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    Shiver Me Timbers, The Cold IS Coming

    1/10/2013, 7:29:47 AM

    Thursday, 11:30 a.m.

    There's a LOT of warm air on the playing field right now from the eastern Rockies to the East. The biggest temperature anomalies today will run from the Dakotas to the Deep South, with many places running more than 20 degrees above average for the 10th day of January. As I have outlined the past couple of days, that incredible, record-breaking warmth will slide eastward in the coming days. Tomorrow, the core of that warmth will extend from the Midwest and Mississippi Valley into the Ohio and Tennessee valleys and the Deep South. By Saturday, that warmth will fan out from the Gulf Coast states through the entire East up into much of Quebec. Even Sunday the springlike warmth will hold fast over the eastern third of the country:

    Two words will suffice at this point. Well, maybe five. Enjoy it (while it lasts). It won't be around for long.

    One of the more hot button topics in the weather realm these days is the term 'Sudden Stratospheric Warming,' or SSW for short. Just what the heck does that mean, and why should you care? Good questions, and I'll try to keep the answers simple and understandable, but it is an important subject.

    When we talk about the stratosphere, we're taking about areas above the troposphere - way up in the atmosphere where the air is so thin we're well above cloud levels, and at a place where weather just takes on a different meaning. However, despite the thinness of the atmosphere, there are measurable things there, like temperature and wind.

    With regard to temperatures, there is an ebb and flow to them, just as we'd see here on the surface of the earth, but those changes are not so much diurnal, but rather over longer period of time. Occasionally, though, we'll see a sudden spike in temperatures, particularly at this time of the year. Take a look at the temperature trace since the first of November over the North Pole at the 10mb level:

    That spike started to occur around the turn of the new year and has probably reached its peak. One of the things that we have observed through the years is that when these SSW events occur, it almost always correlates well with arctic outbreaks around the Northern Hemisphere. These types of things generally induce blocking in the Northern Hemisphere, and when that block is established in a 'traditional' fashion, it can lead to that arctic air being dumped into North America.

    Last winter when something like this occurred, most of the cold air went into Alaska and eastern Asia, as well as into Europe. Virtually none of it ended up in the Lower 48, and there was little evidence of blocking in the North Atlantic. And the winter before, there really wasn't a strong warming event. Most of the cold air that winter remained in Asia.

    This year, it appears as if it is about to be North America's turn to drop into the freezer, as the evidence is mounting for such an event during the second half of the month. How might this happen?

    Let's lay out the string in the next week or so. I've already laid out the current warming. From the one image above of Sunday's departures, it's now turning much colder in the West behind a strong cold front. That front will move out of the Rockies tomorrow, bringing the cold air into Denver, for instance. Saturday, it will move out across the Plains. If you really look at the cold, though, it's more a product of low heights rather than true arctic air. Look at the 500mb forecast and projected anomalies:

    Study that image for a second: That's a very deep trough in the West, hence the cold. But that's a monster ridge, both downstream in the East, and upstream in the North Pacific up into Alaska. Really, it is a highly amplified pattern, period! And note the polar vortex - near the North Pole.

    As we go into the early part of next week, the downstream ridge over the southwest Atlantic will hold its ground, while the upstream ridge off the West coast tries to expand eastward. At the same time, we begin to develop a 'split' in the western trough as it comes eastward through the Rockies, with one piece becoming cut off over the Southwest and northern Mexico, while the rest moves more steadily eastward onto the Plains. Here's the Canadian ensemble 500mb forecast for Monday evening:

    It should be carefully noted here that the European model is the most adamant on this split, and it had the better idea with regard to the storm we see now moving slowly out of Texas.

    With this split developing and the western ridge expanding inland, warming will progress inland and over the top of this ridge and out into the eastern Rockies and western Plains. With the snowcover being wiped out across Kansas and western Nebraska, it'll be easier to warm things up there, and that warming will probably reach its peak on Wednesday. Look at the projected anomalies Wednesday:

    You see the chill over the Southwest and into Texas associated with that upper-level low rolling through, while the warming comes right out of Alaska and through northwestern Canada into the northern Rockies and northern Plains down into the central Plains. The warmth is hanging on for dear life over the Southeast, and the chill has yet to really reach the Northeast, as the front coming through the Midwest into the Lakes this weekend loses its punch coming farther downstream. Essentially, there's still not much in the way of true arctic air. Yet.

    There will be a series of waves coming out of the Southwest and Texas and riding up along the front this weekend and early next week. The last of these probably slides through the Northeast and mid-Atlantic sometime Wednesday. Once it gets out of the way, THEN it gets interesting. Then we start to see how the SSW warming event begins to unfold.

    Typically, there is a lag of 10 to 15 days from the peak of the warming to the dumping of cold air into the pattern. By next Thursday, we're entering that window, and look what we see here:

    That's the Canadian 850mb temperature forecast for Thursday morning, and that's some brutally cold air rolling through Ontario into Quebec behind a storm. And look where that polar vortex ends up by then:

    Suddenly it's on the move. Two other things to quickly point out here. The Arctic Oscillation is a good indicator of the strength of the polar vortex and the availability of cold air. When it goes negative, it usually means arctic air is somewhere on the playing field. It is NOT a guarantee of that cold air getting out of the high latitudes, but it's a good sign that at least it is there. The AO forecast:

    Most of the individual members suggest it is heading that way, some more so than others. Check.

    Another index we look at is the North Atlantic Oscillation, or NAO for short. When it goes negative, it's a good indication of some form of blocking in the North Atlantic ocean, which can lead to cold air being forced into the U.S., depending upon where that block sets up, how strong it is and how it may be oriented. Here's the latest forecast:

    Not necessarily convincing, but it is trending in that direction.

    So, in looking at all of this, the first REAL shot of arctic air is coming later next week and appears to be mostly aimed at the northern Great Lakes, upstate New York and New England. And with that polar vortex rotating away from the northwest shore of Hudson Bay into Quebec, we'll finally beat down that ridge off the Southeast Coast. Ding! Dong! The Southeast ridge is dead! Look at the Canadian ensembles 500mb forecast for next Saturday evening:

    That opens the door for another system to come in behind the first one. Look at the Canadian 850mb forecast for next Saturday evening:

    If the block is in place by then, which I believe it will be, and that ridge is beaten down, then the cold air behind this second system would likely be forced farther south and west, meaning much of the Plains and especially the Midwest into the Ohio and Tennessee valleys could be hearing the arctic hounds nipping at their door. I don't think it's cold enough to warrant concerns for a freeze in Florida from that outbreak.

    That said, if the history associated with these types of events is any gauge for what is to be expected down the road, then the arctic air, once it moves in late next week and next weekend, may be around for a while.

    Shiver me timbers, it's looking cold!

    The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com


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