The center of the storm responsible for today’s weather is currently near the Outer Banks of North Carolina as I write this. Yet there is snow as far north as the coast of Maine. That’s about 500 miles away! How is that possible?
Most of the snow we have been seeing so far today is ocean effect snow. Winds always turn counter-clockwise around an area of low pressure, or storm system, in this part of the world (clockwise in the southern hemisphere). Side note – the force that causes this does not have any impact on small scale things like bathtub drains so don’t believe everything you hear about how toilets flush in Australia. Back to the point…that means the winds are always blowing from the east or northeast on the north side of the low pressure system. That’s why we call them "nor’easters" as you probably know.
As the wind blows over the ocean, it picks up moisture and transports that moisture on land. The longer the wind is blowing over the ocean, the more moisture it grabs. So an easterly wind brings more moisture to the east coast than a north or northeasterly wind. If there is about a 30 degree difference between the sea surface temperature and the air about 4-5 thousand feet above it, the atmosphere becomes unstable (warm, moist air rises, cool air sinks). That instability can lead to showers. If the air temperature is cold enough, those will be snow showers. The water is currently right around 40 degrees F and the air temperature around 4100 feet up is about 10 degrees F and the wind is blowing from the east-northeast at that level. So we are right there today.
The problem with ocean effect snow is that its notoriously difficult to predict. First, just a 10 degree difference in the wind direction can mean the difference between snow somewhere along the north shore and dry conditions on the south shore or vice versa. Second, it tends to be a very localized thing. Meaning, there might be white out conditions in one area and not a flake just a few miles away. Our computer models aren’t very good at predicating the exact timing and location of anything small enough to fit inside a county. We are pretty good at telling you where a big hurricane might go several days in advance, but not so good at telling you where a thunderstorm might go more than a few minutes in advance. In order to predict ocean effect snow precisely, we would need to know the temperature over every square inch of the ocean and the temperature, wind velocity, and humidity levels over every square inch of the atmosphere. That won’t happen.
We are expecting ocean effect snow showers to continue along the coast through the afternoon and into the evening. What makes the forecast even more challenging in this case is that the winds will be shifting around through the afternoon into the evening. The good news is that will help spread the wealth so to speak. It’s less likely that snow will pile up in one location if the winds are shifting. It’s like turning on the garden house and then spraying in all directions.
With all of that in mind, here is my updated guess…