How and Why Nor’easters Form


Storm formation




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.

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 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.

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 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.


Tuesday’s 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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