The polar vortex is stirring up, bringing the earliest hints that the seasons are changing.
That shift is most noticeable in northern Wisconsin, where a frost advisory was in effect early Monday, Sept. 8, as temperatures dipped into the 30s.
Frost advisories also covered other parts of the Great Lakes region, New York and New England Monday morning with Michigan recording the coldest temperature in the U.S. at 27 degrees.
"Temperatures in the low to mid-30s will result in frost formation," the National Weather Service said.
In the short term, this means gradual cool-downs and a more active storm track across the northern U.S. But not everywhere is feeling the chill yet. In the greater Milwaukee area, highs are still expected to reach the low- to mid-70s, with overnight lows in the upper 50s to low 60s.
So don't rush to cover your plants or winterize your faucets just yet -- there's still time to enjoy the outdoors while it's mild and pleasant.
The polar vortex is a swirling mass of frigid air high in the atmosphere, always present near the Arctic.
Its strength and position shift with the seasons, according to the National Weather Service. When the vortex is strong, the coldest air tends to stay near the North Pole. When it weakens or wobbles, pockets of that cold air can sometimes spill south, which can contribute to extreme winter storms or prolonged cold snaps in the U.S.
It's important to note that the polar vortex doesn't directly control U.S. temperatures -- it sets the stage and reflects broader seasonal shifts in the atmosphere around the globe.
How much cold actually reaches the Midwest or other parts of the country depends on other atmospheric patterns, like the jet stream and storm systems. Colder air could eventually move into parts of the U.S. as the vortex shifts, but that doesn't mean immediate blizzards or Arctic outbreaks -- it's simply a sign that the atmosphere is gradually transitioning toward winter.
Over the longer term, forecasters will watch whether the vortex remains stable or weakens later this fall. A weaker vortex increases the likelihood of Arctic air reaching the U.S., while a stronger vortex generally keeps the chill locked farther north. In other words, the polar vortex signals potential cold approaching, but does not guarantee a brutal winter.
Zonal winds -- which track west-to-east wind speeds circling the Arctic at stratospheric levels -- are just above 5 meters per second as of Monday, Sept. 8. That's fairly weak, but typical for the start of autumn.
Over the next several weeks, forecasts show the winds steadily strengthening to 15-20 m/s, a normal seasonal ramp-up as the polar night sets in. Stronger winds generally keep the coldest air locked near the pole, meaning gradual cooling across the U.S.. while the most frigid air sits near the pole.
Even as the vortex strengthens, it doesn't directly push cold air south. This seasonal strengthening simply correlates with autumn weather patterns; it does not cause them. Essentially, the vortex sets the stage rather than controls temperatures.
When the polar vortex weakens or is disrupted -- sometimes during a rare event called a sudden stratospheric warming -- the jet stream can dip south, allowing Arctic air to reach parts of the U.S.
"Many times during winter in the Northern Hemisphere, the polar vortex will expand, sending cold air southward with the jet stream," the National Weather Service said. "This occurs fairly regularly during wintertime and is often associated with large outbreaks of Arctic air in the United States."
For the most part, however, the polar vortex does not directly cause winter weather in the U.S. It just correlates with broader atmospheric patterns that influence how and when cold air reaches the country.
The Climate Prediction Center's outlook shows well-above-normal temperatures across Wisconsin through September, with seasonal temperatures in October and November. This means, much of the state can expect to see its average first frost within the next four weeks.
According to historic data from the National Weather Service, this is when each region sees its average first frost, or temperatures dipping below 32 degrees.
Brandi D. Addison covers weather across the United States as the Weather Connect Reporter for the USA TODAY Network. She can be reached at [email protected].