Polar Vortex Explained

The result

Severe weather

The jet stream, the polar vortex and the Pacific do not build any single storm. They decide where storms are likely to go and how much cold air, moisture and wind those storms have to work with.

See it in 3D In effect now Watch vs. warning

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In effect across the United States

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Severe thunderstorm outlook

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Arctic outbreaks and wind chill

Extremely cold air reaches at least part of the United States every winter.1 It arrives when the jet stream dips south and lets polar air follow it. That is more likely when the polar vortex is weak, displaced or split, and when the Arctic Oscillation is in its negative phase.23

Wind makes cold more dangerous because it strips heat from exposed skin faster. Wind chill is a measure of that heat loss. At 0 °F with a 15 mph wind, the wind chill is −19 °F, and exposed skin can freeze in 30 minutes.4

Threshold
Wind chills of about −20 °F and lower are potentially dangerous as a general rule. The exact criteria for an Extreme Cold Warning are set by each local forecast office.54
Worth knowing
Wind chill applies only to people and animals, and it cannot cause frostbite if the actual air temperature is above freezing.4
Benchmark
February 2021, Texas and the central U.S. Dallas-Fort Worth fell to −2 °F on February 16. Nearly 10 million people lost power at the peak. NOAA counts $24.0 billion in damage and 226 deaths, the costliest U.S. winter storm on record.67 A sudden stratospheric warming that January had disrupted the polar vortex, which encouraged a wavier jet stream for several weeks.2
Also
January 30-31, 2019, Midwest. Chicago recorded −23 °F, its coldest temperature in 34 years, and Rockford, Illinois, set an all-time record low of −31 °F.8

Blizzards

A blizzard is defined by wind and visibility. The official definition says nothing about how much snow falls, and snow that is only blowing, not falling, can qualify.9

Threshold
Sustained wind or frequent gusts of 35 mph or more, with falling or blowing snow that frequently cuts visibility below a quarter mile, for three hours or longer.9
New this winter
The National Weather Service will use "Particularly Dangerous Situation" wording in select Blizzard Warnings and Ice Storm Warnings in the 2026-27 season.10
Benchmark
The "Storm of the Century," March 12-14, 1993. More than 270 people died across 13 states. Mount LeConte, Tennessee, measured 56 inches of snow, and damage came to about $5.5 billion. A powerful jet streak ahead of a deep trough drove it.1112

Nor'easters

A nor'easter is a storm along the East Coast of North America, named for the northeast winds it brings to the coast. They are most frequent and most violent from September to April. They usually form between Georgia and New Jersey, within 100 miles of the coastline.13

Pattern behind it
The polar jet stream carries cold Arctic air south and east, where it meets warm air over the Gulf Stream. In the Weather Service's words, that temperature difference "is the fuel that feeds Nor'easters."13
Prepare
Keep three days of food, water and other supplies on hand ahead of one.13

Bomb cyclones

"Bomb" is a real meteorological term, coined in a 1980 paper. It describes a storm outside the tropics that strengthens explosively, a process also called bombogenesis.1415

Threshold
Central pressure falling at least 24 millibars in 24 hours. That figure applies at 60° latitude. Closer to the equator the bar is lower: about 17.8 millibars at the latitude of New York City.1615
Pattern behind it
It can happen when a cold air mass collides with a warm one, such as air over warm ocean water. Bombs are mostly a cold-season event over the sea, and they form just downstream of a trough in the jet stream.1514
Benchmark
March 13, 2019, Colorado and the central Plains. Pressure fell more than 24 millibars in 24 hours and bottomed out at 970.4 millibars at Lamar, the lowest ever recorded in Colorado. Gusts reached 60 to 80 mph, and locally 100 mph.17

Lake-effect snow

When cold air, often from Canada, moves across the open, relatively warm water of the Great Lakes, it picks up heat and moisture. The air rises and forms narrow bands of cloud that can drop two to three inches of snow an hour or more.18 It can be snowing hard in one spot while the sun is out a mile or two away.18

Pattern behind it
It needs cold air crossing open water, so it tends to follow the same jet stream dips that bring Arctic air south.
Benchmark
Buffalo, New York, November 17-19, 2014. More than five feet of snow fell just east of the city, with "mere inches a few miles away to the north." Thirteen people died. A second storm days later brought totals to nearly seven feet.19

Ice storms

Freezing rain is rain that freezes when it hits the ground, coating roads, trees and power lines in ice.9

Threshold
Ice Storm Warnings are usually issued for about a quarter inch of ice or more.9
Benchmark
January 5-9, 1998, the Northeast and southeastern Canada. More than $4 billion in damage and about 40 deaths. The Weather Service ties some of the storm's origins to that winter's El Niño, which makes the subtropical Pacific jet more active and feeds more moisture across the southern United States.20

Tornadoes and severe thunderstorms

A tornado is a narrow, violently rotating column of air that extends from a thunderstorm to the ground.21 Forecasters look for four ingredients: moisture, instability, lift and wind shear.22

The jet stream supplies the last one. Wind shear means winds at different heights blowing at different speeds or in different directions, and it is one way a column of air begins to rotate.23 A strong trough in the jet stream, with fast winds aloft over a deepening low at the surface, produces a large change in wind with height.24

Severe storm
A thunderstorm is "severe" if it produces a tornado, winds of at least 58 mph, or hail at least one inch across, the size of a quarter.25
How many
About 1,200 tornadoes hit the United States in a year. From 1993 to 2022 they killed an average of 71 people a year.2126
El Niño link
Research finds fewer tornadoes and hailstorms in the central United States during El Niño and more during La Niña, though the signal is noisy. Details.27
Benchmark
April 27, 2011. 199 tornadoes in 14 states in a single day, 316 deaths, and more than $4.2 billion in damage.28 It happened during La Niña.29

The Enhanced Fujita scale

Tornadoes are rated from the damage they leave. The wind speeds are estimates, not measurements.30

RatingEstimated 3-second gust
EF065 to 85 mph
EF186 to 110 mph
EF2111 to 135 mph
EF3136 to 165 mph
EF4166 to 200 mph
EF5Over 200 mph

Atmospheric rivers

Atmospheric rivers are long, narrow bands in the sky that carry most of the water vapor moving out of the tropics. An average one carries about as much water as the Mississippi River discharges at its mouth. An exceptionally strong one can carry up to 15 times that.31

Most are a good thing. Weak atmospheric rivers bring rain and snow that the water supply depends on, and NASA says they deliver 40% of California's annual water.3132 The strong ones cause floods. The "Pineapple Express" is a strong atmospheric river that carries moisture from near Hawaii to the U.S. West Coast.31

Scale
AR 1 to AR 5, based on how much water vapor is moving and for how long. Weak ones are mostly beneficial and strong ones mostly hazardous.33
El Niño link
Weaker than often claimed. NASA research found El Niño does not change how many reach California, typically ten a year, but makes them stronger, warmer and wetter. A 2025 study found atmospheric rivers can override ENSO altogether: in its co-author's words, they "don't dance to the tune of ENSO."3234
Benchmark
December 10-22, 2010. A series of atmospheric rivers brought 11 to 25 inches of rain to parts of the West Coast.31

Heat domes

The same wavy, stalled jet stream that delivers Arctic air in winter delivers extreme heat in summer. A heat dome forms when a persistent area of high pressure sits over a region and acts as a cap. Air sinks, compresses and warms, and heat builds at the surface for days to weeks.353624

Pattern behind it
A blocking high: a ridge in the jet stream that stops moving.24
Benchmark
Pacific Northwest, June 26-29, 2021. Portland, Oregon, set all-time records three days running: 108, 112 and then 116 °F. Lytton, British Columbia, reached 49.6 °C (121 °F), Canada's all-time record. The cause was a high-pressure system stronger than any observed in the region before.3738

Derechos

A derecho is a widespread, long-lived windstorm produced by a fast-moving line of thunderstorms.39

Threshold
A swath of wind damage at least 250 miles (about 400 km) long, with gusts of at least 58 mph along it and several well-separated gusts of 75 mph or more.39
Pattern behind it
Some are driven by strong cold fronts or jet stream disturbances. Others, mostly in summer, can arise with little large-scale forcing at all.39
Benchmark
The Midwest, August 10, 2020. A derecho traveled 770 miles in 14 hours, from South Dakota to Ohio, with widespread winds over 100 mph. It caused $11.0 billion in damage.40

Watch vs. warning

Be prepared

Watch

The hazard is possible in and near the watch area. Have a plan and stay tuned.41

Take action

Warning

The hazard is happening or about to. Act now.41

The same split applies to winter: a Winter Storm Watch means conditions are favorable for a significant storm, and an Extreme Cold Warning means dangerously cold conditions are expected or occurring. Each local office sets its own criteria.942

Cold and winter storm basics

From the National Weather Service:

  • Dress in layers and cover exposed skin. Frostbite can happen in minutes, starting with fingers, toes, nose and ears.43
  • Know hypothermia. A body temperature below 95 °F is hypothermia. It can happen at temperatures as warm as 60 °F, particularly in water or after a long time outside without the right clothing.43
  • If you are stranded in a vehicle, stay with it and wait for help. While running the engine, open a window a little to avoid carbon monoxide poisoning.944
  • Slow down. Roads that only look wet can still be slick.44
  • Go easy on the shoveling. The strain of cold and hard labor can cause a heart attack.44

This page is general education. For a hazard near you, follow your local forecast office and emergency officials.

Is a warming Arctic making all this worse?

This is one of the most argued-over questions in climate science, and it does not have a settled answer.

The hypothesis. The Arctic is warming faster than the mid-latitudes. In 2012, two researchers proposed that this shrinks the temperature contrast that drives the jet stream, weakening its winds and enlarging its waves. Slower waves would mean weather that stays put longer, and more of the extremes that come from stuck patterns: drought, flooding, cold spells and heat waves.45 A 2020 review found that observational studies "overwhelmingly support" a link between Arctic warming and colder continental winters.46

The rebuttals. Other researchers found that the reported trends were likely an artifact of how waviness was measured, and found no increase in blocking.47 A 2020 study found the trend toward a wavier circulation had reversed in recent years even though the Arctic kept warming, and that climate models given a weaker temperature contrast do not produce bigger waves.48 The same 2020 review that found support in observations noted that most modeling results show little connection.46

Where it stands. The Intergovernmental Panel on Climate Change rates this "low confidence" and calls it an example of "contrasting lines of evidence that cannot yet be reconciled." It adds a point that often gets lost: a warmer Arctic, with no change in the jet stream at all, reduces cold extremes in winter.49 NOAA scientists say the same in plainer terms: winters are warmer on average, and cold extremes are less likely than they were a century ago.2

Sources

  1. NOAA National Weather Service. Cold Weather Safety.
  2. NOAA Climate.gov (archived). Understanding the Arctic polar vortex, March 5, 2021.
  3. NOAA Climate.gov (archived). Climate Variability: Arctic Oscillation, August 30, 2009.
  4. NOAA National Weather Service. Wind Chill Chart.
  5. NOAA National Weather Service Glossary. Wind Chill.
  6. NOAA National Weather Service, Fort Worth. February 2021 winter event.
  7. NOAA Climate.gov (archived). 2021 U.S. billion-dollar weather and climate disasters in historical context.
  8. NOAA National Weather Service, Chicago. Record cold January 30-31, 2019.
  9. NOAA National Weather Service. Winter Storm Warnings, Watches and Advisories.
  10. NOAA. El Nino looms large over U.S. Winter Outlook, October 15, 2026.
  11. NOAA National Centers for Environmental Information. On This Day: The 1993 Storm of the Century.
  12. NOAA National Weather Service, Wilmington NC. Superstorm of 1993.
  13. NOAA National Weather Service. What is a Nor'easter?.
  14. Monthly Weather Review. Sanders and Gyakum, Synoptic-Dynamic Climatology of the “Bomb”, 1980.
  15. NOAA National Ocean Service. What is bombogenesis?.
  16. NOAA National Weather Service Glossary. Bomb.
  17. NOAA National Weather Service, Boulder. Bomb Cyclone of March 13th, 2019.
  18. NOAA National Weather Service. Lake Effect Snow.
  19. NOAA National Weather Service, Buffalo. Lake Effect Summary: November 17-19, 2014.
  20. NOAA National Weather Service, Burlington VT. 25th Anniversary of the Devastating 1998 Ice Storm in the Northeast.
  21. NOAA National Severe Storms Laboratory. Tornado Basics.
  22. NOAA Storm Prediction Center. The Online Tornado FAQ.
  23. NOAA National Severe Storms Laboratory. Tornado Types.
  24. NOAA National Weather Service, JetStream. Basic Wave Patterns.
  25. NOAA National Weather Service, Binghamton. Severe Weather Definitions.
  26. NOAA Education. Tornadoes.
  27. Nature Geoscience. Allen, Tippett and Sobel, Influence of the El Niño/Southern Oscillation on tornado and hail frequency in the United States, 2015.
  28. NOAA National Weather Service. Service Assessment: The Historic Tornadoes of April 2011.
  29. NOAA Climate.gov ENSO Blog (archived). ENSO and tornadoes, April 27, 2017.
  30. NOAA National Weather Service, Norman OK. The Enhanced Fujita Scale (EF Scale).
  31. NOAA. What are atmospheric rivers?.
  32. NASA Scientific Visualization Studio. AGU El Niño Press Conference Release Materials, December 15, 2015.
  33. Bulletin of the American Meteorological Society. Ralph et al., A Scale to Characterize the Strength and Impacts of Atmospheric Rivers, 2019.
  34. Scripps Institution of Oceanography. Atmospheric Rivers Explain Atypical El Niño and La Niña Years, February 13, 2025.
  35. NOAA National Weather Service, JetStream. Heat Index.
  36. NOAA NESDIS. Extreme Heat and Severe Weather Plague Parts of North America.
  37. NOAA Climate.gov (archived). Astounding heat obliterates all-time records across the Pacific Northwest and Western Canada in June 2021, June 30, 2021.
  38. World Meteorological Organization. June ends with exceptional heat, 2021.
  39. NOAA Storm Prediction Center. About Derechos.
  40. NOAA Climate.gov (archived). 2020 U.S. billion-dollar weather and climate disasters in historical context.
  41. NOAA National Weather Service. Tornado Watch vs. Warning.
  42. NOAA National Weather Service. Cold Weather Warnings, Watches and Advisories.
  43. NOAA National Weather Service. During Extremely Cold Weather.
  44. NOAA National Weather Service. During a Winter Storm.
  45. Geophysical Research Letters. Francis and Vavrus, Evidence linking Arctic amplification to extreme weather in mid-latitudes, 2012.
  46. Nature Climate Change. Cohen et al., Divergent consensuses on Arctic amplification influence on midlatitude severe winter weather, 2020.
  47. Geophysical Research Letters. Barnes, Revisiting the evidence linking Arctic amplification to extreme weather in midlatitudes, 2013.
  48. Science Advances. Blackport and Screen, Insignificant effect of Arctic amplification on the amplitude of midlatitude atmospheric waves, 2020.
  49. IPCC. Climate Change 2021: The Physical Science Basis, Chapter 10, Cross-Chapter Box 10.1, 2021.

Last updated November 12, 2026