Blocking Patterns 101

Western Wisconsin can bring every kind of weather and often big swings in the weather conditions can happen day to day. One day you’re in the 50s and the next, 20s with wind chills close to zero. We've probably all heard the saying, "if you don't like the weather, just wait a minute".

But every once a while we get into a weather pattern that brings the same weather for days, if not weeks on end. We refer to those as blocking patterns and here is the explanation of those.

Blocking patterns are created by the location and strength of the jet stream. The jet stream is a column of air that moves around the world at highs speeds. At times the winds can up to 200 mph and the jet stream is usually about 6 to 9 miles from the surface of the Earth.

The first blocking pattern we’ll talk about is the cut off low. Below you’ll see a picture of the jet stream winds from Wednesday March 21st.

The cut off low occurs when the jet stream shifts closer to a higher latitude (closer to the North Pole) and leaves a circulating low pressure behind it. With that low center being cut off from the rest of the jet stream, it can stay in one location several days and bring several days of rainfall and flooding underneath the places they sit(just ask the people in Texas, Arkansas and Louisiana from the past couple of days!). These commonly happen off the coast of California but can develop anywhere.

The next blocking pattern is called a Rex block. The picture is below.

A rex block sets up what a ridge of high pressure is located north of a large trough. Air flows north of the ridge and then south of the trough, and the centers of the high and low will stay in the same spot for long periods. The blocking pattern resembles a half figure 8 or a backwards S.

Another blocking pattern is called the Omega Block. That is shown below.

Omega blocking patterns usually set up across the Unites States. At the center of the pattern is big area of highs pressure over the center of the country. On either side of that high, there are centers of low pressure. Under the high pressure, along period of dry, warn and light winds will be experienced. While by the low centers rain showers and thunderstorms are likely and could potentially lead to flooding. Areas on the right side of the blocking pattern usually see below normal temps while on the left, above normal temps are likely.


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