I must warn you this is a rather along read, so I apologize in advance.
First of all, in my previous discussion I talked about the possibility of one or two storms affecting us around the 8th and 12th of January. It appears there will be only one storm, and it will likely be a big rain producer for anyone from the Appalachian mountains east, which would include I-95, mid week next week. It could end up being a VERY powerful storm, as is often the case with storms that immediately precede pattern changes. However, after that, well, let’s just say after that it looks like winter is coming back with a vengeance.
Let’s review a few things that were discussed in the previous blog.
The biggest key to this pattern change is the sudden stratospheric warming (SSW) event, which is PRESENTLY very impressive:
Remember, that warming over Alaska breaks down the cold polar vortex that’s been stubbornly present there for the last several weeks, and has prevented true Arctic air from invading the U.S. for any sustained period of time, and instead flooding North America with relatively mild Pacific air:
But, with that warming over Alaska, the vortex breaks down and retrogrades west, allowing for ridging and development of cross polar flow, bringing Arctic air into North America including the northern U.S.:
You can easily see the ridge over Alaska. But look at Greenland. There’s no ridging – YET. What do I say yet? Well, because the European ensemble model run from yesterday is advertising an even bigger SSW event in the coming days:
That bright white shading over the pole is literally off the scale regarding the degree of warming. More importantly the area which is warming now includes Greenland. Just as the ridging developed over Alaska due to the current stratwarm, warming over this large of an area including over Greenland would result in ridge, and blocking along the East coast of the US:
This blocking prevents storms from exiting the eastern seaboard stage left. Currently, we’re in what’s called a very progressive pattern – storms come and go in a matter of hours, and cold snaps and warming periods last a matter of days. But with blocking, everything slows down. The Arctic air is funneled across the north pole and into the central and eastern parts of North America, and it sits. Meanwhile storms come along and are prevented from quickly moving out to sea, increasing our chances for snow, and in some cases big snow events.
But there’s more at work here than just the SSW event, ridging in Alaska and Greenland, and the negative NAO. We’re currently in a weak, east based La Nina. La Nina refers to cooling of the waters in the eastern Pacific ocean, which has a profound impact on the weather pattern over North America. I’m not going to go into an entire explanation on the El Nino Southern Oscillation (ENSO) here, that’s another discussion for another time. But just so you know what I’m talking about, here’s another pretty picture:
This is in contrast to El Nino, as depicted by the late Chris Farley:
I can never get enough of that.
There are two relevant things one needs to know when it comes to a weak, east based La Nina and this pattern. The first is the southeast ridge, which will fight the cold air bleeding in from the north – remember the picture above:
And the second is the subtropical jet (STJ) stream:
which is relatively active BECAUSE the La Nina is indeed weak. If this were an El Nino, the STJ would be raging, becuase the WARM waters of the eastern Pacific allow for greater convection in the eastern Pacific. That being said, a weak La Nina allows for some convection, and an additional source of fuel for storms as they cut across the southern US. Look at the convection currently in the eastern Pacific west of Mexico:
The southeast ridge will fight back against the advancing Arctic air, while the STJ provides an extra fuel source.
Here’s a modified image from the afternoon run of the GFS, for next Thursday morning, to tie everything together:
Now, first of all, I’m not saying anything is going to happen next Thursday. I just used this image as a way to illustrate what I’m talking about here. The arctic air and southeast ridge will be at odds, creating a storm track somewhere between TX/OK and the Mid-Atlantic/eastern New England. Where exactly this sets up is still a little up in the air, but you get the general idea. Storms will be able to gather moisture from both the Pacific (via the STJ) and the Gulf of Mexico (also courtesy of the southeast ridge). As blocking develops later, the storms slow down and can’t exit as quickly as they have been doing. The warm air from the south rises up and over the colder Arctic air just north of the storm track, causing overrunning precipitation:
Notice how there’s not just snow in this depiction. Ice is also a possibility. But in areas where the precip remains snow, it can really pile up, even though such storms aren’t true nor’easters, at least not to start. The President’s Day Storm of 2003 was largely an overunning event:
Again, this is for illustration purposes only, to show potentially where this pattern could go. I’m not saying something like this is a certainty, but it can happen.
So that’s it for now. Currently I expect a large, probably pretty powerful rainstorm to affect the eastern US mid week next week, followed by Arctic air, which should be sustained rather than transient starting late next week into next weekend. And then the real fun should begin.
Winter is far from over.