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What Is An Upper-Level Disturbance?

Updated: Friday, August 30 2013, 09:51 PM EDT

While strong fronts are often the main reason for big snows and severe storms in the Tri-State, a lot is going on the upper-levels of the atmosphere to make a storm. In fact, changes in the upper-levels of the atmosphere often help to create or strengthen areas of low and high pressure at the ground.

Simply put, an upper-level disturbance is an area of rotating air found around 18,000 feet above the ground that forms as a result of wind speed differences aloft. These wind speed differences form as a result of low and high pressure differences above the ground. Upper-level disturbances can have a big influence on the weather we see (or don’t see).

An example of an upper-level forecast, showing ridges and troughs and also vorticity maximums

The meteorological term for an upper-level disturbance is a vorticity maximum; this is a fancy term describing rotation thousands of feet above the ground. This area of rotation is frequently found at the base of a trough, or an elongated area of low pressure. When you look at the upper-level forecast map over the United States to the right, a trough is the dip in black lines (extending south) over New England and also over the Pacific Northwest. You can also see a ridge – an elongated area of high pressure – in the upper-level forecast map as the black lines extending northward into south-central Canada. Winds in the upper-levels of the atmosphere tend to move parallel to these black lines on the map; in other words, winds aloft generally flow from west to east but will move more north and south where there are strong troughs and ridges.

When these black lines on the map are closer together, the wind aloft moves faster. Because winds at the base of troughs tend to be faster, an area of rotation forms. This is similar to moving your hand through a bucket filled with water; water will move faster near your hand compared to water farther away from your hand, and an eddy (or area of vorticity) will form. Areas of rotation in the atmosphere are important to meteorologists because they help to lift air in the atmosphere, especially ahead of the disturbance. Air that is being lifted is more much likely to create clouds, showers, and storms compared to air that is sinking or not being lifted. Knowing where upper-level disturbances are in the atmosphere can often tell a meteorologist where clouds and precipitation are more likely to form.

An upper-level forecast shows a trough and embedded vorticity maximum over North and South Dakota.

Suppose you are a meteorologist looking to see where rain and storms may form in the near future. You are given the upper-level forecast at the left. An upper-level disturbance (colored in yellow, brown, and red) in the base of a trough is moving through North and South Dakota, heading east. Where do you think clouds and precipitation may develop soon? If you guessed Minnesota, Wisconsin, or anywhere in the Great Lakes, you are correct! Clouds and precipitation tend to develop on the nose (or ahead) of upper-level disturbances, where air is rising, cooling, and condensing. Showers and thunderstorms don't always form on the nose of an upper-level disturbance; other conditions (such as instability, moisture, and wind shear) are often needed for precipitation to develop.

An upper-level forecast shows a trough and embedded vorticity maximum over North and South Dakota.

In the image at the right, the same model that was forecasting an upper-level disturbance to be over the Dakotas is forecasting precipitation over the Great Lakes 24 hours later. Also in the image at the left, you can see lines of equal pressure (isobars, in black) tightly packed around a center of low pressure (the black “L” on the map”), which has deepened in response to the approaching upper-level disturbance. This sample exercise also shows the importance of looking at weather information at all levels of the atmosphere. While the movement of air near the ground is very important in forecasting, meteorologists need to know where upper-level disturbances are forming or may form to know how the weather may change here at the ground.

What Is An Upper-Level Disturbance?

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