There is an old weather proverb that says, “A year of snow, a year of plenty.” Rooted in agriculture, it refers to the traditional process of cold winters providing snow cover for fields that prevent crops from sprouting too early. That snow then melts in the spring and provides moisture for a successful growing season.
As we now know, this winter has been anything but traditional with most of the United States experiencing uncommonly dry and mild conditions. In fact, today the U.S. Drought Monitor showed that 38% of the country is experiencing moderate drought. This is up from 28% in December. Some of the states most seriously affected by the lack of precipitation include, Texas, Georgia, Florida, and parts of California.
One of the reasons for the quiet weather this winter is the northern position of the Jet Stream. Also known as the “storm track”, the Jet Stream usually dips south in the winter bringing cold air and snowstorms to the continental U.S. The current La Niña episode, however, has pushed the Jet Stream north keeping most of the lower forty-eight states warmer and drier than normal.
Forecasters are predicting an end to this pattern in the spring when La Niña is expected to subside.
The local forecast is calling for a big dip in the jet stream this weekend. This will drive cold air south over New York City and send temperatures plummeting from their above average readings. You may wonder, what is this meteorological phenomenon that is bringing winter back to the city so abruptly?
Jet streams are bands of strong winds in the upper atmosphere that mark the location of the strongest temperature contrast at the surface. They are born out of the complex interactions among a variety of atmospheric conditions, including the position of warm and cold air masses, and the location of areas of high and low pressure.
In the U.S., the polar jet stream, one of four major global jets, marks the divide between cold arctic air and mild mid-latitude air. Its position is a function of temperature contrasts and therefore shifts throughout the year, depending on the season. In the warm summer months the jet is typically located around 50° to 60°N latitude. In the winter, when temperature contrasts are increased, it usually shifts to the south. It has been known to plunge as far south as 30°N latitude.
This winter, however, the jet stream has been staying well to the north with only a few arctic outbreaks so far. When they do come though, the sudden drop in temperature can be very jarring.
Average seasonal positions of the Polar Jet Stream in the northern hemisphere.
Image Credit: ncsu.edu