WASHINGTON – Lately, the jet stream isn’t playing by the rules. Scientists say that big river of air high above Earth that dictates much of the weather for the Northern Hemisphere has been unusually erratic the past few years.

They blame it for everything from snowstorms in May to the path of Superstorm Sandy.

And last week it was responsible for downpours that led to historic floods in Alberta, Canada, as well as record-breaking heat in parts of Alaska, experts say. The town of McGrath, Alaska, hit 94. Just a few weeks earlier, the same spot was 15 degrees.

The heat wave in the Northeast is also linked. “While it’s not unusual to have a heat wave in the east in June, it is part of the anomalous jet stream pattern that was responsible for the flooding in Alberta,” Rutgers University climate scientist Jennifer Francis said Tuesday in an email.

The jet stream usually rushes rapidly from west to east in a mostly straight direction. But lately it’s been wobbling and weaving like a drunken driver, wreaking havoc as it goes. The more the jet stream undulates north and south, the more changeable and extreme the weather.

In May, there was upside-down weather: Early California wildfires fueled by heat contrasted with more than a foot of snow in Minnesota. Seattle was the hottest spot in the nation one day, and Maine and Edmonton, Canada, were warmer than Miami and Phoenix.

And here is what federal weather officials call a “spring paradox”: The U.S. had both an unusually large area of snow cover in March and April and a near-record low area of snow cover in May. The entire Northern Hemisphere had record snow coverage area in December but the third lowest snow extent for May.

“I’ve been doing meteorology for 30 years and the jet stream the last three years has done stuff I’ve never seen,” said Jeff Masters, meteorology director at the private service Weather Underground. “The fact that the jet stream is unusual could be an indicator of something. I’m not saying we know what it is.”