The polar vortex is beginning to stir, signaling the first signs of seasonal change.
In the short term, that means gradual cool-downs and a more active storm track across the northern U.S. as fall settles in.
The shift won’t be abrupt, but overnight lows are expected to dip more steadily by mid-October. For now, temperatures are still holding in the low to mid-50s, so there’s no need to cover plants or pipes just yet.
Still, it’s a good time to start thinking about which fruits and vegetables do well in cooler weather — and to keep an eye on the forecast so you’ll know when to protect your favorite flowers.
When will cold air move in? Average first frost dates in New Hampshire
The Climate Prediction Center’s outlook shows normal temperatures across New Hampshire through September and well-above-normal temperatures across the state in October and November.
According to historic data from the National Weather Service, these are the average frost dates in New Hampshire.
Earliest: Sept. 24 (1953)
What is the polar vortex?
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.
What does the polar vortex look like right now?
Zonal winds — which track west-to-east wind speeds circling the Arctic at stratospheric levels — are just above 5 meters per second as of Friday, Sept. 11. 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.
How to protect plants and winterize pipes
Most plants can survive brief bursts of cold weather, but prolonged freezing temperatures may end the growing season for your garden. If you still want to grow something or plan a fall garden, here are some tips:
Protecting plants:
Cover sensitive plants: Use sheets, blankets or frost cloths overnight. Remove coverings during the day so sunlight can reach them.
Move potted plants indoors: Bring pots inside or into a garage for the night if possible.
Water plants: Moist soil holds heat better than dry soil, helping reduce frost damage.
Mulch: A layer of mulch around plant bases helps insulate roots.
Winterizing pipes:
Drain outdoor faucets and hoses: Disconnect hoses and drain water from outdoor faucets.
Insulate exposed pipes: Use foam sleeves or pipe wrap on pipes in unheated areas.
Seal drafts: Check for cracks or openings in walls and around pipes where cold air can enter.
Let faucets drip: On extremely cold nights, letting indoor faucets drip slightly can prevent freezing.
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Brandi D. Addison covers weather across the United States as the Weather Connect Reporter for the USA TODAY Network. She can be reached at baddison@gannett.com.
This article originally appeared on Portsmouth Herald: Polar vortex stirring up. Should New Hampshire expect frost? What data says
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