Your Polar Vortex Questions Answered
After my post about the polar vortex back on Saturday, I have seen a few questions in the comments, on the Facebook page and on Twitter that I thought were worth answering here and expanding upon some of them. All are pretty good questions but some will take longer to answer than others.
Our first came via Facebook from David Eliff who lives in Lexington, TN. This was in the comments on my Facebook fan page.
Would the 1899 cold snap that hit the eastern continental U. S. be considered the vortex breaking free and heading south?
It's hard to say for sure. The problem with looking at weather events from so long ago is that we don't have many observations compared to today and nearly all of the observations that we do have are of surface weather rather than upper air plots. So, we really don't know where the polar vortex was around the time of this cold snap. It would be interesting to know what was going on in Alaska during this time as well, because normally when it's very cold in the lower 48, it's quite a bit warmer than normal there. If we knew it was warmer than usual in Alaska, that might give us a hint that the polar vortex had been pushed out of place somewhere. But, the period of record for the Alaskan stations only goes back to around 1905. My gut feeling is that the polar vortex had been shoved into Siberia but a large chunk of it had broken off and indeed did move into the U. S. By that, I mean that the part of the polar vortex that is in the troposphere had sort of decoupled itself from the part in the stratosphere and had been forced across Canada and into the U. S. This same sort of thing going on now. In the stratosphere, the polar vortex is centered over the arctic north of Russia currently. But, the upper trough currently supporting the extreme cold into the U. S. is trackable from the same general area several days ago.
The cold wave of mid-February 1899 was the worst one on record. Here's what the weather map looked like on February 12, 1899:
Notice the very large, sprawling arctic high over the middle of the nation this day. Notice the large area of pressures greater than 31" of mercury as well (this is before the old Weather Bureau switched units to something more science-friendly). This is equal to about 1050 millibars. This is a huge area of 1050+ pressure and much farther south than you typically see with some of the truly epic cold snaps in history! I went back and looked at previous days and found that pressures with this high were well over 1060 millibars as the high crossed the Canadian border into the U. S. You ask, "Why is this important?" Well, cold air is more dense than warm air, so colder air masses will usually produce higher barometer readings than warm ones. Very cold arctic sort of air masses will generate extremely high surface pressures and anything over 1050 millibars, about 31 inches of mercury, is a very high pressure. You can compare this to the current cold wave, where the highest pressures reported in Montana late Tuesday afternoon are running at about 1049 millibars, 30.97 inches of mercury. For reference, at sea level the average barometer reading is about 29.92 inches of mercury, 1013 millibars. At the other extreme, an very powerful hurricane may have a pressure under 900 millibars, 26.58 inches of mercury.
This cold snap resulted in below zero readings all the way into Florida. It was -2 in Tallahassee on the morning of February 13, 1899, which still stands as the coldest reading ever in Florida. In fact, daily and some all-time records still stand from this cold wave all across the South. Some were tied or broken during the 1985 cold wave that I mentioned in my original post. However, during the 1985 cold wave the highest pressures reported were only around 1050 millibars. So, the pressure of a given air mass does not tell you how just cold it is, just that it is probably very cold.
The second question I'll address is via Twitter from Kevin Reid. Y'all can find me on Twitter as @AccuFrank, by the way.
Is the polar vortex same as Siberian express?
No. A Siberian Express is just a colloquial term to describe a very fast moving arctic air mass. It tends to be extremely cold once it gets to the South because it moves so fast that it doesn't have much time to be "modified", that is, to be warmed by daytime sunshine, become mixed with other warmer air masses, or absorb heat from the ground or bodies of water on the way south. The 1985 cold wave would probably be the best example of a Siberian Express. The name comes from the fact that the cold air mass usually originates over Siberia and is dislodged northward over the North Pole and then across Canada and into the United States. A setup such as the one we currently have across the Northern Hemisphere is conducive to these kind of events. As I mentioned in a blog post late last week, the western Pacific Typhoon Nuri transitioned into a midlatitude type of storm by this past weekend and became very strong. This resulted in warm Pacific air getting drawn northward into Alaska and this, in turn, forced the jet stream north into the polar regions in this area. This then led to flow going over the North Pole (cross-polar flow, weathermen call it) and so very cold air from Siberia and the North Pole was shoved southward into Canada and the United States. So, the current cold wave might be described as a Siberian Express. It sure did move fast across the Plains states. Monday morning the leading cold front moved through Denver and reached Deep South Texas, a distance of about 1000 miles, yesterday evening, in about 31 hours. That's an average speed of about 32 mph. The really fast moving Siberian Expresses can move at more than twice that rate!
Finally, here's a question from the blog comments section (y'all will find that below each post) from Gorman Bel of Cave Spring, VA
So if the polar vortex is an upper air feature, how does it drive cold temperatures at the surface? Does a low-level surface low develop below the PV?
This question is a little more complicated and will take some explaining. This is part of the reason why I showed so many different slices of the atmosphere in the post back on Sunday. The main point I wanted to get across was that the polar vortex is in no way a surface feature. It exists over the top half of the troposphere and into the stratosphere. So, no, it would not cause a surface low to form beneath it. Conditions should usually be favorable for a surface low to form over to the east right side of the polar vortex, though, while surface highs and sinking air should be found to the west.
What is actually driving the cold air at the surface is dependent upon a number of different factors and there's more than one way to get a cold snap into the eastern U. S. For example take the chain of events that led to the cold wave we are seeing now. That started out because a strong West Pacific typhoon met up with an upper trough over east Asia. This allowed the typhoon to transition into a very powerful midlatitude type storm moving poleward. It brought a lot of heat and mass northward with it. Like I mentioned days ago in a video, a highly simplified explanation is that the equal and opposite reaction to that was cold air heading south somewhere. Other factors present (negative NAO, a blocking ridge over the North Atlantic and a strong ridge over the East Pacific ahead of the storm and into western North America... positive PNA and negative EPO in weather geek speak) were favorable for the very cold air to head south into central and eastern North America. Thanks to the large amount of blocking present in the current weather pattern around the world (large dips and rises in the jet stream, in some cases these dips and rises are closing off from the flow) the setup that has resulted will be persistent for a while, more than a week, perhaps two weeks. Another factor present is that we have cross polar flow currently, with the flow going from Siberia across the North Pole and into North America. That also will contribute to very cold air available to come south into the U. S. and the next good cold shot coming early next week currently looks to be even colder than the current one.
When looking at the stratospheric polar vortex, you actually want that to be as far away from you as possible if you want it to be very cold. The reason for that is because where the polar vortex is present in the stratosphere, it is cold in the stratosphere. That results in the stratosphere being a thinner layer than it usually is (cold air is more dense, so it takes up less space). This means the troposphere, where our weather happens, can be thicker and that usually means it's warmer directly under the polar vortex in the stratosphere. Sure enough, the polar vortex is actually currently over northern Russia and it's been getting into the 40s in Moscow, 5-10 degrees above current normals. But on the other side of Russia, considerably removed from the polar vortex and where it's much warmer in the stratosphere ... it's 44 below in Jakutsk. The current chart for that high up in the stratosphere looks like this:
and there's no mistaking where the polar vortex is on this. Note the cold stratosphere temperatures over Europe ... well not only is Moscow running above normal right now, so is the rest of Europe. Berlin, Paris, London and other cities were all above normal today. Meanwhile, you saw see how it's relatively warm over Siberia and the lower 48 states? Well, it's cold in those areas. This, of course, isn't a hard and fast rule ... it's warm currently in East Asia and Alaska.
Interestingly, take a look at what the GFS is forecasting for Thanksgiving week:
Note the wobble toward North America of the polar vortex while the stratosphere warms quite a bit over Siberia. This means the GFS is showing another straospheric warming event late in month. These tend to be precursors of extreme cold snaps somewhere and often into North America because the warmer area of the stratosphere tends to migrate eastward toward North America (currently, models are showing this, too). We actually saw one of these 2-3 weeks ago and that was one of the many signs that some big cold air was coming south well in advance. Other models are in general agreement about this happening. So, while it appears that in 10 days or so we will finally see the cold across North America ease, there is cause for concern that in the early to middle part of December, we get another bad cold snap. So, a stratospheric warming event is another way we can get severe cold into the U. S. It's worth noting that there was a very big stratospheric warming event right before the January 1985 cold wave and before many of the epic cold snaps we have seen.
Hopefully, the above explanations and tangents don't confuse y'all too much. It's really hard to simplify the polar vortex into something that's easily digested by the general public but I did my best and tried to show examples to help with the explanation. Hopefully, my answers to your questions don't beget too many more questions!
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