Fronts defined: How they form and what weather they may bring

Weather Glossary

ROCHESTER, NY (WROC) – Frontal boundaries are the separation of two air masses. Those air masses are defined In between them will often result in clouds and precipitation. These boundaries are often guided by the jet stream aloft and shape low pressure at the surface. These frontal boundaries can occur during any time of year. Depending on the characteristics of the front, we can categorize them into four main categories. Cold front, warm front, stationary front, and occluded front.

COLD FRONT:

Cold air advancing into warmer air that is forming a boundary would be a cold front. Colder air is more dense than warmer air, so it will (most of the time) ride underneath the cold air and there can be an abrupt change in temperature when a cold front moves through. Note in the graphic below that the cold front extends high up in the atmosphere. As warmer air rises, it cools and expands to form clouds. Those clouds can lead to rain and thunderstorm formation along the cold front.

Image courtesy NWS

Cold fronts usually drag along the southern part of low pressure systems with cold, Canadian air advancing behind the front. Expect wind directions to change at your location from a warm south wind to a cooler west or northwest wind. Dewpoint ussually drops with the passage of the front as well. Behind the front, pressure will increase. Most cold fronts

A cold front is blue on a surface map with triangles indicating the direction in which the front is moving. Most of the time it is moving generally west to east or north to south.

WARM FRONT:

Warm fronts can be much more broad and will often extend eastward from low pressure. Since this is advancing warm air, it essentially can look like a cold front that is retreating. Precipitation associated with this front will be stratiform (widespread) and rarely will bring thunderstorms in the Northeast.

Image courtest NWS

Warm fronts can often be difficult to forecast for because of their broad nature. A gentle rise of an air parcel can mean lighter precipitation that may not reveal itself within certain model runs.

A warm front is red on a surface map with half circles indicating the direction of travel. This is generally from west to east or south to north as warmer air originates to our south.

STATIONARY FRONT:

A stationary front represents two air masses that are locked in place. Generally you will find precipitation along the front and it will be sitting underneath a steady or slowly moving jet stream. Stationary fronts can lead to flooding situations as moisture may linger over one region for a long period of time and continually bring rain. Stationary fronts can form when an advancing cold front stops moving.

A stationary front is a combination of both a cold and warm front with protruding shapes that show where the warm and cold air are located in relation to the front.

OCCLUDED FRONT:

Cold fronts move faster than warm fronts. Along a low pressure system, when there is an advancing warm front east of low pressure, the cold front to the south can approach and eventually take over the warm front, creating an occluded front.

An occluded front on a weather map is represented in purple as alternating cold and warm front shapes indicating the direction the front is moving.

The most common occluded front is where the advancing cold air rides underneath both the warm air associated with the warm front as well as the colder air. This is considered a cold occlusion.

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There are a few other honorable mentions when it comes to frontal boundaries. A dry line is a separation between dry and moist air masses that may be close in temperature. A very common dry line found in the United States is one that clashes the dry and warm airmass from the desert southwest with the moist and warm airmass from the Gulf of Mexico. Thunderstorms that can sometimes be strong to severe may form on this line.

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