For the past week, more than 200 million Americans have felt above-average temperatures.
Some locations saw daytime highs almost 20 degrees above normal for this time of year.
But a change in the weather pattern will bring a winter-like chill to most of the United States.
Why Has it Been so Warm?
Lately, we’ve seen a very zonal pattern in the jet stream. This means it’s been pretty flat– no major ridges or troughs in the pattern. And that meant calm conditions and unseasonable warmth.
This is a pattern much like what we would expect to see in early fall.
Really the only reason we’ve seen temperatures dropping at all is the change in sunlight. But that lack of sunlight is playing a big role in the change to come.
Kiss the Warmth Goodbye
We rely on sun for daytime heating. That’s why it gets much warmer during the longer days of summer.
The response is lagging. This means that the hottest days of summer follow the longest days of the year, and the coldest days of winter follow the shortest days of the year.
Water is able to hold onto heat much better than land, so as the days grow shorter land masses cool off much quicker.
Since the jet stream flows along areas of greatest temperature differences, the flux between land and sea temperatures sets up the drastic ridges and troughs we see in the winter-time Polar jet.
And this coming week will bring about a big trough.
That dip in the jet will usher in some of the coldest air of the season, and an end to the warmth we’ve all loved.
The Temperature Drop
I could explain this with words all day long and not describe what this sequence of maps below will do with a quick glance.
Needless to say, it’s time to get out the parka.
For WeatherNation — I’m Meteorologist Jeremy LaGoo