Snow may just be snow to those affected by winter storms Garbanzo Bean (last weekend) and Horseradish (this weekend). But the mechanisms that produce the snow are as different as fruits and roots.
All Precipitation Comes From Rising Air
Meteorologists have a hard time predicting precipitation. Not to make excuses, but there is a reason: Vertical motion in the atmosphere (rising air) is what produces rain and snow, and that motion is tiny compared to the horizontal wind. An updraft of a fraction of a mile per hour can release a lot of moisture. Such tiny motions are hard to predict.
The atmosphere can only hold a limited amount of water vapor; the amount depends on the temperature. Warm air can hold more water vapor; cold air can hold less.
Pressure decreases as you go up in the atmosphere, simply because there’s less air above pushing down. So when air rises, the pressure decreases, and since temperature is directly related to pressure by the ideal gas law, the temperature decreases.
Simply put: Rising air cools. As it cools, it can hold less water vapor. When rising air cools to the point at which it can no longer hold all the water vapor present, the water vapor begins to condense: A cloud forms. As the air continues to rise and water droplets (or ice crystals) coalesce, they become heavy enough to fall as rain or snow.
What Makes Air Rise?
Ah! Now we come to the root and fruit of all weather.
If one were to write down the equations that govern the atmosphere (don’t worry , one isn’t going to), one could derive an equation for the vertical motion. The two main components are called vorticity and warm advection.
The fruit (Garbanzo Bean) primarily got its updrafts from vorticity. This sinister sounding word means, approximately, spin: In the northern hemisphere, counterclockwise is positive and clockwise is negative. Positive vorticity is correlated with rising air.
Last week’s winter Storm Garbanzo Bean was associated with a tight swirl in the atmosphere at and some distance above the ground. That swirl translated to positive vorticity and a blinding blizzard.
This week’s storm, Horseradish, is a different sort of weather phenomenon. Horseradish will get its uplift from warm advection. This term doesn’t sound as sinister, but it can produce nearly as much precipitation as vorticity. Warm advection is simply warm air advancing via wind to where the air is colder. Cold air is denser and holds its ground, so the warm air must rise up over the cold.
The upper level flow connected with Horseradish shows very little spin; the snow will come from warm air advection.
The Battleground States
No, we’re not talking about the contested electoral votes in a presidential election. A swath of states from the midwest to the Atlantic coast will be in the zone where cold and warm air are battling for supremacy. The cold air is reluctant to move and warm air will be persistent in trying to push it away. The result will be a narrow band of relentless snow stretching from Nebraska and Iowa to the east coast.
Battleground states will include Illinois, Wisconsin, Michigan, Indiana, Pennsylvania, New York, and all of New England.
When the storm reaches the east coast, the snowfall will become heavier as moisture from the Atlantic Ocean enters the system. In addition, a low pressure center may form on the coast, so that some lifting due to vorticity will exacerbate the upward push of warm advection.
Another foot or more of snow is not out of the question for Boston.
A Kinder, Gentler — But Heavier — Snow
Despite the National Weather Service’s posting of blizzard warnings for Chicago, Horseradish is a relatively tame storm, lacking the wind-whipped fury of Garbanzo Bean. Under the new definition of blizzard, winds off Lake Michigan may briefly produce the required frequent 35 mile-per-hour gusts, but they will pale in comparison with Garbanzo’s hurricane-force winds.
Since the temperature will be relatively warm, the water content will be high and the snow heavy. As opposed to Garbanzo Bean, which featured temperatures generally below 20 degrees and a snow-to-melted-precipitation ratio of 15-to-one, Horseradish’s temperatures will be near freezing and its snow-to-water ratio will be closer to ten-to-one. The snow will be dense, heavy, and hard to shovel.
The Horseradish Timeline
The heaviest snow is now falling in Iowa and Illinois, and will spread eastward during the day. Though today’s storminess will have minimum impact on Super Bowl partiers, the snow will undoubtedly affect Monday morning rush hours in New York, Boston, and Hartford. Washington and Philadelphia will avoid the snow entirely, and New York will get a break as the precipitation will fall partly as rain.
By Monday night the snow should be confined to Down East Maine and residents can dig out under clear skies but plummeting temperatures on Tuesday.