Weather Wisdom

How and Why Nor'easters Form

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 isnít the case. Let me explain why the storm formed and then why itís hitting and moving so slowly.

Storm formation
It's important to keep this in mind when thinking about a storm. It's all about nature's attempt to bring balance to the atmosphere. Storms try to blend cold and warm air together to make them even. Once this happens, a storm will weaken and die. As the storm is forming the contrast in air masses is at it's most intense. We are seeing nature's attempt to bring warm air north and cold air south, the result is an intense coastal storm, we call a nor'easter.


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 doesnít 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 whatís 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 noríeaster.

500mb 12615.png

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 wonít 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 noríeaster 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.

low pressure east of areas125.png

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. The means the storm canít 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.


Tuesdayís 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.

Predicting the state of each of these levels of the atmosphere is still a challenge. While it's certainly much better, we will have lots of room for improvement.

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