Waves of cooler, less humid air to reach the Midwest, Northeast

As the change occurs, the jet stream will dip southward over the Northeast and Midwest, and that will pave the way for rounds of cooler and less humid air.

“This change in the pattern will first lead to an increase in thunderstorm activity across the Northeast and Midwest regions,” AccuWeather Senior Meteorologist Brett Anderson explained, “Cooler air will first dip into New England while the worst of the heat gets squeezed into the Middle Atlantic and Southeast.”

Anderson noted that the meaning of cooler and less humid air in this context only refers to conditions more typical of the historical average for late June and not a return of the unseasonable chill that occurred during parts of the spring.

The pattern flip may be a bit of a shock to those who have become accustomed to sweltering humidity and temperatures of 10-20 degrees above the historical average for much of the past week. For millions of others, the change may feel refreshing, if not a substantial relief, especially for those who do not have air conditioning or access to beaches or pools.

Thunderstorms helped to break the heat in parts of New England on Thursday. Following highs in the upper 90s F in Boston on Wednesday and Thursday, high temperatures reached 82 in Beantown on Friday and will be in the 70s on Saturday.

However, two main cool fronts will direct more widespread changes to typical late-June conditions in the Midwest and the Northeast.

“One front will slice southeastward from the Upper Midwest to the mid-Atlantic and New England coast into early week,” AccuWeather Senior Long-Range Meteorologist Joe Lundberg said, adding, “A stronger front will progress southeastward from the early to the middle part of next week.”

The stronger push of cool air will be preceded and accompanied by locally severe thunderstorms from the Midwest to the East.

Temperatures and humidity are forecast to surge briefly to high levels ahead of both fronts then dip in their wake. High temperatures are forecast to reach the triple digits in Baltimore and Washington, D.C., on Saturday, Sunday and Wednesday, then trend downward into the upper 80s later in the week. By Thursday and Friday of next week, high temperatures will be in the middle 80s in New York City and close to 80 in Pittsburgh.

Western heat to surge again

“The core of the heat will become re-established from the southern Plains to the deserts and Great Basin of the West next week, similar to where it was earlier in June,” Anderson said.

Some parts of the interior West, such as around Fresno, Calif., may begin to challenge daily record highs as early as this weekend.

Other cities, such as Las Vegas, may approach record highs by the middle of next week, with temperatures hitting 110 or more.

The high pressure area may not be strong enough to prevent the daily eruption of spotty thunderstorms over the interior West. Where little rain falls from the storms, lightning strikes could spark new wildfires.

According to the National Interagency Fire Center, wildfires were active on Saturday in California, Utah, Arizona, New Mexico and Colorado. Fires in the Ruidoso, New Mexico, area alone have killed at least two people, forced the evacuation of 8,000 people and burned at least 1,400 structures.

Meanwhile, isolated torrential downpours can lead to highly localized flash flooding, especially in some of the arroyos in New Mexico and Arizona.

Short-lived relief from the dangerous heat in sight for part of the Northeast

Flash flooding and hail left dozens of vehicles stuck along State Road 42 in New Mexico on June 19.

Not much change for the Southeast

As the weather changes in the Northeast and the West in the coming days, people in the Southeast region are unlikely to experience much change. Most days will remain hot and humid. Some cooler nights and slightly lower humidity levels may be felt in the southern Appalachians and Piedmont areas for a day or two.