If you’ve lived on the east coast long enough, you probably know that there are several ways we can get snowfall, but only one of them will deliver our big, crippling snowstorms. If you’re a snow lover, you’ve probably also realized that we can suffer some disappointment with these “big” storms, and it’s not because the meteorologists over-hyped the storm (well, at least for the most part). The fun begins looking at how all this comes together.
Before we get to the classic storms, let’s look briefly at the other ways we get snow, and why they don’t produce nearly as much snowfall.
The simplest way we get snow is via snow squalls. A snow squall is a small, intense band of snow, usually accompanied by gusty winds that can reduce visibility, sometimes to near zero. Snow squalls are fueled the same way pop-up summer thunderstorms are – instability. What happens is behind a cold front, the atmosphere usually cools off fairly quickly, but the surface of the earth is much slower to do so. As the sun heats the Earth, we get little bubbles of air that form near the ground, which are much warmer than the rest of the air. Since warm air is very light, it rises. As long as that bubble stays warmer than the environment, it will continue to rise – this is the beginning of the clouds forming/precipitation processes. You may notice that the day after a cold front we usually have the big puffy cumulus clouds in the sky – this is why. If there is sufficient moisture, we can get precipitation to form, and many times in the winter: snow squalls. Snow squalls are quick movers, and as such, unlikely to deliver more than a dusting.
And then of course we have our clipper systems, you’ve probably heard the term Alberta Clipper. Clipper systems typically form in Alberta Canada, just east of the Rocky Mountains. They then follow the upper-air flow off to the east and can bring a shot of rain or snow, depending on how cold. Because clipper systems form so far north, they lack sufficient moisture. In addition, they are usually quick movers. The lack of moisture and faster motion leads to lighter snow amounts. In our area specifically, because we have the Appalachians to our west, the air has to descend down the mountains, which causes it to dry out – which can also contribute to small accumulations of snow.
One thing clippers can do, however, is reinforce a shot of cold air. Often times, their cold fronts also stall along the gulf coast, or just off the SE US coastline. This can become a focal point for coastal lows in the coming days.
And that brings us to our “classic winter storms.” These storms always involve a coastal low, and therefore have something the other methods of snow do not – abundant moisture, since the counter-clockwise flow around the low draws moisture straight off the Atlantic. This is the reason our “big” snow storms come up from the south. However, this can also provides a source for warm air to enter the system, creating the dreaded rain/mix/snow lines.
Take a look at the graphic below, which shows everything we need for a classic winter storm on the east coast- and I think one glance will show you why it’s difficult to actually get a good East coast storm. If any one of these pieces is missing, it’s incredibly difficult to get a blockbuster snowstorm.
West Coast Ridge: The purpose of having a ridge in the Jet Stream over the west coast is simply to put a trough over the east coast. Troughs bring colder, and generally more active weather with them – which you need to get a storm.
Polar Vortex Displaced South: Generally the polar vortex lives in the Arctic, but when it weakens, lobes of it can drop south. The polar vortex is home to the some of the coldest air on the planet, and when it drops south it can make that bitter cold available to places outside the arctic. This gives us a source of arctic air.
Arctic High: In order to get the arctic air from Canada down into our area, we need an area of high pressure to funnel that air down. The high has to remain in place throughout the storm, or we lose our cold air source and the warmer air brought in by the storm takes over and we get something other than snow, i.e. Rain.
50/50 Low: The job of the 50/50 low is to keep that Arctic High from sliding out to sea. It can also help in dragging cold air south, as well as, increase the strength of the Greenland block. It’s called the 50/50 low because it tends to sit over 50° latitude/50° longitude.
The Greenland Block: The purpose of the Greenland block is to buckle the Jet Stream in a way that keeps the low riding up the coast from moving too quickly or moving out to sea. The easiest way to think of it is think of the Jet Stream like a highway, and the Greenland block as a construction area. Instead of the nice, open highway where traffic is moving at highway speeds, it’s blocked by the road work and traffic has to slow down. The same thing happens in the atmosphere with a block.
Stalled Front: This is not always necessary, but in many cases, stalled fronts along the Gulf Coast, or SE US Coast can be focal points for new storms to form.
So there’s the big picture of what is needed for these storms to form. Now let’s look at how they actually form.
There are two types of winter storms, termed Miller Type A, and Miller Type B. They refer to the method in which the coastal low forms along the east coast.
Miller A storms form in the Gulf or off the SE US. Miller A’s form from the stalled frontal boundaries of the Gulf coast/SE US Coast. The two branches of the Jet Stream tend to converge upon one another in this region in a way that leads to increased dynamics. This gets a little more complicated, so I’m leaving the details out, but these increased dynamics lead to cyclogenesis – or the formation of an area of Low Pressure. Miller A storms then follow the upper air flow, tracking up the coast. The March 1993 Superstorm was as Miller A storm.
Miller B storms work a little different – and the winter storm from Monday night was a Miller B storm. In a Miller B storm, we get an initial area of low pressure (typically an upper low) that tracks across the Midwest. This low eventually dies out and this old low transfers its energy to the coast and a new low forms off the East Coast, usually around the Carolinas. Now the energy doesn’t just Apparate from one place to another like it lived in the Harry Potter world. But the process of how this happens is complicated, so I’m not going to go into it. Miller B storms can also be less favorable for snow because of that initial low. If that low across the midwest is too far north, it drags in too much warm air and we see the mix/rain into the I-95 corridor. If the low tracks farther south, this effect is reduced. For comparison, the Blizzard of 2016 was also Miller B storm, but unlike Monday’s event, it was almost entirely snow because the primary low dropped farther south.
So there you have it. A complete lesson in East Coast Winter storms. Winter storms are by far one of the hardest things we have to forecast as meteorologists, primarily because there tends to be an over-emphasis of amounts over impacts. Obviously 1″ of snow isn’t going to have as big of an impact as 6″, but we tend to spend too much time focusing on how much we see, rather than the impacts the storm can bring. Just a closing thought.