across the Anaconda Range, and then northward through Glacier National Park (dividing the park in half).

Areas west of the Divide include all ranges in Idaho, most of the Bitterroot Range, the Mission Range near Missoula, and Whitefish and the Flathead Lake region.

Areas east of the Divide include Big Sky, the Madison Range, Gallatin Range, Tobacco Root Range, Bridger Range (including Bridger Bowl), and Beartooth Range, the latter of which contains the highest peaks in Montana. 

Image courtesy of the Center for Land Use Interpretation

British Columbia and Alberta

The easiest geographic boundary relative to the Continental Divide is in Canada, where the Continental Divide is located right along the British Columbia/Alberta border from Waterton Lakes into the far north, to the point where the Alberta/BC border takes an abrupt turn to due north.

At this point, the Continental Divide turns northwest across Northern BC and into Yukon.

Major ski resorts Lake Louise and Banff Sunshine are located just east of the Continental Divide crest in Alberta, while all ski resorts in BC are located west of the Divide. 

Bottom Line

Prevailing winds from the west to southwest mean that mountainous areas west of the Continental Divide tend to experience wetter climates than areas east of the Continental Divide, whereas areas east of the Divide are prone to more extreme temperature fluctuations (including arctic cold fronts in the winter).

The differences between these wetter and drier climates are more pronounced in the cooler months compared to the warmer months.

Also, the "typical" patterns can quickly reverse depending on short-term weather patterns for a given day or weak, no matter the time of year.

Therefore, it's always important to keep up with our 10-day and hourly forecasts (available for any location) and Daily Snow expert discussions so that you know what to expect before you hit the slopes or the trails. 

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Alan Smith