One question you might ask during this major storm is how and why did this form? If you saw a forecast as recently as Friday, meteorologists thought this storm would stay mostly out to sea, obviously that isn’t the case. Let me explain why the storm formed and then why it’s hitting and moving so slowly.

Storm formation
You have likely heard of the jet stream. This is the band of winds at 30,000 feet that moves storms and air masses across the middle latitudes of the planet where we live. This jet stream doesn’t blow in a straight line. Instead it has curves and loops and these change the way the air flows within this stream of air. Basic meteorology tells us that if the air up at 30,000 feet is spreading out and speeding up it in turn aids in pulling the air upward off the surface of the planet. We call these concepts diffluence and divergence and they are two major components to why winter storms form. That upward motion or lift brings the air from the ground higher and higher until it cools and forms clouds and eventually precipitation. The more lift you have, the bigger the storm.

When storms are formed and becoming more intense all the levels of the atmosphere work in tandem to help build the storm. The diagram below shows three such levels of the atmosphere. Notice how what’s occurring at the highest levels impacts what goes on below.

At 18,000 feet the atmosphere also has something called vorticity, or spin. This helps the storm to rotate and enhances the lift even further. The map below shows a very large amount of vorticity associated with our current nor’easter.

At around 5000 feet above the ground warm moist air is clashing with the cold air over New England. This clash creates additional lifting of the air and more snow. Sometimes the warm air at this level moves far inland and this is why the coast changes to rain. That won’t happen in this storm.

Here at the ground the air is rushing into the center of the storm from all directions and then turning counter clockwise. On a weather map, the L you see stands for Low pressure and it the center of the storm. The less air at the center the lower the pressure and the stronger the storm. This nor’easter is going to have a pressure around 976 millibars which is quite low. In comparison, Sandy was in the 940 millibar range, the lowest ever so far north. Average sea-level pressure is 1013 millibars.

The present storm storm is going to move very slowly because the steering currents are going to cut off and create a loop above the storm. That means the storm can’t move very far or moves in a circle for a period of time. During the Blizzard of 1978, the storm did a loop south of Nantucket and kept heavy snow in Boston for two days. In 1969 a storm became stalled under an upper level flow for so long it snowed for 100 hours without interruption.
This storm will not become stuck very long, but will take a full day to finally move far enough east to allow the snow to stop.

Last week, the computer models predicted the steering currents during this storm would take it far enough out to sea so the heaviest precipitation would remain offshore. On Friday, we began to see hints the present scenario might occur and by Saturday we knew this was going to be a big snow maker.

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