A row of homes in Scituate, Massachusetts, is surrounded by high-tide water at midday on Saturday, 4 March 2018. Photo: Ralph Karl Swenson, Amateur Radio SKYWARN Spotter (N1YHS) / NWS Taunton Skywarn

By Bob Henson
5 March 2018
(Weather Underground) – The rugged coast of New England has never recorded a one-two high-water punch like it’s gotten this winter with the nor’easters dubbed Grayson (4 January 2018) and Riley (2-3 March 2018). These storms produced two of the three highest water levels ever measured in Boston Harbor, and both of them produced widespread damage along the Massachusetts coast, with many water rescues carried out. Nearly a million people along the East Coast remained without power on Monday, reported weather.com.At least two more nor’easters are in the pipeline for New England, one later this week and another early next week. Neither of these should be on par with Grayson and Riley in their coastal effects, but they will prolong the misery and delay recovery efforts for thousands of residents along and near the shore.In the longer range, there’s a more ominous outlook. Sea level is expected to rise even faster along the Northeast U.S. coast than in most places around the world, thanks in large part to effects related to a weakening Gulf Stream. The renowned ferocity of nor’easters will thus play out atop a progressively rising sea surface, making coastal impacts progressively worse unless adaptation efforts can keep pace.

Two blockbuster events in the space of two months

In records going back to 1921, here’s how the two big nor’easters of 2018 rank in terms of water levels in Boston Harbor:

  • Grayson: 4.88’ above MHHW (mean higher high water), highest on record; previous record 4.82’ on 7 February 1978 during the infamous Blizzard of ’78
  • Riley:  4.4’ above MHHW, third highest on record, behind only the Blizzard of ’78 and ahead of the 3.92’ observed on 2 January 1987.

As shown in Figure 1 below, water levels on par with Grayson would be expected only about once every 100 years in Boston Harbor in today’s climate. The odds of getting two storms of the magnitude of Riley and Grayson in the same year are in the ballpark of several thousand to one—if we assume that the climate is not changing. Climate change makes such events more likely, though, through rising sea levels. Whether or not climate change makes intense nor’easters like Riley and Grayson more common is uncertain, as we discuss below.

Extreme water levels in Boston Harbor since 1921. Added on the right-hand side are the water levels achieved by Winter Storm Grayson in January 2018 (1.49 meters) and Winter Storm Riley in March 2018 (1.34 meters). The plots show the monthly highest and lowest water levels with the 1%, 10%, 50%, and 99% annual exceedance probability levels in red, orange, green, and blue. The plotted values are in meters relative to the Mean Higher High Water (MHHW) or Mean Lower Low Water (MLLW) datums established by CO-OPS (1 foot = 0.3 meters). On average, the 1% level (red) will be exceeded in only one year per century, the 10% level (orange) will be exceeded in ten years per century, and the 50% level (green) will be exceeded in fifty years per century. The 99% level (blue) will be exceeded in all but one year per century, although it could be exceeded more than once in other years. Graphic: NOAA Tides and Currents

The record set by Grayson would not have been reached without the help of long-term sea level rise. Water levels along the Massachusetts coast increased by roughly an inch per decade over the 20th century, almost twice the global average rate of about 0.54 inches per decade. Subsidence is exacerbating global sea level rise along many parts of the U.S. Atlantic coast. Since the 1990s, global sea level rise has been accelerating; it’s now running at around 1.2 inches per decade.The 2017 report, Global and Regional Sea Level Rise Scenarios for the United States, had this to say about the future of sea level rise along the Northeast U.S. coast: “From Virginia through Maine and along the western Gulf of Mexico, sea-level rise is projected to be greater than the global average in nearly all global average sea level rise scenarios. For example, the sea in these regions would rise 1 to 1.6 feet higher than the global average rise of 3.3 feet under the intermediate scenario by 2100.” […]

Why the Gulf Stream matters to Massachusetts

The Gulf Stream plays a surprisingly large role in modulating sea level. As it wends its way northward, arcing away from the U.S. coast well before it reaches Massachusetts, the Gulf Stream separates colder waters along the Northeast U.S. coast from warmer water further offshore. It’s not unlike a strong atmospheric jet stream separating colder air near the poles and warmer air near the tropics. […]“The storms we’re seeing now, people thought this was decades in the future,’’ Emily Norton, who directs the Sierra Club’s Massachusetts Chapter, told the Boston Globe after Riley struck. [more]

A Weaker Gulf Stream Means Trouble for Coastal New England