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The Omega block: Feast or famine

1:45 PM, Sep 25, 2013   |    comments
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While we've been enjoying sunny, cool and pleasant conditions here in the DC Metro area, it has been quite unsettled in the northern Rocky Mountains as of late, with thunderstorms in the northern Plains states. And, it looks like this pattern is going to basically stay the same all the way through this upcoming weekend! It's due to a weather pattern we refer to as "blocking" in the upper atmosphere. In this case, it's an Omega block that is responsible. 

The Omega block gets its name from the Greek letter, which is shaped like this: O. The image above was pulled from data derived from the North American Mesoscale model (NAM). I outlined the pattern in blue to make it easy to spot. Notice the highly amplified ridge, with a deep trough on each side. While there is nothing unusual about a trough and a ridge next to each other- it is a wave pattern, after all- an Omega block is not your typical crest-trough couplet. Because of the highly amplified nature of the ridge in the center, and the strong counterclockwise flow around each trough, this pattern moves slowly and changes slowly, too. So, dry areas stay dry. Hot areas stay hot. And, wet areas stay wet. Especially if you live in the center of one part of the pattern (in the base of the trough or the crest of the ridge), your weather will not change much from one day to the next. 

Blocking patterns are fairly common. The Omega is the easiest to spot on forecast model data because it's the most distinctive-looking of any blocking pattern. It's also the type of blocking pattern that is most likely to affect our weather here in the DC Metro. We are just as likely to get nice weather (in the ridge of the block) as we are to get bad weather (in the easterly trough) out of this pattern. 

In the Spring and Fall, blocking patterns often have trouble maintaining their strength and duration. These patterns last longer in the winter and summer if they are put in place. A wintertime Omega block can easily last for more than a week or more with practically no change! The Omega that's in place right now isn't as static. Even though our weather will stay nice, it does appear that the pattern will break next week. 

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