It has been clear since Friday, when Decoded Science named Winter Storm Xenia, that an unusual diffluence (outflow) at jet stream level would create a hurricane-strength low pressure center off the east coast of the United States on Tuesday and Wednesday.
The only question about this storm was, and still is, whether it will be close enough to the coast to bring heavy snow to any major metropolitan areas.
The likelihood of blizzard conditions on Cape Cod was, and still is, a virtual certainty, and the National Weather Service issued a blizzard watch on Sunday.
The Ingredients of Winter Storm Xenia
The juxtaposition of warm and cold air is a high potential energy state. With the polar vortex continuing to direct arctic air into the eastern United Sates, the potential energy for storms has persisted the entire winter.
Winter storms relieve the energy stress by redistributing air to a lower energy state and converting potential energy to the kinetic energy of wind. This time there is an added ingredient: diverging branches of the jet stream.
Similarity Between a Hurricane and an Extra-Tropical Storm
Though they have different energy sources, tropical and extra-tropical cyclones have something in common: they intensify when there is a diffluence of air in the upper levels of the storm. Readers are probably familiar with the clockwise outflow from a hurricane on satellite imagery. The same thing takes place in a developing mid-latitude low pressure system, but the visible evidence is lost in the general cloudiness associated with the storm.
Winter Storm Xenia’s Strong Diffluence
Two branches of the jet stream are converging in the eastern U.S. and diverging again off the coast. Below the zone of diffluence the pressure falls and low pressure forms. As the pressure falls, the pressure gradient (the difference in pressure from one location to another) increases, and pressure gradient is what causes wind.
It has been apparent since Friday that the divergence in the jet stream would persist and the storm would become very powerful.
Who Will Feel The Effects of Winter Storm Xenia?
The major cities of New York and Boston will be on the outer edge of Xenia. The exact track of the storm will determine how much wind and snow the cities receive. The most recent computer forecasts from the two most reliable sources show a small difference in the path of the storm, less than 100 miles. However, this is enough so that the forecasts provide guidance of 12 inches of snow in Boston from one model and an inch or two from the other.
What seems certain is that the winds will be very strong along the coast, probably in excess of 50 miles per hour over Cape Cod. Some flooding could occur, though the astronomical tide (the tide that would result from the moon’s and sun’s gravitational pull alone) is about average. A few days later and the tides would have been at least a foot higher.
Why Did The Weather Channel Wait So Long to Name Winter Storm Xenia?
Discretion may be the better part of valor — and weather forecasting. But there is a difference between discretion and procrastination, and the latter is rarely a good idea. The Weather Channel, which pioneered the naming of winter storms, has been beating the drums since last week and warning of a potentially powerful storm. Yet at this writing, only a day or so before the storm is expected to strike, the Weather Channel still has not named Xenia.
Xenia Will Have an Effect on the Coastline, Name or No Name
Xenia will bring some snow to the northeast, possibly large amounts to Cape Cod and Down East Maine. Along the coast, winds will reach gale force, with hurricane-force gusts possible offshore. In addition, blizzard conditions will likely develop on the Cape and downeast. Offshore interests all the way from Cape Hatteras to Nova Scotia should prepare for the very strong winds which will be caused with Xenia.