During the winter we often see a struggle between two pieces of jet stream energy; the northern branch, or polar jet, and the southern branch, known as the subtropical jet. Typically, one of these is more active than the other and dominates the pattern for a time while the other takes a back seat. But, in between the change from dominance in one branch to another, there are occasions when both streams are active, causing unusual patterns and storm tracks. That shift appears to be taking place during the next seven-to-10 days.
The southern jet has been much more active during the last couple of months as the storm track has generally been over the southern United States. Storms moving into the Pacific Northwest would dive into the Central or Southern Plains and hook through the East Coast, producing moderate to heavy precipitation along the storm track. This jet looks to remain active as it produces yet another system across the Gulf of Mexico next week.
Forecast models indicate that the northern stream will become more active during the next 10 days. Our friend the polar vortex remains weak and susceptible to bringing Arctic air south from the North Pole. It finally appears that it will tap into some colder air around the pole and bring it southward, but not until late next week.
In the meantime, the northern jet will remain active. A couple of storm systems will move through the northern tier of the U.S. during the next week. One of these systems may produce moderate snowfall and strong winds across portions of the Northern Plains and northern Midwest Jan. 13-15. More isolated showers are expected until the end of next week.
The models will have a difficult time piecing together storm systems as the two jet streams duel each other. But when we hit the end of next week, that all looks to change as the northern stream takes over.
This is when the Arctic air will really start to move into at least the northwestern half of the contiguous U.S. A large temperature gradient will set up around the middle of the country as a ridge of high pressure develops in the Southeast. Cold Arctic air will set up in the Northwest while warm tropical air builds over the Southeast.
This will look similar to the winter of 2019-20 when we saw many systems ride along this gradient. This could produce moderate precipitation to the east of the Rockies and put some dents into the ongoing drought. However, the chances will be best east of the Mississippi River as we saw last year, where drought does not exist in any significant sense.
This general pattern of a trough Northwest-ridge Southeast is indicated for the end of January and at least the front half of February on extended versions of the European, American and Canadian models, leading to a high likelihood and confidence in the overall pattern.
Pushes of colder Arctic air would move over the northern half of the country while the south would stay mostly warm. Precipitation will be more variable and heavier near the largest temperature gradient. In a real sense, this is what La Nina typically looks like for wintertime. After months of being in a La Nina, the pattern is finally beginning to take shape. How long it lasts, as La Nina weakens through spring, will be a major question going forward.
John Baranick can be reached at firstname.lastname@example.org
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