What is the Jet Stream and What Does it Mean for the Forecast?

Boston ended a seven-day stretch of 90-degree days on Monday, and here we are with a midweek “cool” down with highs in the 80s and very comfy air. 

But in our 10-day forecast we see signs of another warmup and maybe another heat wave setting up in New England.  Why? It’s about the jet stream.

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What is the jet stream?

The jet stream is a river of air that flows around the northern hemisphere with strong winds in the center.  This river is about 30,000 feet in the sky, or where cross-country "jets" fly.  (During the summer months the jet stream can be as high as 38,000 ft).  Warm air is to the south of the jet stream, and cold air is to the north with the jet dividing the airmasses.  The winds flow from west to east around the globe, with ebbs and flows as the stream amplifies or weakens.  When we have a powerful low-pressure system riding along the jet stream, and an associated cold front, severe storms sometimes move through and the change in temperatures can be quick.    

How did the jet stream affect our recent weather?

When we had the hot stretch in the northeast, the jet stream was located to our north and west and there was a large ridge.  We were in the hot and humid airmass. When we had a cold front move through, the jet stream dipped over New England on Monday and this brought in some at least milder temps and much lower dewpoints.  Yesterday and today the cooler temps are around as the jet stream is right across New England.  We continue to see the jet located across the northeast this weekend as humidity stays low Saturday and Sunday.  By the end of next week, another ridge of high pressure, or dome of heat, builds across the western, then middle of the U.S.  The jet stream slowly retreats north of New England again, and we’re looking at another 90-degree stretch possibly starting midweek.