Just in time for the beginning of August, the jet stream is undergoing a readjustment that will bring a noticeable change in the weather over the United States for the next couple of weeks. The shift should bring cooler weather to much of the eastern U.S., while the West Coast stays as hot and dry as ever.

An unusually strong low pressure system that formed on the U.S./Canadian border earlier this week—the one that brought snow to the Rockies and extremely strong winds to much of the Upper Plains—marked the beginning of the realignment. That low, which we’ll call the “Montana low,” is currently over Hudson Bay (pictured above) and dragging with it a cold front that stretches all the way down to the southeast states. This cold front is responsible for the lines of showers and thunderstorms affecting the East Coast this afternoon, including the storm that prompted the flash flood warning in New York City.

Elsewhere, we’re in that stretch of the summer where there’s a deep ridge of high pressure over the south-central part of the country. This ridge, which is responsible for the brutally hot weather seen in the Plains during the summer, is usually located over Texas, leaving the jet stream (and everything else) to go around the edge of the high.

The above animation (be warned: the gif is 7.02 MB) shows the progression of winds at the jet stream level over the next ten days as predicted by the GFS model. If you watch the animation all the way through, you can see the influence of the Montana low sticking around all the way through to the middle of the month, forcing a steady trough to develop in and around the Great Lakes and Northeast. This feature will help nudge the ridge that’s over the southern United States, forcing it to move out of Texas and into the southwest for a couple of days.

Confidence in model forecasts goes down with time, but since last week they’ve consistently shown this pattern taking place during the first two weeks of the month.

What does it mean for you?

Normalish Temperatures in the East

When the jet stream shifts next week, it will allow cooler air to filter down from Canada and start to pour over the northern states. Temperatures will dip to or slightly below average for much of the country east of the Plains and north of the 37th parallel. “Average” doesn’t sound too cool to most of us right now, but upper 70s in August is pretty nice even in areas where that kind of weather is normal for this time of the year.

Temperatures will likely tick back up a little bit for the second week of August, but it shouldn’t be anything unbearable.

Hot and Dry in the West

Parts of Oregon and Washington are experiencing the hottest weather they’ve felt so far this year, with high temperatures in much of western Oregon exceeding 100°F this afternoon. Seattle-Tacoma International Airport is about to record the most 90°F days they’ve ever seen in one year, blowing past the record of 9 set back in 1958. Temperature records at SeaTac go back to 1945.

After a brief reprieve from the heat late next week, temperatures should steadily climb back up into “ick” territory and stay there for a while. Warm temperatures are bad enough—especially where people don’t have adequate access to cooling—but where there’s heat, there’s usually not any rainfall. Add to that the fact that the Pacific Northwest usually sees a drier- and warmer-than-normal winter during an El Niño year, and things look like they’re probably going to get worse over the coming months. There’s always hope, though.

[Images: NOAA, WeatherBELL]

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