Today is the first day of fall, meteorologists say. But it won't feel like it in the West

Sick of sweltering summer heat? Well, you are in luck, because fall starts Thursday, at least according to meteorologists.

Here in the Northern Hemisphere, "meteorological" fall (aka autumn) is defined as the months of September, October and November. (It's spring in the Southern Hemisphere).

But while some consider Sept. 1 the first day of autumn, crisp, cool temperatures will be hard to come by for many across the nation. And if you are in California, you won't come close: Get ready to roast.

Most people mark the first day of autumn on the autumnal equinox, which this year is Thursday, Sept. 22. That's because, astronomically, the sun will be directly over the equator that day as the Earth's tilt begins to point the Northern Hemisphere away from the sun.

Astronomical fall is still three weeks away. People often say the "official" start of the seasons are the astronomical dates, though there is no legislative body that makes such a designation.

Meteorologists and climatologists need specific dates for the four seasons, so it makes sense for the seasons to follow our familiar monthly schedule. Scientists require set time periods to calculate averages and do year-to-year and decade-to-decade seasonal comparisons, WeatherBug meteorologist Mark Hoekzema said.

NOAA's National Center for Environmental Information puts it this way: "By following the civil calendar and having less variation in season length and season start, it becomes much easier to calculate seasonal statistics from the monthly statistics, both of which are very useful for agriculture, commerce and a variety of other purposes."

So the three hottest months of June, July and August constitute summer, and the three coldest months of December, January and February are winter.

As for fall, September does usually tend to be more of a "fall-ish" month, while December is more wintry, so the meteorologists seem to have it right.

Though temperatures will be near average in the north-central and northeastern U.S., it will be much warmer than average across the western U.S., where record heat is likely Thursday and over the next few days.

In fact, an excessive, prolonged heat wave will further parch large swaths of California in the longest and hottest heat wave of the year, according to forecasters at the National Weather Service.