I received a question (er, request) on Facebook to discuss in a little more detail about how/why storm systems and fronts seemingly 'jump' across the Appalachian Mountains. Actually, this process is not really all that rare, and can happen multiple times in the evolution/advancement of a storm system. A storm center spawning a new low in a new location is similar to a front-jump, but it's got its own peculiarities. This is a bit wonky, so feel free to ignore it and continue on to the updating tracking maps below ;-)
The shortest way to explain this is two-fold: (1) The upper-air dynamics that foster the storm on the surface are unaffected by the mountains, but the surface layers are; (2) A storm system approaches and connects to such a wondrously more attractive source of energy that causes a rapid intensification at that location in which the parent system can no longer benefit. Here's a video that will also help explain what I'm talking about, although it might seem (WAY) out of place to start ;-)
(How has this thing been viewed 500,000 times?!)
Anyway... so let's look at the present situation: Upper-air first, then the surface.
|NAM - 500mb Chart - Monday Night (past)||NAM - 700mb Chart - Monday Night (past)||NAM - Surface Chart - Monday Night (past)|
One of the things I want you to notice first, is that the center of lowest pressure in the Northern Plains is in more-or-less the exact same spot (meaning, from left to right, the lowest on the 500mb chart is near over the lowest on the 700mb chart which is near over the lowest pressure at the surface. As far as storm systems go, this is a bad thing. It is a situation called being "vertically stacked". When a storm system is vertically stacked it begins to fill in and weaken. What you want to see in a strengthening storm is a more staggered vertical profile, so vertical air motion has room to breath, encouraging even more upward air motion, instead of a big fat lid being placed on everything. It is near-inevitable that all strong storm systems suffer the same fate, but that's not the end.
I want you to see on the left-hand chart (the 500mb-chart), the deal with the "X's" again. These refer to impulses of "vorticity" (I've talked about this before, but I understand if you are new to the blog). Vorticity is basically a relative twisting going on in the atmosphere, and an instigator of storm formation wherever they go. Whenever you have a stacked upper air pattern, it is often that along with it you have a little flare of vorticity rotating around the core. And, much like the Polka in the above video, they swing around each other, propelling dynamic changes at the surface.
As the secondary vorticity maximum rotates around the base of the primary (denoted in the left-hand graphic), a new low pressure center is instigated. A helping factor here is the newfound access to the Gulf of Mexico and it's huge source of moisture/energy. This one will begin to take over, as the parent low is left behind to continue to fill in and dissipate.
|NAM - 500mb Chart - Tuesday Night||NAM - 700mb Chart - Tuesday Night||NAM - Surface Chart - Tuesday Night|
In this next stage, the original low in the northern plains has largely faded, and this new low has taken over. The 500mb vorticity complex is still driving the events at the surface, but the air is not vertically stacked. On the 500mb chart at left, the low is in Missouri, the 700mb low is over IL/IN, and the surface low is near Lexington, KY. Ergo, it is still a healthy system. And it would be too, would it not be for the looming Appalachian Mountains. As the upper air energy continues to do the Polka around and around, the surface low will stall up at the base of the mountains. Once the storm energy in the upper levels of the atmosphere (above the mountains) cross harmlessly to the other side, the bountiful storm energy of the Atlantic Ocean awaits. A new low will form on the eastern side of the Appalachian Mountains, and the original begins to languish behind and fill in, no longer the focus of the upper-air energy. This gives the appearance of a "jump", one that happens all the time when the mountains are involved.
|NAM - 500mb Chart - Wednesday Morning||NAM - 700mb Chart - Wednesday Morning||NAM - Surface Chart - Wednesday Morning|
So here's the money-maker... A pretty tight stack, but it's also a rapidly strengthening low pressure (that seemingly 'jumped' over the mountains). Windflow across the storm from the northeast and behind the system from the northwest both contribute to lowering temperatures and snow showers. As the air pushes up the mountains (this time from the east toward the east-facing slopes), that's where we'll see the highest snowfall totals.
So, that, in a hopefully not completely confusing manner, is how storm systems can jump and hop around at the surface, even though the upper air properties retain some element of consistency.
If you have any questions with this, I completely understand :-) Post comments below and I'll try to get to them.