Cold air waves do not come by white snowstorms or power lines collapsing. Their first signs appear silently on high-altitude weather maps, where twisted colored bands show a ice belt of air in the Arctic extending, accelerating and distorting in a way that makes many veteran forecasters confused when comparing with winters in the past.
More than 30km above the Earth's surface, a rare anomaly of the polar vortex is happening at a speed faster than anything that many climate scientists have witnessed in decades of winter data.
Polar vortex is a giant "tornado" of very cold air surrounding the two poles, mainly existing in the stratosphere, at an altitude of about 15-50km.
The US National Weather Service describes this as a large low pressure area containing icy air, often acting as a barrier to keep the Arctic cold "locked" near the pole, like the lid of a giant freezer.
Usually, extreme whirlpools rise and then weaken slowly seasonally, as energy waves from the low atmosphere spread upwards.
However, the phenomenon of "sudden stratospheric heating" can occur when the temperature at high altitudes spikes in a short period of time, weakening or pushing the polar vortex.
This season, data from forecasting centers in Europe and North America show an early event, appearing from the end of November. Temperatures at a gas pressure of about 10 hectopascals are already 30-40 degrees Celsius higher than average, while winds are almost at a standstill.
Instead of forming a neat circle on the Arctic, the polar vortex is stretched into eccentric "vessels" - a rare shape in meteorology textbooks a few decades ago.
When the polar vortex weakens or shifts, the Arctic cold air "lenches" may spill southward, bend the jet stream into deep turns and slow down the movement of weather systems.
Current forecasts show that polar vortex disruption may send consecutive cold air waves to North America, Europe and Asia in the coming weeks. Instead of a severe cold snap that ends neatly, meteorologists are concerned about the weather "snack" scenario: deep cold, short-term thaw, then temperature plunge - a phenomenon that recent research calls the "weather whip" in an increasingly "game-changing" winter.
For Vietnam, according to the weather forecast of the National Center for Hydro-Meteorological Forecasting, the cold air wave this weekend is moderate to strong, causing rain in many areas.
The Northern region from the night of February 6th - 9th, is forecast to have rain, showers and thunderstorms in some places. From February 7th, the weather will turn cold, especially in the period from February 8th - 9th, there is a possibility of severe cold, with dangerous cold in some mountainous areas.
The lowest temperature in the upcoming cold air wave is commonly 12 - 15 degrees C, the highest is commonly 15 - 18 degrees C.
Around February 11, the northern provinces may be affected by a wave of weak cold air intensifying to the east.
