The Weather Channel has named the 19th storm of the winter season. Though it is handy to have the names for reference, the criteria are vague and subjective; this storm will be different from its predecessors.
The Polar Vortex and the Jet Stream
The story of this winter has been the southward displacement of the polar vortex and a related powerful jet stream across the United States. The result has been a battleground between warm Gulf of Mexico air masses and cold ones from Canada.
Any ripple in the jet stream could set off a snow and ice storm on the boundary of the air masses, and several have developed into strong nor’easters. With the temporary retreat of the jet stream to the north, accompanied by a flow of warm air into the Great Plains and midwest, the snowy action temporarily shifts to the northern tier of states and Canada – but there will be a southern component of Seneca that could bring violent weather to the south and midwest.
Winter Storm Seneca: A Very Powerful Low Pressure Center
The forecast calls for Seneca to rapidly develop a deep low pressure center. Since pressure gradient (change of pressure with distance) creates wind, Seneca will produce winds up to hurricane force in some places. Where the precipitation falls as snow, blizzard conditions will develop.
The Spring-like Aspect of Seneca
Seneca’s circulation is drawing warm, humid air northward into the Mississippi and Ohio River Valleys. At the same time a drier flow from the plains will move eastward at several thousand feet elevation. This combination produces thunderstorm and tornado activity. Dry air above humid air is a potentially unstable mix; daytime heating or the lifting provided by a front can set violent weather in motion.
What Areas Will Seneca Impact?
Snow should be confined to Nebraska, Minnesota, Wisconsin, UP Michigan and Iowa. Snow totals will likely be less than 6 inches in most areas — higher in the Arrowhead of Minnesota and northern Wisconsin — but drifting caused by the wind will make it seem like more. As the low pressure system deepens and the wind picks up, the weather will meet the technical definition of a blizzard: wind in excess of 35 miles per hour; visibility below one-quarter mile in snow or blowing snow.
Flooding will be a concern in some areas that receive rain, especially those with melting snow cover. At this time of year, the frozen ground cannot absorb any water; several inches of rain combined with several inches of water-equivalent melted snow could be more than local streams can handle. The northern parts of Illinois, Indiana, and Ohio appear to be most susceptible to flooding.
More significant, perhaps, will be the threat of heavy thunderstorms and tornadoes in a wide area moving from west to east through Friday. Currently there are thunderstorms in the Chicago area associated with Seneca’s warm front. Later today, the activity will shift south to the Mississippi and Ohio River valleys as Seneca’s cold front sweeps through, reaching the Carolina coasts Friday afternoon.
Some Normally Snowy Places Catch a Break
As the storm passes into Canada, winds across the Great Lakes will shift to the west. Earlier in the winter, this would have been the setup for heavy lake-effect snow. But the lakes are almost entirely frozen over, so lake-effect totals will be on the order of inches rather than feet.
Can We Expect More Senecas?
The jet stream weather pattern is changing back to the January flow of a strong trough in the central and eastern United States. Any thunderstorm and tornado activity will be depressed to the south for the next couple of weeks. The new storm pattern favors Alberta Clippers, a series of which will move through the plains and midwest, and one or more of which could blossom into nor’easters with yet another bout of heavy snow for the major cities of the east coast.
Spring inevitably brings a retreat of the polar vortex and a relaxation of the jet stream — but not yet.
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