As much as it's fun to learn about meteorology through real weather events, please be careful and I hope everyone stays safe through this serious storm.
It seems like just yesterday we were scoffing at the wild divergence in weather models for the approaching snow and now we're facing yet another historic weather event. Where the heck did this come from and why has it been so hard to predict?
First off, let's review what we know about nor'easters. We know it's warm, moist air from the south running into cold air from Canada. Right?
What else we know is that the Jet Stream is what's driving both the cold and warm. Specifically, the Polar Jet for the cold and the Subtropical Jet for the warm. Wikipedia gives us some handy aids for visualizing those particular forces
From my colleague Alastair I learned that the cold ingredient is an Alberta Clipper. That means warm, moist air started from the Pacific and headed across Canadian mountain ranges all the way to where the Rockies reach into the south of Alberta (ahem... that's the second province from the left). The air dumps its moisture on the way up the mountains and warms on its way down the east side. As it comes off the Rockies (see also, Chinooks winds) it hits the cold air that's been sitting in the Canadian prarie and a storm forms that heads across the U.S.
(I know from my past weather posts that many of you are well versed in weather phenomena, so hopefully you can help me understand the role of Arctic Oscillation in this whole concoction. It looks like we're in a generally negative trend so does that explain the trajectory of the Clipper running into the U.S. instead of keeping a more northerly bearing?)
Ok, that's enough theory, let's see if we can go back and watch the birth of Nemo.
First, I want to take a look at the Jet Stream model here. This is going to update over time, so I've taken a couple of images of what happened earlier in the week before they disappear from the model's time range.
Let's start with Monday. If you're struggling with the orientation, the perspective is looking down at the North Pole. The U.S. is in the lower right. We see the Jet Stream making a dip from the South Pacific and running right up to British Columbia. I don't know for sure how quickly this weather travels across the continent, but if I had to guess when the warm, moist Pacific air that began this storm first made its way to Alberta, that's what I'd pick.
Moving ahead to Tuesday, we get a good look at that Subtropical Jet in the Gulf of Mexico that's going to give Nemo its warm, moist ingredient.
I'm assembling these images in the wee hours of February 8, so the two storms, the Alberta Clipper and the rain storm coming up the coast from the Gulf, are still distinct. By the time I publish this later today that may not be the case.
Here's a pretty look at the two from around 3 a.m. The low on the Atlantic Coast is actually in the southern portion of all that rain. As all the meteorologists are explaining this morning, the storm is spinning water in off the ocean. The low in the Midwest is way up at the top of that front line, like somewhere between Toledo and Cleveland. That's already giving some snow to the folks there.
What's going to happen is that the two lows are going to crash into each other and make a super low. ("Super low" is a term I just made up. If you know what the actual term is, let me know. Is it a bomb?) The cold is what's going to make the snow, but the pressure drop is what's going to make the extreme wind.
Presumably at some point a weather site or TV guy will give us a cool animation of these two moving on a collision course. One place we can see it briefly in advance of it actually happening is in the forecast models.
Before we go on, let me point out that if you're into these weather maps, you'll have a really great time exploring this page where NOAA keeps the model animations.
For the purposes of watching these storms collide, I chose the first box of the Global Forecast System and then selected the Atlantic view. From there I chose the first box again, but that doesn't produce a permalink I can share, so you're on your own.
Again, these models are constantly updating, so it may be that by the time you click these links the days leading up to Nemo are no longer stored. That's why I've saved these individual frames. I should also make clear, these images are from a forecast model, not from the actual weather. They're just so we can get a sense of the storm's motion and formation and appreciate why it's tricky to figure out the paths of intersection.
In the image below we see them starting to merge. Notice also how the number of rings around the storm increase and are closely packed together. They increase because the barometric pressure is dropping in the center. When they're close together like that, it means the pressure changes rapidly over a short distance, and that means strong winds.
And out to sea...
As ever, I appreciate any insights or expertise that you can offer to help make this a more thorough and accurate item.