Why Do Nor'easters Cause Such Bad Flooding?

A boston firefighter wading through floodwaters in the aftermath of the ‘bomb cyclone’ on January 4, 2018
A boston firefighter wading through floodwaters in the aftermath of the ‘bomb cyclone’ on January 4, 2018
Photo: Michael Dwyer (AP)

The East Coast is bracing for another nor’easter this week, with the National Weather Service in Boston warning that the city and nearby coastlines could see three to four feet of storm surge flooding. This, in the same winter that the bomb cyclone turned many of Boston’s streets into frozen soup.


Is it the latest sign of the end times? Not really. Winter storms like nor’easters, it turns out, routinely deliver worse flooding than hurricanes to this part of the world, a reality confirmed by a study published earlier this year, the Washington Post’s Capital Weather Gang noted today.

Why’s that, you might ask? The reason has everything to do with the difference in how nor’easters and hurricanes form—a difference that allows nor’easters to maintain hurricane-like central pressures and rip-roaring winds at latitudes where their tropical cousins typically fizzle out.

“By the time a hurricane gets off New England, it’s over very cold water and starts to decay,” Jeff Masters, meteorologist and blogger with Weather Underground, told Earther.

Hurricanes get their strength from oceanic heat, forming in warm equatorial regions where there’s plenty of evaporating water to condense into storm clouds. Nor’easters get their energy from an entirely different source: as Masters put it, the contrast in density between warm, moist air from the ocean and cold, dry air from the land. When these contrasting air masses collide, the cold dry air sinks, and releases energy. The technical term for the atmospheric set-up that drives the formation of a nor’easter is ‘baroclinic instability.’

Unlike hurricanes, which form and thrive in tropical waters and feature a warm core, nor’easters form at mid-latitudes, and their cores are cold. Because they don’t rely on ocean heat for their energy, they can maintain hurricane-like central pressures and wind strengths at latitudes where hurricanes struggle to.


This can lead to some very bad flooding.

“Occasionally you get a strong hurricane up there [in the northeast], but they’re pretty uncommon,” Masters said. “It’s the big ‘nor’easters that dominate the record storm surge,” for the region.

Models are calling for this week’s nor’easter to deliver damaging winds, heavy rain, and snowfall to the Eastern seaboard beginning on Thursday night, although the storm’s exact track remains uncertain, the Weather Channel reports. A coastal flood watch is in effect from Friday morning through Saturday afternoon for eastern Massachusetts, with the NWS warning that “moderate to major” coastal flooding that could inundate roads and basements and tear up beaches. Coastal flood watches are also in effect for much of Delaware and New Jersey, and parts of Queens and Long Island.


Exacerbating the flood risk is the fact that this storm is due to coincide with the astronomical high tide—the full moon rises tomorrow. When the Sun, Moon and Earth are perfectly aligned, the gravitational tug-of-war causes the most extreme tides of the month. The coastal flood risk is expected to persist over three high tide cycles, thanks to the long-lived nature of this storm.


“All three tides are expected to be in the top ten on record [for Boston], maybe in the top five,” Masters said said.

Lastly, just in case you thought we were going to get through an entire weather post without mentioning climate change, you should know that for nor’easters as for hurricanes, sea level rise is adding inches to storm surge and making the coastal flooding threat worse. A recent study in Nature found that sea levels along the northern half of the Eastern seaboard have risen about an inch per decade since 1980, three to four times the global average rate.


“That makes it a lot easier to get a record-high water level causing substantial damage,” Masters said.

[h/t Washington Post]


Maddie Stone is a freelancer based in Philadelphia.


Dense non aqueous phase liquid

Man, I’m obsessing on baroclinic instability. Mark that under, “Huh, I did not know that.” Maddie’s link was nicely informative. Then there’s this deep dive from Caltech: “Large-Scale Inhomogeneous Thermodynamics: And Application for Atmospheric Energetics” Talk about a dog trying to learn algebra, here.

We could have Scott Pruitt and some of Trump’s greatest climate science minds read it to us. Slowly. Kind of like story time in grade school, where we sat indian style around the teacher.

My niece lives in Beacon Hill. I’ll warn her of the forthcoming instability of the large-scale kind - other than her uncle’s usual silliness.