I’m sure you’ve heard of the threat for winter weather coming this weekend. Lot’s of uncertainty sits in just how this event will unfold. Will the storm track too far north, throw in some warm air, and make this more of a rain event, will it track too far south we either get grazed or see nothing at all, or will everything work out perfectly and we get a decent march snow storm? Let’s discuss…
We’ll be rather mild through Thursday with highs still in the 60s (50s for some north) before another arctic front drops southward across the area. This will usher cold air back into the area, and it will probably be on the lines of the coldest air this season with highs only in the 30s (Sunday could only be in the 20s if there is a snow storm, but that is yet to be seen).
By Saturday, the front has moved well south, and the cold air is establishing itself over the east coast. On Sunday, the cold air is still around*. I put the asterisk there because the track of the storm makes this an uncertainty. If the low tracks too far north, it will draw up warm air and make this a rain or wintry mix event, instead of snow.
So we have the cold air in place, do we have an appropriate atmospheric setup for this that will allow for an optimal storm track?
Current Atmospheric Setup
If we take a look at the current setup, there is a rather large ridge over the east coast that extends into the Canadian Maritimes – hence all the warm weather, and an upper low pushing across south-central Canada/northern US. This low has the cold front responsible for the showers today/Wednesday, and all the severe weather yesterday in the Mid-West/Great Plains.
If we look back to the west, we see a ridge over Alaska, which is part of a block. You can think of a block like a construction zone on a highway, that closes the flow of traffic and forces a detour. The block forces the Jet Stream to take a path around the block, a “detour” if you will. But, because we have a ridge, there has to be a trough immediately downstream – physics. To understand why, imagine trying to put two mountains next to each other without a valley or low point in between – it doesn’t make sense, right? Same principle here. So, the downstream trough is represented by the upper low just south of Alaska. And this low has a big implication – it prevents a ridge from forming on the west coast. If you look at the black lines over the west coast, notice how they are straight, and don’t curve upward (ridge) or downward (trough). We call this a zonal flow. But what this means is any piece of energy that comes in from the pacific is forced to take an eastward path, and the result is that the storm stays north of us (rain).
You’ve probably heard the saying in our area that our big snows come from the south. In order for a storm to take that southern route, it either has to originate from the south (which this one won’t), or follow a pattern that directs it south. In other words, we need a ridge on the west coast. This would then force the energy to dive southeastward (going down the “mountain” using the example from above). So can we get a ridge to form? Yes.
The models show that the upper low over western Canada will move westward, and this will allow a slight ridge to build on the west coast. It’s enough that as the piece of energy associated with our storm system, will take a more southeastward track – which is what we wanted.
Now what determines how far south the low tracks is the strength of the cold air over the eastern US. If the cold air is too strong, it forces the low too far south and we either get grazed or see nothing. If the cold air isn’t as strong, the low will track further north, but there is more marginal temperatures, which could mean a mix or rain instead of pure snowfall.
On Tuesday, all the models had a rather suppressed storm (or way south). The GFS model is slightly further north than the European, but both are generally misses for the area.
The GFS does get some snow into southern Maryland, and maybe a couple tenths of an inch in northern Maryland, but it’s not that big of a deal.
The European model on the other hand, is further south than the GFS, and has no snow over our area. In fact the European model, as a result, allows temps to get up to around 40 on Sunday. I don’t think that’s right, even if the Euro has the right track, just given the cold air in place. However, it has to be watched.
One thing to keep in mind is that we often see a northward trend in the models 72-84 hours prior to the event. So it may not be a bad thing the models are south with the storm. If we see a north trend with this system, it gives room that the storm may track in a more optimal path, that’s not too far north. These northward trends, I should note, are never significant, so don’t expect to see the low suddenly in Virginia. It would be a slight shift.
Stay tuned for updates on this.