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Saturday, January 19, 2013
Nature ... Science ...

Wintertime is never fun, at least not for me. If you are really into winter skiing or live way down south or don’t need to drive back and forth most every day (with a place to live big enough to prevent cabin fever), then you might enjoy a long season of arctic cold. Let it snow, let it snow! But when you have to wait outside early every morning for a bus or train, or push your car over miles of dangerous roadways every day, the winter season gets old real fast. Given that I am a fan of short, mild winters (like we got here in NJ last year), I constantly scan the radio and TV stations for forecasts and the papers and web-sites for articles on what to expect in January, February and March.

There are two times of year when the weather gets a lot of attention from me: during the mid and late-summer months, when thunderstorms and sometimes even hurricanes (as in 2011 and 2012) become atmospheric events here in Jersey. And in winter, when cold blasts and monster snowstorms threatens my own comfort and safety. Between the two, I still favor summer’s humid discomfort over winter’s dirty snow and bitter winds. At least there is a lot more daylight in summer, and if something unexpected were to happen where you had to stay outdoors for a long time, most summer weather wouldn’t kill you. Whereas a 10 degree night truly does put your life on the line, if you can’t successfully find a way to insulate yourself sufficiently from the cold. Most homeless people and street people learn ways to survive, but once in a while one of them doesn’t and the body is removed from the street in the morning.

So it’s become a point of interest for me to understand a bit more about winter weather and what are the signs of its longer-term trends. Yes, I’ve heard of the folk wisdom about the tails of squirrels and bird migration patterns. In fact, where I live, we see junco “snowbirds” arrive from the woods in December and usually stay with us thru March or April. But during the warmer winter years we don’t see too many. To be honest, I haven’t seen very many lately — I’m hoping that’s a good sign for an early Spring, even though I enjoy seeing the little snowbirds on the driveway and tree branches.

But there are some scientific talismans that can also be consulted, if you are sick of “headline grabber” weather writers who vie for attention by over-dramatizing a quick snowfall or a few days of cold air. “Arctic blast on the way”, “significant accumulations coming”, “an icy, treacherous mix over much of the area”, on and on. The problem is that they use the same language whether a real blizzard is on the way or just a 2-inch “harassment” storm is passing through. They love to tell you that “it looks like we’re in for a really rough one this year, folks”. Given their mixed track record, I decided to learn a bit about what the real meteorologists look at in identifying longer-term trends during the cold season.

So here is my quick review of what to watch for, at least here in the NJ-NY area. It probably also mostly applies to the mid-west region, east of the Mississippi anyway.

Basically, our winters here in the middle-latitude eastern USA are determined largely by how the jet stream behaves. This stream basically blows across the USA from west to east, and then hooks north or north-east over the Atlantic towards Europe. When the jet stream is forced down into southern latitudes and broadens out, the eastern USA (and even the mid-south states like South Carolina and Alabama) are in for some frigid and potentially snowy months. When the stream is drawn up into the north, it blocks the arctic air flowing down from the dark no-sun regions near the North Pole, and we have milder weather. So what helps to shape the jet stream?

There is a list of maybe 7 or 8 trends regarding very large swaths of land and ocean, and either temperature or pressure or both, which add up to give us easterners our winter work-out. But three of them appear to be quite important; and more importantly, can be tracked by daily or weekly releases from the federal NOAA agency, i.e., the government weather bureau. First, there is the “ENSO”, more famously known as El Nino / La Nina. This trend is determined by a complex set of events out in the Pacific Ocean involving wind directions and ocean temperatures. When an El Nino event occurs, the jet stream tends to “lower itself”, allowing arctic air “into the bowl” of the eastern USA. A La Nina event tends to do the opposite. Some winters, like the one we are in right now, the ENSO stays mostly in neutral, there isn’t a strong El Nino or La Nino trend. Thus, this year the ENSO factors are basically having the jet stream “split the difference” between warm and cold periods.

Next in importance (IMHO) is the NAO, the North American Oscillation. This has to do with air pressure patterns over a wide swath of the USA, Canada and the Atlantic Ocean. When the NAO is said to be “positively biased” or positive, for short, the jet stream stays high in the north and blocks out the cold waves from the frozen north. Obviously, a negative result indicates that the jet stream will drop, and down comes the cryogenic air into Philadelphia, Memphis and Atlanta (maybe even Jacksonville!). Some years the NAO strongly trends one way or the other; this year the NAO has been wavy, some ups and some downs, but no big move from average. Again, like the ENSO, the NAO has been splitting the difference so far this year.

Number 3 on my list is the PNA, the Pacific-North American pattern. Again, there is a PNA index of pressure bias covering the Pacific Ocean and the continental USA, and once again, there are positive and negative bias trends. When the PNA is positively biased, winds over the eastern USA overall tend to blow from west to east, longitudinally; when the PNA goes negative, the wind directions start tipping towards the northeast and even straight south to north. This obviously blows warmer air up into the colder regions of the USA north of the Ohio and Potomac Rivers. The jet stream may or may not be pushed north by a negative PNA, but even if it is not, the northward PNA-driven winds blow warmer air into the cold waves that the jet stream may be letting slip past the Great Lakes. Thus, the overall trend of a negative PNA is towards a milder winter in the USA “middle-East”.

You can go to the US NOAA web site and view the weekly reports on the ENSO “El Nino / La Nina” status; and also check-out the plotted-out daily index values for the PNA and NAO. Oh, there is also an index for the “AO”, the Arctic Oscillation, which roughly reflects the “weight” of the cold air mass over the Arctic; the heavier (more negative) it gets, the more likely Arctic air is to “slide downhill” through Canada and over the Lakes into middle-USA and maybe even the southern USA. What are these indicators saying right now?

As of today, the ENSO report is still “neutral”. The AO is in a small negative downswing, so cold air is ready to charge southward. The PNA is in the middle of a small positive bubble, so once that cold air gets over the Canadian border it is going to be blown towards the east coast. But the NAO is also in a positive blip, tending to keep the jet stream from dropping down too far. Add these things up, and we basically get what the forecasts say over the next 10 days: frigid cold air is on the way down into the heartland and will reach good old New Jersey shortly, but after a few sub-freezing days will tend to moderate. We will then be in early February, mid-winter, and neither of the trend factors has been so strong as to pre-determine what will happen in the second half. Although I am not a meteorologist, I would opine that February will basically be more of the same; some warmer spells, a few cold spells. And spring will be in the air by mid-March (with a few last snowfalls and frosty mornings, here and there).

But you too can keep an eye on these indexes and cobble together your own outlook on where the winter of 2012-2013 has been, and where it is going. Without all the overly-dramatic headlines! A little guide-book to all this was kindly put together for the citizens of North Carolina by their State Climate Office. Even if you live far from the Tarheel State, this guide will be of at least some help for everyone east of the Mississippi, who wants to know what to get ready for from Old Man Winter.

◊   posted by Jim G @ 4:53 pm      
 
 


  1. Jim, I would say a good exposition of the meteorology of winter, and I can’t disagree with any of it. I have noticed that often the weather in the Midwest eventually gets to the East – except! If you get a “Nor’easter” which barrels up the coast. That will block out anything coming in from the Midwest. I do tend to think that in general your winters on the East Coast tend to be somewhat less cold than ours here in the Midwest.

    Yet, I have a question behind all this meteorology (and I admit that I have never taken the time to check out and study the phenomenon for myself): I wonder just what effect the shift that took place some years ago in the magnetic pole had to do with all this meteorological “stuff”. I remember it being on the news that there had been a shift in the magnetic North Pole, I believe. It set me to wondering what effect, if any, that might have on the earth. I tho’t it must have some effect. But no one had more to say about that shift.

    I tend to take the “easy” way out: Early on in late autumn, close to winter, I check where the cold air from the Arctic is landing. Is it landing in Montana? If it does, then the West always gets the brunt of the cold, before it moves east and is attenuated somewhat. If the Arctic cold comes down thru Canada on to Wisconsin and Illinois, then the Midwest is due for a very cold winter. This winter the cold started dropping on Montana.

    Then too, very unscientific of me, I know: I watch the squirrels’ tails. This year they were more bushy than last years’ “warm” winter, but not as bushy as some of the really cold winters we’ve had in the Midwest. So . . . here we are: A kind of mix — just as you describe.

    So, science for the scientists. I guess old wives’ tales (is that what they are?) for the less scientific of us. MCS

    Comment by Mary S. — January 20, 2013 @ 3:38 pm

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