When evaluating ridges and troughs, meteorologists look at wind and pressure both at the surface and in the upper levels of the atmosphere. However, it is in the upper levels where wind, pressure, and temperature drive global weather patterns which in turn influence what happens at ground level.

Jet streams form in the middle latitudes of the earth (between 30 and 60 degrees) as a result of sharp boundaries in temperature, which in turn results in relatively narrow bands of strong winds in the upper atmosphere at approximately 30,000 feet above sea level.

The jet stream generally moves in a west-to-east direction due to the tilt and rotation of the earth, but as boundaries between cold and warm air ebb and flow, this causes the jet stream to meander to the north and south, forming ridges and troughs.

How do ridges influence the weather?

Ridges form when the jet stream builds northward in an arc-like shape, and are associated with warm air, high pressure aloft, and a more stable atmosphere. This tends to result in dry, fair weather conditions underneath the ridge. Sometimes, a closed circulation of high pressure develops under the center of the ridge with winds aloft blowing in a clockwise direction around the high pressure center.

Depending on the time of year and the region, ridges can produce periods of pleasant, sunny weather along with warm temperatures and light winds. However, during the summer months at lower elevations, ridges can also lead to uncomfortably hot temperatures. Moisture rotating around the periphery of a ridge can also lead to afternoon thunderstorms in the summertime.

During the winter, ridges are often associated with inversions when cold, dense air sinks into valleys and stable conditions above prevent the cold air from mixing out and rising above the valley floor during the daytime hours. When wintertime inversions persist, fog and low clouds sometimes develop, while warmer temperatures and sunny skies prevail higher up in the mountains. 

How do troughs influence the weather?

Troughs form when the jet stream dips southward into a bowl-like shape, and are associated with colder air, low pressure aloft, and a less stable atmosphere. This tends to result in cooler and more unsettled weather conditions. Sometimes, a closed circulation of low pressure develops within and near the base of the trough with winds aloft blowing in a counterclockwise direction around the low-pressure center.

Approaching troughs are associated with frontal storm systems that can produce all different types of weather depending on the time of year, from snow in the winter months to thunderstorms and heavy rain in the spring and summer. Troughs tend to be deeper and more pronounced during the fall, winter, and spring and weaker during the summer when the average jet stream position retreats northward.

Leading Edge of a Trough

The region at the leading edge of an approaching trough (the downstream side of a trough and backside of a ridge) is typically where the most widespread “stormy” weather occurs. This is the region where energy and moisture increase and stability decreases, often in advance of an approaching cold front. These factors can lead to rain, snow, thunderstorms, and/or strong winds. 

The intensity of weather systems associated with troughs varies depending on the nature of the trough – for example, whether or not the trough is deepening to the south or lifting to the north and the moisture and energy associated with this movement, among other factors. 

If moisture is limited in regions on the downstream side of an approaching trough, dry and windy conditions along with warm temperatures (before the cold front arrives) can result in elevated fire danger. 

Backside of a Trough

On the backside of a trough, colder and more stable air typically arrives but gusty winds may persist. In some cases, moisture can wrap around from the north and northwest, resulting in additional clouds and precipitation. 

In mountain areas, lingering moisture on the backside of a trough often results in orographic (terrain-driven) showers in the mountains while valley areas stay comparatively dry. These patterns can be beneficial for skiers in areas where the wind direction is favorable, as heavy snow can fall while temperatures are also colder, resulting in lower snow levels (if rain was a factor before) and higher snow-liquid ratios.

The movement, or lack thereof, of ridges and troughs

In more progressive weather patterns, ridges and troughs move across North America and other mid-latitude regions relatively quickly with periods of cool and wet or snowy weather interspersed with periods of dry/stable weather. 

However, it’s also common for blocking patterns to develop where ridges and troughs become entrenched over regions for 1-3 weeks at a time, and occasionally longer. This can be a good or bad thing depending on the time of year and the activity of your choice.

Blocking Ridges

When blocking ridges develop over an area with little movement, dry weather can last for a week or more. Sometimes, troughs will move through and break down the ridge with temporary periods of stormy weather, only for the pattern to revert back to ridging.

Blocking ridges in the winter are a bad thing for skiers as this can lead to extended dry spells with little snow. During the fall, blocking ridges may be more welcome as they can support sunny weather patterns with stable conditions for hiking and climbing before winter sets in.

In the summertime, broad high pressure ridges are more common due to the jet stream retreating further north as warm temperatures from the tropics extend northward with the changing seasons. However, sharper (or taller) ridges can lead to heatwaves, and in the West, this often leads to an increase in fire danger.

The strength and location of high pressure centers underneath a ridge are key factors in the summer across both the West and the East as subtropical moisture rotating around high pressure centers influences when and where thunderstorms are most likely to occur.

Persistent Troughs

On the flip side, persistent troughs associated with blocking patterns can lead to prolonged stretches of cool, wet, and snowy weather. Embedded storm systems developing within a relatively stationary trough can lead to endless days in a row with precipitation. If you catch this type of pattern in the mountains during ski season, then consider yourself lucky as you could be looking at multiple powder days in a row.

However, persistent “troughing” can be more of a nuisance for hiking and other non-snow outdoor activities, as these types of patterns can result in cool and showery weather for days on end. These patterns can also result in multiple days of enhanced thunderstorm potential on the warmer downstream side of a trough. 

Cut-Off Lows

Occasionally, a trough will become "cut off" from the mean flow of the jet stream and will develop into what is known as a cutoff low. These types of lows have a closed circulation and can meander over an area for several days or longer. The movement and associated weather impacts of cutoff lows are notoriously difficult to forecast.

Learn More → Cut-Off Lows

---

Download the OpenSnow app and stay tuned to our forecasts for the latest weather updates. 

Alan Smith