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Coming next week: the collapse of a heavenly weather pattern

GFS model evolution of flow pattern at around 18,000 feet next 6 days (

A wonderful “blocking” pattern has established itself over the Mid-Atlantic region – protecting us from stormy weather surrounding us.  By early next week, this pattern, sadly, breaks down.

Currently, tranquil high pressure is stuck over the region, with areas of low pressure on both sides (see the orange and red areas in the animation above).  We call this pattern an “omega block” as the configuration of flow over the region resembles the Greek letter.  When an omega block sets up, it’s difficult to dislodge, like a boulder in a fast flowing stream.

But this weekend, the weather system – which has produced unprecedented May cold and snow in the central U.S. – will lean on our friendly block, nudging it offshore slowly but surely.

This system – an area of upper level low pressure – arrives in our region Monday, and will be in no hurry to move away.  It’s likely to pester us for several days next week with clouds and the chance of showers.

On the plus side, the low is forecast to be focused just to our south, sparing us from steady, heavy rains.

Another plus? You may notice that over the weekend – in the animation above – this low gradually becomes divorced from the fast flowing river of air over Canada, i.e. the jet stream (where the black lines of equal pressure are packed closely together).  The system is transitioning into what we call a cut-off low, since it becomes separated from the main flow. What this means is that it will lose its primary source of cold air, so we don’t have to worry about any real wintry weather next week, even if clouds hold temperatures in the 60s.

The take home message: enjoy the sparkling sunshine the weekend brings courtesy Mr. Omega Block.  He will be history when we return to work next week.

(CWG’s Rick Grow will have further analysis of this cut-off low next week)




Jason is the Washington Post’s weather editor and Capital Weather Gang's chief meteorologist. He earned a master's degree in atmospheric science, and spent 10 years as a climate change science analyst for the U.S. government. He holds the Digital Seal of Approval from the National Weather Association.
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