Summer just doesn’t want to quit this year. As the calendar flips from September to October, it will feel more like July in many eastern U.S. locations.

The National Weather Service’s Climate Prediction Center is assigning “high confidence” in unusually warm weather persisting through at least the start of October. Temperatures may be more than 15 degrees above average in spots on multiple days.

“For those who like summer, you’re going to like the six-to-10-day outlook,” said Scott Handel, a meteorologist at the National Weather Service’s Climate Prediction Center in College Park, Md.

Forecast details

Warm weather in the East builds late this week. Temperatures by Saturday could sneak into the mid-80s in Philadelphia and New York City. In Washington, 90 degrees is reasonably likely. These temperatures are generally 10 to 15 degrees above normal.

After a couple of cooler days with an easterly breeze between Sunday and early next week, Philadelphia and New York could climb even higher Oct. 2, nearing 90. In the District, temperatures that warm could be realized Oct. 1, too. There’s an outside chance Washington could see its first October heat wave, defined as three straight days hitting at least 90, since 1941 between Oct. 1 and Oct. 3.

Even Boston could come close to hitting 90 Oct. 2, with a better chance just west of downtown — as a sea breeze may keep the shoreline a touch cooler. And Portland, Maine, could enjoy a Wednesday high in the mid-80s — beach weather at a time of year where highs should be in the mid-60s. Even in the heart of summer, Portland’s average highs stay below 80 degrees.

Sometime between Oct. 2 and Oct. 4, a cold front may abruptly end this heat, starting in the Northeast and then pushing south into the Mid-Atlantic, although the exact timing of this air mass change is still to be determined.

In the Southeast, which is enduring one of its hottest Septembers on record, the heat is likely to be most persistent and intense. The predicted strength of the heat dome over this region next week only occurs once every 10 to 30 years, according to one analysis. From Mississippi to Georgia, highs most days over the next week are expected to average between 90 and 95 degrees with pockets of upper 90s.

This heat in the Southeast will be made worse by a lack of rainfall.

Many cities in the southeast and Mid-Atlantic have seen less than a quarter inch of rain this month and are experiencing one of their driest Septembers on record. Not only is that causing drought conditions to intensify and expand, but the drier ground is easier to heat up — which makes temperatures head even higher. It’s a vicious self-reinforcing cycle.

Behind this toasty pattern

The abnormally warm weather is all thanks to a sprawling heat dome that will engulf the eastern third of the nation for at least the next one to two weeks — and maybe longer.

“We’re seeing … strong ridging [in the East],” said Handel, referencing the initial blast of heat slated for the next six to 10 days. “But on the flip side, that means strong probabilities for cold in the western U.S.”

The seesaw weather is thanks to a “much more amplified pattern than usual,” said Handel. Meteorologists refer to amplitude to describe how wavy the jet stream is.

This week, the jet stream is expected to soar north, clipping Alaska’s North Slope and drawing in a surge of warm air. That “ridging” over Alaska has been there a while — leading to a summer of extremes in the Last Frontier.

From there, the atmospheric conveyor belt dives south, spilling a pool of chilly air across the West and Northern Tier. With one last kink, the jet races north again over the Mississippi Valley, opening the doors to southerly winds and impressive heat and moisture for the Ohio Valley and the Eastern Seaboard. In addition to generating impressive temperature contrasts, the pattern is ripe for storm systems to develop over the Rockies, Plains and Northern Tier, where the air masses clash. Up to three feet of snow are forecast in the Montana Rockies.

The Climate Prediction Center is most confident in East Coast warmth sticking around for the first part of the eight-to-14-day period; after that, Handel mentioned a subtle “retrogression of the surface high,” which means the axis of warmest temperatures may get shunted southward and westward late in the first week of October

Brad Pugh, another meteorologist at the Climate Prediction Center, says that shift could favor more back-door cold fronts across the northeast and Mid-Atlantic. That would cool coastal cities a bit. That may be the case Monday and later next week in the northeast and Mid-Atlantic.

Part of a trend toward toastier autumns

As the climate continues to warm, heat domes such as the one predicted over the next week are expected to become more intense. That keeps summerlike weather around deeper into the fall, and data show that this is already happening.

Since 1960, the annual growing season is now four weeks longer every year in Chicago due to later fall frosts and earlier spring thaws. In Philadelphia, it’s a 16-day difference. In Washington, about 12 days.

At the same time, instances of record high temperatures outpace record lows — the United States has set 171 record highs this year, compared with just 108 record lows.