From The Weather Channel:
When Will the Real Cold Air Return? It’s been a strange start to the winter season in the Lower 48.
Arguably the most significant cold air blast in the nation happened in late October and the first half of November, smashing all-time October records and more daily record lows into the Deep South in November. Since then, the cold has been rather weak. Over 100 daily record highs were tied or set Christmas Day in the Midwest and Plains, leaving some in the southern Great Lakes with a snowless Christmas.
We’re headed into the coldest time of year for many east of the Rockies. But the latest outlooks from The Weather Company, an IBM Business, and NOAA’s Climate Prediction Center suggest it may be difficult for sustained colder-than-average air to hold in the eastern half of the country in January.
Instead, colder air is expected to be most persistent in January from the Rockies westward. This could change in February, with the best chance of colder-than-average air across the northern tier.
Two areas of warm water, one in the northeast Pacific Ocean known colloquially as “The Blob” and another in the central Pacific southwest of Hawaii known as a Modoki El Niño, favor “a steady stream of cold air into Canada and parts of the northern U.S,” Dr. Todd Crawford, chief meteorologist with The Weather Company, said in a December outlook.
Another wild card for winter cold outbreaks is a sudden stratospheric warming (SSW) event that could weaken the polar vortex.
The stratosphere is a layer of the atmosphere about 6 to 30 miles above the ground – above the troposphere, where most of the weather with which you’re familiar occurs.
An SSW is when the stratosphere warms sharply – 50 degrees Fahrenheit or more in just a few days – miles above the Earth’s surface.
When this happens, the typically strong stratospheric polar vortex that keeps the coldest air fenced in over the Arctic weakens, the jet stream becomes more blocked-up with sharp, southward meanders, and more persistent cold air plunges deep into the Lower 48.
One such event triggered the coldest outbreak since the 1990s in parts of the Midwest late last January.
For now, there is no such SSW event on the horizon, but we can’t rule it out in February or March.
