CNN
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Autumn is a season of transitions: scorching summer days give way to crisp autumn evenings, green leaves take on colourful hues and the fresh summer flavours are drowned out by the warmth of the spices used in baking.

But while pumpkin spice is already taking the country by storm, it may be much longer before the familiar feeling of fall sets in, as the warmth of an intensely hot summer in the United States shows signs of carrying over into the new season – a trend as the world warms due to fossil fuel pollution.

“The fall season is increasingly seen as an extension of summer,” said Jennifer Marlon, a climate scientist at the Yale School of the Environment. “It’s warming up quickly.”

Intense, frequent and long-lasting warming will likely make this summer the warmest on record for nearly 100 cities in the West, South and East, including Las Vegas, Phoenix and Washington, D.C.

In some places there has been some autumnal weather in recent weeks, but now the scorching temperatures of this summer are returning rapidly.

Take Philadelphia, for example. High temperatures in the low to mid 70s made it feel like late September for a few days last week. But the heat index — what the combination of heat and humidity feels like to the body — will reach triple digits there on Wednesday.

Tens of millions of people were under a heat warning on Wednesday. Record high temperatures can be expected in the east, with temperatures above 32 degrees Celsius and below 40 degrees Celsius in large parts of the country.

This temperature rollercoaster could be a seasonal theme. Despite persistent cold snaps, autumn warmth looks set to persist for much of the U.S.

Fall technically begins in September, but the more consistently cooler temperatures that come with it become more widespread across the country in the second half of the month. The problem is, the summer heat isn’t going away without a fight, according to the latest forecasts.

The most unusual warmth forecast this month extends from the central and southern Rockies to the Southeast, through the south and then north along the entire East Coast.

According to the Climate Prediction Center, temperatures are most likely to remain closer to normal in the northern parts of the country, from the Pacific Northwest to the Upper Midwest.

According to the CPC, temperatures in October and November will remain largely above average.

The entire fall won’t feel exactly like summer, even with higher average temperatures. Typical high temperatures drop by about 70 to 78 degrees from mid-July through mid-October for much of the U.S.

Denver, for example, where temperatures drop from a normal high of around 32 degrees Celsius in July to a high of around 17 degrees Celsius in October.

The East Coast could get some relief from the abnormal heat this fall, if an active hurricane season has its way.

Despite some quiet periods, the Atlantic hurricane season is expected to resume soon. La Niña is expected to intensify in the fall and could increase tropical activity in the Atlantic. The natural climate pattern of cooler-than-average ocean temperatures in the tropical Pacific Ocean is the greatest influence on U.S. weather in the winter, but signs of that could emerge later this fall.

Meteorologists are monitoring possible tropical disturbances early in the month; the climatological peak of the hurricane season is less than two weeks away and the most active part of the season lasts until mid-October.

Any storms moving along the U.S. coast through the end of the hurricane season could also bring rain and “result in periods of cooler than normal temperatures for these areas this fall,” the CPC said.

The active hurricane forecast also means the East Coast could end the fall with a wetter than normal weather.

The other extreme is possible in much of the western and central U.S., where below-normal precipitation is forecast. A lack of needed rain could worsen drought conditions in areas that have already spent the summer drying out.