We’re just about through with our three-fer, the trilogy of winter storms Pax, Quintus, and Rex.
The jet stream flow is changing temporarily to allow warm air penetration into the midwest and northeast, and the next low pressure system should produce mostly rain, with severe storms a possibility over a wide swath of the Mississippi Valley.
The forecast includes some snow for the upper midwest on Thursday, but this is likely to be more of a wind event as the low pressure deepens over Canada.
After that, global forecast models suggest a return to the early winter pattern of snowstorms in the midwest and east, and dry weather on the west coast.
Long Waves in the Jet Stream
The jet stream averages a west to east motion, but within this motion there are troughs and ridges. Theoretically any length wave is allowable, but in practice certain wavelengths dominate. The long wave global pattern normally has four or five troughs which move very slowly, and are frequently anchored by mountain ranges.
In the winter, a trough tends to develop over the central United States in the lee of the Rocky Mountains. When this trough becomes pronounced, as it has been this winter, cold air spills far to the south. Storms form where the cold air from Canada meets warm and humid air from the Gulf of Mexico.
Short Waves in the Jet Stream
Within the long wave flow of the jet stream, shorter waves ripple along, generally moving at around 20 miles per hour. These short wave troughs provide the energy and spin needed for cyclone development. The typical wavelength is about 3,000 miles, so storms normally arrive at six or seven-day intervals. Last week’s storms had shorter wavelengths and traveled faster, so the time between storms was only a couple of days.
The Jet Stream in the Last Two Weeks
After storm Maximus, the pattern became more zonal, that is the long wave flow was pretty much west to east, without a pronounced trough-and-ridge pattern. The ridge that brought unusually warm and dry weather to the west broke down and storms hit the west coast; much-needed rain fell as far south as southern California.
Forecasters followed storm Nika from the west coast across the Rockies, into the midwest and across the east coast out to sea. Now, however, the pattern is changing again. The ridge of early winter is returning and eventually this will help establish a trough in the middle of the country. This fold in the jet stream will create a northwest flow from Canada that should reach deep into the southern states.
The Jet Stream Around The World
The accompanying jet stream forecast for February 25 shows a well-established ridge on the west coast, while the downstream trough is in the midwest. This pattern produces Alberta Clippers, which can drop snow on the midwest and blossom into nor’easters when they reach the coast. At this time of year, the lake effect snow that accompanies Clippers is reduced because the Great Lakes are largely frozen over.
A powerful jet stream also can be observed traversing the Atlantic Ocean. This is the pattern that has brought so much rain to the United Kingdom, and more can be expected.
Will Spring Ever Come?
Well, it does every year, except when an asteroid hits the earth or a massive volcano erupts. The south and parts of the midwest, plains and northeast will get a taste of spring this week. But Punxsutawney Phil forecasted six more weeks of winter, and the jet stream appears determined to make a prescient prognosticator out of him.