We interrupt your spring-leaning inclinations to bring you this winter weather warning.
This image from CNN Weather illustrates the typical air and moisture flow in a nor’easter. This is just a screen capture of the video clip, so the “play button” does not work. However, they also did not animate this image in the forecast video. If they had, this storm would look very like a hurricane storm progression, but with ridiculously low temperatures. This image is, to my knowledge, illustrating the European forecast for this storm. The European model shows the storm center hitting precisely where it can draw up moisture and throw it onto land without bringing substantial warming and therefore rain rather than snow. The American models from the National Weather Service are showing a storm track further offshore so that very little of the coast will be affected. Right now, Weather Underground is following the American predictions — still bad for Boston and Providence, but not much happening either to the south or to the west. Here’s hoping they’re right!
Even so, Boston is probably in for a blizzard on Saturday.
So what is a Nor’easter?
The time around the Wolf Moon — late January into early February — is the coldest time of year in the North. This is likely to remain true even as we head deeper into climate change. At this point in the solar year, the northern hemisphere has experienced short days for many months. Stored surface heat has dwindled and can’t moderate temperature as effectively as it did in the long nights leading up to the solstice. This moderating effect is why we see a time lag between the length of daylight and the seasonal temperature peaks. It’s colder after the winter solstice; it’s warmer after the summer solstice. It takes a while for heat to build up on the surface in the long days of summer and then a while for it to dissipate again in the short days of winter. So it’s coldest around the Wolf Moon.
(Incidentally, the annual points of temperature maximum and minimum coincide with early August — Lughnasadh — and early February — Imbolg. I say “coincide” but there are no coincidences. The “Celtic” calendar has these temperature shifts — dubbed thermstices by these people —built into it. And you thought our ancient ancestors were clueless ninnies afraid that the sun wouldn’t rise.)
After the Wolf Moon, when the North has cooled down as much as it’s going to, Greenland glacier melt is at its minimum. The cold southward flow from Greenland does not inhibit warm northward flow from the Gulf so much in the late winter and early spring. That warm air and water flows up the East Coast and interacts with the Northern jet stream and any polar vortex air the jet stream might have picked up on its travels over the North American continent. Thus, this is prime time for the weather phenomenon we’ve named the nor’easter.
A nor’easter is a storm along the East Coast of North America, so called because the winds over the coastal area are often from the northeast due to the storm’s counterclockwise rotation. These storms may occur at any time of year but are most frequent and most violent between September and April. Nor’easters usually develop in the latitudes between Georgia and New Jersey within 100 miles of the East Coast. These storms travel generally northeastward and typically reach maximum intensity near New England (yay…) and the Maritime Provinces of Canada. They are nearly always associated with heavy precipitation — all kinds, often mixed up in a horrifying slurry — as well as gale force winds, rough seas, and coastal flooding.
Nor’easters are a product of winter air and ocean flow along the East Coast of North America. During winter, disruption of the circumpolar air flow sends cold Arctic air southward across the plains of Canada and the United States, where this cold air mass gets picked up by the jet stream and is pushed eastward toward the Atlantic. This cold and dry air current slams into the warm and wet air flow of the North Atlantic Current as it flows northward from the Gulf of Mexico. As you know, warm and wet mixed with cold of any moisture level will generate storms. It’s that difference between warm and wet air over the Atlantic and cold Arctic air flowing off the coast that is the fuel that feeds nor’easters.
The storm’s rotation and the resulting wind flow are simple geometry. Air flow from the northwest is slamming into air flow from the southwest, so about at a right angle. Both flows are fairly strong. Picture a jet of water from a hose gushing into a river. The eddies will swirl in loops running downstream (the river current) and looping back toward the hose. Downstream in this case is the air heading north with the North Atlantic current. The polar vortex flow is the hose. So the storm swirls in counterclockwise eddies that we on land experience as wind from the northeast. Out at sea, on the other side of the storm’s rotating center, the winds are flowing toward the northeast. Just to confuse things, occasionally, the storm center will wander far enough inland that land lubbers experience nor-easters with wind from the southwest (or any other direction). This is a problem of our human-centric naming though, not an inconsistency in the storm type.
All this is to say that February and March are likely to see increasing winter storm activity even as we head deeper into climate change. Bostonians still rightfully shudder at the winter of 2015. That sort of extreme winter weather straddling the end of winter and the beginning of spring will likely be a feature (and a bug) as the planet heats up.
So the next time your annoying uncle scoops up a snowball and claims that snow proves “global warming” is a whiney liberal hoax, you can toss all this right back at him. And tell him it’s not global warming — it’s climate change. Terms matter.
The National Weather Service. https://www.weather.gov/safety/winter-noreaster, accessed 6 February 2021 at 11:30am.
©Elizabeth Anker 2022