Jet stream finds a shortcut and Canada goes cold. We explain

Thursday, April 23, 2015, 1:26 PM - As you follow the forecast this week, you may hear this unfamiliar weather term coming up over and over again: upper level low. Here’s what it means.

Meteorologists talk a lot about the upper levels of the atmosphere, and you may find yourself wondering why you should care about what’s going on so far above the earth's surface. After all, we spend the vast majority of our time down here on the ground. So, unless you’re a pilot (or a bird), why would you be interested in what’s going on so far above us?

The answer is that what goes on high up in the atmosphere directly drives the weather that we experience. So understanding what’s going on up there provides insight into why things are happening down here. This week is an excellent example of how this is the case.

This Week’s Pattern

I’ve drawn a line to highlight the flow of the jet stream at this level. You’ll notice that path is very contorted with lots of sharp curves, including a trough over the B.C. coast, a steep ridge centered on Alberta, and a deep trough over the Great Lakes. As a general rule, the jet stream would prefer to flow directly west to east. But as you can see this heavily amplified pattern causes it to divert wildly to the north and south.

In scenarios such as this, you’ll often find that the jet stream wants to look for a shortcut, to carve out a more direct path from west to east. In this image above, I’ve highlighted two areas where a shorter path is available for the jet stream, (1) and (2). In the case of (2), you can see how the shortcut is especially appealing. If you want to cross Hudson Bay, there are much easier ways of doing it than to go through Kentucky.

So let’s take a look at how the ground-weather pattern develops in response to this situation, in the following image.

This shows the pattern (as forecast by a model) for the morning of April 23, about 36 hours after the first image. The jet stream has taken advantage of the two shortcuts, to form a more direct path from west to east. But in doing so, it has left something behind. What was formerly a pair of troughs, or southward dips in the flow, have become cut off from the main jet stream and are now separate lows. These are the features we are interested in – the upper level lows – that will be driving much of the weather across Canada for the remainder of the week.

Because these lows have been cut off from the main jet stream flow, we also refer to them as “cutoff lows.” Without the connection to the jet stream, they tend to stall in place, and a cutoff low like this can hang around for several days until it is eventually reabsorbed into the jet stream flow.

As you can see, the smaller West Coast low has been reabsorbed into the Gulf of Alaska trough, but the East Coast low has made very little eastward progress by the weekend.

Weather impacts for Canada

In addition to being slow moving, cutoff lows have another characteristic that is causing some unusual weather across Canada this week. Because they originate in the arctic air mass north of the jet stream, these cutoff lows are associated with a pool of very cold air. And adding a slug of cold air to the upper atmosphere is a sure way to create weather chaos.

When we have a situation in the atmosphere with cold air above warm air at the surface, the result is instability. The warm air is less dense, so it wants to rise like a hot air balloon. In this situation, with the cold air aloft being provided by the cutoff low, and the relatively warm air at the surface being provided by the strong late April sun (yes, temperatures are well below seasonal, but they are still warm compared to the very cold air aloft), we have a situation that tends to produce a lot of widespread rising air.

The result is a radar map that looks like this below.

Widespread rain and snow showers are spread across the entire area influenced by the upper level low. But because these showers aren’t being influenced by a single strong forcing mechanism, like a cold front, they are widely scattered and don’t really show any organizational pattern. As you can imagine, a weather system like this makes for a particularly complicated forecast.

What is Graupel?

On a day influenced by an upper level low, the weather tends to change quickly. You can see sun, clouds, downpours, snow flurries, and everything in between in the space of an afternoon. You can even see an unfamiliar precipitation type: graupel.

Graupel are small, white, opaque clusters of ice crystals, by definition less than 5mm across.

By the look of it, you might think graupel is a type of winter weather, maybe a cousin of ice pellets or snow. But in fact, graupel is caused by convective storms, and is much more closely related to hail. Remember that cutoff lows, because of their cold air aloft, tend to produce a lot of instability and rising air. This leads to convective storms that act much like summer thunderstorms, though not as strong.

The updrafts of these convective storms can contain a layer ice crystals (snowflakes) and a layer of supercooled water droplets. The diagram below shows how these layers form, based on the temperature profile. As the ice crystals fall through the storm, through the layer of supercooled water, the water droplets freeze onto the ice crystals forming the small, brittle pellets called graupel. This is the exact same process that forms hailstones, except that in the case of hail, the storm is stronger so the hailstones can grow much larger and more solid before they eventually fall to the surface.