Saturday, May 30, 2015

Does warm-up signal winter retreat?

After today, looking warm from here to Presidents Day.

Does warm-up signal winter retreat?

It's been quite awhile -- maybe since before last winter -- that the atmosphere has been this quiet, with not so much as a remote storm rumor on the horizon.

It would be premature, to place a white flag over the defeated, darkened snow. If winter has shown any consistency, it has been in its defiance of those would dare predict its behavior.

But a major warm-up is due to start tomorrow, with temperatures at least approaching the 50s on Monday. (This is not good news for motorists, by the way, and we'll be talking more about that later.)

It may cool down a bit at midweek, but it could get even warmer by the end of next week, although this is clearly not the winter to put too much trust in extended forecasts.

What is going on? For more than two months -- the middle of November to the end of January -- the atmosphere was locked into a pattern that delivered cold air into the East.

Nothing in the atmosphere occurs in isolation, but a big factor in this winter has been the so-called Arctic Oscillation.

Pressure differences between the Arctic and mid-latitudes vary over time, a phenomenon tracked by the Arctic Oscillation index.

When pressures over the Arctic are relatively higher, the index is said to be negative, and cold air is exported southward.

Since the 1970s, the index has tended to be positive, but it went dramatically negative right before the winter and stayed there until the end of last month.

It has been positive in February and looks to stay that way, at least for awhile.


Inquirer Weather Columnist
About this blog

Everyone talks about the weather, and here we write about it.

When we’re around and conditions warrant, we’ll keep you updated about what’s coming, but we will do our best always to discuss weather and climate developments in context and remind you that nothing in the atmosphere happens in a vacuum.

Tony Wood has been writing about the atmosphere for The Inquirer for 26 years.

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Tony Wood Inquirer Weather Columnist
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