Several cold fronts have affected the United States and Canada this winter, and an additional cold front is forecast in early to mid-January.
While the timing and intensity of the new cold front is unclear, there are signs that the atmosphere is changing, and cold weather will spread across the United States and southern Canada.
Weather is a global phenomenon, and events in other parts of the world can affect the weather in North America. One such factor is the La Nina phenomenon, which is developing in the Pacific Ocean.
Although La Nina has been forecast for months, it has not yet fully developed. Although sea surface temperatures near the equator east of the Date Line are lower than normal, they are not low enough to be considered a full-blown La Nina condition.
However, temperatures have dropped rapidly over the past few weeks. Ocean waves bringing cool water up from below, combined with strong trade winds pushing warmer water westward, caused sea surface temperatures to drop below La Nina thresholds on December 16, according to a report from the Climate Prediction Center of the National Oceanic and Atmospheric Administration (NOAA).
At the same time, a series of storms moving along the equator will change weather patterns. These storms, called the Madden-Julien Oscillation (MJO), will change direction and energize the atmosphere, changing air currents. The MJO will move across the Americas, Africa, and back into the Indian Ocean in early January.
When the MJO is active in the Northern Hemisphere winter, it typically creates an atmospheric trough around Alaska and northwestern Canada, bringing cold air from the Arctic. The strength and location of this trough will determine the cold snap that will come.
Another important factor is the emergence of a high-pressure dome moving into the Himalayas and Tibet, called the East-Asian Mountain Torque event (EAMT), which will strengthen the cold trough over western North America and Alaska, assisting the MJO in creating this cold spell.
A storm is expected to bring stronger cold air, and models show the cold snap will hit on New Year's Eve or early New Year's Day.
What’s particularly noteworthy is that this cold snap is not linked to a disruption of the polar vortex, a phenomenon that often causes prolonged cold weather in many areas such as Asia, Europe, and North America. If this cold snap were not affected by the polar vortex, the weather would likely be colder, and the cold snap in January would likely be worse.
The La Nina phenomenon has a clear impact on the weather around the world, including Vietnam. La Nina causes cold air from Siberia and the Asian continent to strengthen and move southward. As a result, cold air waves flow into Vietnam more strongly and more frequently, causing temperatures to drop significantly, especially in the northern region.
During La Nina years, winter in Vietnam often lasts longer with more severe cold spells occurring.
