Anatomy of a Nor’easter
Thermodynamic Solutions

Anatomy of a Nor’easter

What goes into this significant real-life weather event often sounds a lot like science fiction. TDS Meteorologist Beth Carpenter separates fact from fiction of this week's major winter event..

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December 18, 2020

If you live in the Mid-Atlantic or Northeast, you’re likely very familiar with the impacts that nor’easters bring to the region. However, many in the Midwest and Plains have never experienced such an event. What is a “nor’easter” and how does it form?

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The first step in the development of a Nor’easter is a buckling Polar Jet Stream. When the jet stream “buckles”, it pulls cold air equatorward into the United States and out over the Atlantic Ocean. Cold air then becomes situated over the warm Gulf Stream waters along the East Coast. The colliding air masses (cold, dry air from Canada and warm, moist air from the Atlantic) cause air to rise, and eventually aid in the development of a low pressure system. This low pressure system typically needs to form within 100 miles east or west of the East Coast for a nor’easter to develop.

Next, if upper level conditions are supportive, the low pressure system will continue to strengthen as it travels up the East Coast. As it does, the storm system will develop strong gale-force winds and pull moisture from the Atlantic into the northeastern United States. Because low pressure systems rotate in a counter-clockwise manner, the winds that impact the northeastern United States in these events come from the northeast- thus the name “Nor’easter”.

The constant flow of moisture from the ocean, combined with strong winds and ripe upper level conditions, bring the threat for flooding rains and/or heavy snowfall to the region. Heavily populated areas from Washington D.C. to New York City to Boston are typically impacted by these events- basically along the I-95 corridor. The impacts range from heavy precipitation, to high winds, rough seas, and coastal flooding. The proximity of the low pressure system to the coastline determines who gets these impacts and how intense they’ll be.

Nor’easters are most powerful from September through April due to the greater difference in air masses, but can happen anytime throughout the year. Significant Nor’easters that come to mind include the Blizzard of ’78 and more recently the January 2015 New England blizzard. In these events, 2-3 feet of snowfall was observed.

Nor’easters pose a significant challenge to meteorologists, even at less than 24 hours before the storm. The exact track of the low pressure system and proximity to the coastline have significant impacts on precipitation types, amounts, and locations. There’s typically also a very sharp rain/snow cutoff on the south side of the storm, and this area could also see icing. Temperature fluctuations of 1-2 degrees can cause the entire forecast to “bust” or “boom”. Narrow, heavy banding of precipitation can also develop with the support of mid-level frontogenesis and strong upper level energy, which can really add to snowfall totals. Weather models have a hard time picking up when and where this will occur.

The historic nor’easter that impacted the Mid-Atlantic and New England over the past 48 hours had all of these ingredients and more. The low pressure system at the surface was relatively weak for a nor’easter, but the supply of old air and upper level dynamics were very ripe for big snowfall- and that certainly verified! Widespread snowfall amounts of 10-18” occurred across the region, with higher amounts of 24-36” in a swath from north-central Pennsylvania to southern New York and into southern Vermont and New Hampshire. The “winner” for highest snowfall from the storm was Binghamton, New York, where several measurements of 40-44” were observed! At times, NWS employees measured snowfall rates of 6” per hour- an incredibly rare event. Note that the snowfall total image below only includes snowfall through 7am Thursday, and does not include the full event for some areas. Many daily and monthly snowfall records were broken.

CLICK HERE for a summary of the event by NWS Binghamton.

Beth Carpenter is Co-Owner and Meteorologist at Thermodynamic Solutions, and a frequent Snow Magazine contributor.