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Don Paul: Winter weather might be postponed, but not canceled

Anyone remember the weather in December 2015, when the mean temperature was an astonishing 12 degrees above average, and there was a total of 1 inch of snow? That will not be the story of this December.

This is the look of the winds at about 18,000 feet late Monday, according to the National Weather Service. Note the orientation of the lines, running across the Pacific and into the lower 48 mainly from west to east.

This kind of a pattern keeps the arctic air bottled up to the north and keeps Pacific air behind passing cold fronts instead of arctic air.

Next, we look ahead to the projected look of those winds after the first week of December, also from the National Weather Service.

Note the new orientation of the lines, which ridge up over western North America, forcing a dip, or a trough in the polar jet near the Great Lakes, Ontario and Quebec. This delivers arctic air that has been forced up and over that western ridge. What you’re looking at is an “ensemble” of many model runs that are showing unusually good agreement with each other so far in advance.

When we look at projected lower level winds, at 5000 feet, watch the trend in the loop (hit the right arrow at the bottom) of this American model/GFS courtesy of from Dr. Ryan Maue.

Here is the European ensemble output, which displays only to Dec. 7, before the full pattern change. But even here, you can clearly see that strong western ridge building up that will force the trough near the Great Lakes to amplify, or deepen, in the days that follow.

As I said, it is unusual for virtually all ensembles to show such good agreement nearly two weeks in advance. However, another thing most meteorologists can agree on this far in advance is we cannot even begin to guess snow or lake-effect snow potential when this pattern change is so far out in time. We cannot know how the low-level winds will be directionally oriented over Western New York; we cannot know how much water vapor there will be in the arctic air (dry air over the lakes greatly diminishes accumulation potential); and we cannot know how much winds will shift in direction from the surface up to about 10,000 feet (too much shifting – called directional wind shear – can tear lake effect bands apart).

We CAN say the presence of true arctic air, replacing the Pacific air now in place, will generally increase the probability of more snow during this coming pattern, for however long it holds in place. On the latter point, several long-range specialists using different predictors from one another are projecting this colder pattern – with some normal ups and downs – will have more persistence. Just don’t ask me if it’s going to snow on the afternoon of Dec. 12 in Sloan … not without a certified check, my friend.

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