The jet stream, a river of air a few miles up in the atmosphere, controls weather in the middle latitudes. Like a river, the jet stream, in circling the globe from west to east, meanders. It also flows at different speeds. The meanders and the speed of the jet stream determine the weather near the ground.
The jet stream forms in response to the differential heating between pole and equator, which is greatest in the winter. This is when the jet stream is most frisky; summer tends to make it sluggish, like some people I know. The change can be quite abrupt, with blustery conditions replacing placid days in the fall.
The Current Pattern
For several weeks, a broad and fast-flowing trough, or dip, in the jet stream has covered the continental United States, bringing stormy weather to much of the country. The fast flow tends to support a strong temperature gradient — a large horizontal change in temperature — near the surface. Currently, daytime highs are struggling to get to zero in the northern plains, while Florida has daily highs in the 80s. This is a high potential energy state, and this potential energy can be transformed into kinetic energy of wind, accompanied by precipitation, at any time.
Ripples in the Jet Stream
The broad trough over the United States is considered a ‘long-wave’ feature. Typically between two and five of these waves circle the globe, and they tend to persist for days or weeks, sometimes even months, in the same location. Presently there are three major long-wave dips in the jet stream: One over the United States; one over the Middle East and central Asia which recently brought Jerusalem its heaviest snowfall in 50 years; and one in the Western Pacific Ocean.
Within these long-wave features, shorter waves ripple from west to east at around 20 miles per hour. The shorter waves, spanning about 1000 miles from west to east, cause individual storms. Some waves amplify and crest much like ocean waves as they splash along, while others subside. It is the bane of forecasters to try to determine when a short wave will amplify and suddenly become a dangerous storm.
Winter Storm Electra
Winter Storm Electra is now rippling through the long-wave trough over the United States. With the strong temperature gradient still in place, amplification is possible. However, the conditions that produced the freezing rain associated with storms Dion and Cleon have mostly disappeared. Electra’s precipitation will fall as rain or snow — a mix on the boundary — but except for a short time today in Missouri, not freezing rain.
Snow amounts will be appreciable (up to 8 inches) from Illinois through Ohio today, and from Ohio and Pennsylvania through central and northern New England on Saturday and Sunday. Up to a foot of snow is likely in some inland areas; however, the major cities along the eastern seaboard will be spared because the wind will be mainly from the east, off the (relatively) warm water of the Atlantic Ocean. The temperature will remain above freezing, and the precipitation will be all or mostly rain from Washington to Boston.
And Then? And Then? (as the Coasters Would Say)
None of the computer models suggests a change in the current pattern of a strong long-wave trough over the United States, so the conditions are in place for vigorous storms (Falco is next, followed by Gemini) for at least a couple of weeks. Right now, there are indications that an amplifying short wave will cross the United States the weekend before Christmas. Plan accordingly and listen to updated forecasts.