Residents of New England will endure another snow event this weekend and beyond, adding additional accumulations of a foot or more to the already several feet on the ground. Boston is again in the crosshairs of the maximum snowfall target.
The Weather Channel has named the event Winter Storm Marcus, but this stretches definitions, for despite the fact that it is snowing over an elongated stretch of real estate, there is as yet no low pressure center.
Decoded Science is naming this Extended Snow Event Cayenne.
Cayenne, another case of lifting by warm air advection like last week’s Super Bowl Sunday Storm Horseradish, will cover a long, rather narrow span from the midwest to the northeast.
Reviewing Why It Snows
The ingredients of a significant snow event are simple:
- Moisture. Enough water vapor in the atmosphere to provide the essential — basically the only — ingredient of snow.
- Upward vertical motion. Enough to force the atmosphere to release moisture as it cools.
- A long enough period of snowfall to allow accumulation.
The Atmospheric Setup For Cayenne
The jet stream is a reflection in the middle levels of the atmosphere of the battle on the ground between warm and cold air masses.
The earth radiates heat in the microwave portion of the electromagnetic spectrum. With some variation, the outgoing radiation is relatively uniform from equator to pole.
The incoming solar radiation, primarily in the visual range of the spectrum, varies with the angle at which the sun’s rays strike the earth. At low latitudes, the sun is direct and hot; at high latitudes the sun is oblique and weak. In the winter, there is no sun at all in far northern latitudes.
The warm air from the tropics and subtropics meets the cold air from the higher latitudes in the middle latitudes; the contrast is especially sharp in the winter and in North America the battleground can be anywhere from Florida to central Canada.
The axis of the jet stream is aligned with the zone of maximum temperature contrast, and currently this zone extends from Wisconsin to Massachusetts.
The Gulf Of Mexico And Atlantic Ocean Provide Water Vapor
Very humid air from the Gulf of Mexico frequently invades the eastern United States, and when it collides with the southward moving cold air the precipitation can be significant. When storm systems reach the coast, the Atlantic Ocean is an additional source of moisture. Just an inch of melted precipitation — a summer thunderstorm’s worth — converts to a foot of snow.
Rising Air Causes Precipitation
The small vertical motions in the atmosphere are responsible for precipitation. Whereas the horizontal motion — what we call wind — tends to blow in the range of 10 miles per hour, the vertical motion is normally only a fraction of a mile per hour. Though the process is simple, measuring and forecasting the vertical motion is not.
What Causes The Air To Rise?
Air generally rises on a large scale for one of three reasons:
- It encounters a change in land height (such as a mountain).
- It encounters denser air at the surface which it cannot dislodge.
- It participates in a process that reduces the potential energy of the atmosphere.
Though the snowfall will certainly be enhanced as it falls on higher elevations of the Adirondacks and other mountain areas, upslope is not the primary driver of rising air in Cayenne.
The battle lines between warm and cold air have been drawn through the midwest, extending eastward through Pennsylvania, New York, and New England. As the cold air stubbornly holds its ground, and the warm air determinedly advances, a long-duration overrunning event is taking place. With little change in the dynamics for several days, the snow will continue to fall.
Eventually A Low Pressure Center Will Intensify
Currently, the only low pressure is over Wisconsin, and barely discernible at that. However, the continuous production of potential energy from heating at the equator and cooling at the poles demands a release. As the low pressure in Wisconsin, associated with a minor ripple in the jet stream, moves east, it will eventually begin to develop into a cyclonically rotating mass of air. Cyclonic rotation (vorticity) is correlated with ascending air (the math physics are complicated), so as the low deepens, the vorticity will exacerbate the overrunning. Much of New England is likely to feel both effects and get the largest amount of snow.
The Southern Boundary Of Cayenne
Parts of Pennsylvania and New York will be out of the zone of maximum snowfall, but will still face difficult winter weather conditions. While the atmosphere is below freezing at all levels in the area affected by Cayenne, near the 32 degree line the warm air riding over the cold may be above freezing. In this case, the falling snow melts in the warm layer and re-freezes when it hits the ground.
This is a continuation of Dangerous Weather Event Fennel, which is ongoing and will last until the polar vortex relaxes its grip on parts of the eastern United States.
Will It Snow More In Boston?
By the time Cayenne is done and departed on Tuesday, Boston will have added another foot and more to its total of several feet remaining on the ground from storms Horseradish and Garbanzo Bean.
Boston will go into the deep freeze this week, with record temperatures possible, and though the phrase is often ridiculed, there is some truth to “too cold to snow.”