Historic Nor’Easter to Wallop New York, New England

Observations and model output over the last few hours continue to point toward a snowstorm/blizzard tonight and Tuesday that will affect tens of millions of Northeasterners with potential record snowfall, extremely high winds, and significant coastal flooding. Snow was quickly developing on Monday morning across the New York metro area and southern New England. Below is a summary of the potential impacts from this major weather event. We will provide frequent updates as the storm unfolds through a live blog created by Weather Underground’s Shaun Tanner.

Snow amounts: holding firm
Computer models continue to vary somewhat in the placement and intensity of this storm’s heaviest snowfall. However, the big picture has not changed appreciably. A strong upper-level wave of low pressure in the atmosphere moving toward the mid-Atlantic coast is the main driver of the impending blizzard. On Sunday, a fast-moving, intense line of thunderstorms developed over Mississippi and Alabama, bringing severe wind gusts (58-59 mph) at the Tuscaloosa and Birmingham airports. The fact that such strong thunderstorms could develop in relatively cool, dry air suggests how powerful this upper-level wave is.

As the wave pivots around a much larger upper-level low stationed in eastern Canada, it will generate a surface low off the mid-Atlantic coast that will rapidly intensify tonight and Tuesday. Together, these features will generate very strong dynamics and rapid snowfall rates, with the heaviest amounts expected along a broad swath from northern New Jersey to southern Maine. Throughout this region, a foot or more of snow is likely, and some areas could receive 24” or more. The placement of the overall swath of snow will be determined largely by where the upper-level and surface features evolve, and especially by how far east or west they are. While there is still some model uncertainty here, the surface low is expected to move very slowly while at peak intensity just southeast of Long Island and Cape Cod. The New York area is toward the southwest end of the snow swath, so the overall east-west placement of the overall storm will be vital to how much snow falls there.

Within the overall snow swath, the heaviest amounts will hinge on where persistent bands of snow (mesoscale bands) develop. It is still too soon to know exactly where these will take shape. In a Monday morning discussion, National Weather Service forecasters in Boston noted the possibility of parallel bands of very heavy snow, with an outer arc located inland (perhaps from central Massachusetts to interior Maine) and another band closer to Rhode Island and eastern Massachusetts, where a coastal front will separate milder marine air from the frigid inland air. Later today, the new High-Resolution Rapid Refresh model will provide increasingly detailed guidance on where the mesoscale bands may develop. The HRRR model is updated every hour using radar data and other current observations.

Here are some current ranges of snowfall being predicted in NWS local forecasts:

New York: 18 – 24”
Providence: 20 - 30”
Boston: 20 – 30”

These numbers suggest that the storm has a good shot of placing in the top-ten list for heaviest snow accumulations at all three of these cities. For a summary of top snowfall events at New York, Boston, and Providence, see our live blog post.

As indicated by the ranges above, no single number can capture the range of possibilities in a rapidly evolving, highly dynamic storm like this one. Another way to view the potential is through probabilistic maps, such as the experimental ones posted by NWS/Boston on its winter weather page. Drawing on model guidance, these maps indicate the likelihood of a given amount of snow in two ways:

--What are the least, most, and most probable accumulations one could expect?
--What is the likelihood of exceeding a certain accumulation?

In these experimental maps, virtually all of eastern Massachusetts and Rhode Island show a 100% chance of at least a foot of snow, and the odds are 80-90% that most of these areas will receive at least 18”.




Figure 1. Experimental guidance on potential accumulations from the unfolding nor'easter, posted on Monday morning, January 26, by the National Weather Service forecast office in Boston. The maps show the lowest 10% (top) and highest 10% (bottom) of potential accumulations that one might expect from this storm based on computer model guidance. Image credit: NWS/Boston.

A storm surge of 2 - 4 feet and waves up to 31 feet high
The blizzard will feature a powerful low-level jet stream of strong winds that will dip close to the surface over the offshore waters of New England on Tuesday, bringing sustained winds of 40 - 50 mph with gusts as high as 75 mph to the waters of Southeast Massachusetts, including Cape Cod, Martha’s Vineyard, and Nantucket. These winds will create huge waves 26 - 31 feet high along the east coast of Cape Cod, and 7 - 13 feet high along more sheltered parts of the coast. Ocean-facing barrier beaches of Long Island, New York will see waves of 7 to 12 feet. A storm surge of 2 - 4 feet is expected along the coast from Maryland to Maine. The combined action of the storm surge and waves will cause coastal flooding and severe beach erosion, and could cut new inlets through exposed east and northeast facing barrier beaches in Southeast Massachusetts. Moderate coastal flooding will occur at many coastal locations during the Tuesday morning and Tuesday afternoon high tide cycles in Southeast Massachusetts, with a few areas of major flooding possible during the Tuesday morning high tide. The difference in water levels at Boston between low and high tide is twelve feet, so a storm surge arriving at low tide will do little damage, while a storm surge arriving at high tide has the potential to cause significant damage. According to the Monday morning runs of the National Weather Service's experimental storm surge model for extratropical storms, the peak storm surge in Boston is predicted to occur about 2 - 3 hours after the 4:30 am EST Tuesday high tide. A storm surge of 3.0 - 3.5 feet is expected, which should cause mostly moderate flooding. However, if the timing of the storms’s strongest winds shifts just 2 - 3 hours earlier, the storm surge could arrive at high tide, causing widespread major flooding along the coast and considerable damage. Even in this worst case scenario, the coastal flooding in Southeast Massachusetts should not be as great as that of the great Blizzard of 1978.

Significant storm surges of 3 - 4 feet are also predicted to occur along the coasts of New York and Connecticut in Long Island Sound, causing mostly moderate coastal flooding. Monday morning storm surge model runs show a storm surge of about four feet hitting Bridgeport, Connecticut and Port Jefferson, New York near high tide on Tuesday morning.


Figure 2. Forecast wave heights for 10 am EST (15 UTC) Tuesday, January 27, 2015 from the 2 pm EST (18 UTC) Sunday January 25, 2015 run of NOAA's Wavewatch III model. Significant wave heights of 7 - 8 meters (23 - 26 feet, yellow green colors) are predicted for the waters just offshore from Boston, Massachusetts.

Bob Henson and Jeff Masters