December outlook: below average snow likely

December 1, 2011

The start of December is upon us and it’s time to start sniffing for snow. To do this, we need to examine the state of two key players: La Nina (and its associated weather features) and the existence (or lack) of blocking high pressure systems at the higher latitude to direct cold air south. The current and forecast evolution of these players are not conducive to the development of any major snowstorms for the Washington and Baltimore areas.

That doesn’t mean we won’t see snow during December, but the the pattern looks like it is a below average one for snowfall. In addition, CWG’s winter outlook call for a colder than average December looks to be in jeopardy.


Part of the problem for snow lovers is that we are fighting typical La Nina climatology where cold snaps are often short-lived, if existent at all.

La Nina schematic (NOAA)

But this schematic shows just one variation of La Nina. How far the jet stream extends northward into Alaska and the exact position of the ridge over the Pacific varies during the year. When it is relatively flat and does not extend into Alaska (as shown above), the pattern across the lower 48 can be quite mild especially over the eastern two thirds of the county and cold air outbreaks are close to non-existent.

When it is amplified like the pattern shown on the schematic, cold air plunges south from northwestern Canada which can lead to shots of Arctic Air shifting into the middle of the country with some of that air making it into the East Coast. When the ridge shifts a little east of the position shown the Arctic Air can impact our region. However, usually, such Arctic intrusions are transitory as the tropical forcing tends to reconfigure the ridge back to its normal position farther to the west. Then the mid-Atlantic and especially the Southeast can get quite warm.

The typical storm track during La Nina winters is from the lee of the Rocky Mountains into the Ohio Valley and Great Lakes area unless there is high pressure blocking in place across eastern Canada and Greenland to force the storm track south of us. Without the blocking, when we do get cold shots, they usually moderate as the next storm approaches from the west and delivers southerly or southwesterly winds as it tracks to our north and west. Remember that the flow around low pressure system is counter-clockwise; therefore, a low to the southwest and northwest brings us warm, southerly winds.

Another signficant player this December will be the state of the high latitudes. Presently, the polar vortex, a deep pool of bitter cold air and low pressure, is much stronger than normal which has led to the Arctic Oscillation (AO) index to be very positive.

In the Washington/Baltimore area, to get major snowstorms, we usually need the AO index to be negative. A negative AO signals higher pressures at high latitudes than to the south (right now we have the reverse). Such a configuration of pressures forces cold air to shift south since air moves from areas of higher pressure to lower pressure. The southward shift of the cold air has the important effect in shifting the frontal boundaries and storm track southward. For snowstorms in our region, we need storms to track to our south.

The probability of getting two inches of snow in the Baltimore/Washington area is almost three times more likely when the AO is strongly negative versus strongly positive. Unfortunately for snow lovers, the current forecasts of the AO are for it to remain very positive.

Since October, forecasts of the AO index have generally been too low. Given the state of the polar vortex (deep low pressure at high latitudes), it is hard to see any switch to negative before late December or even more likely January. (Note that Judah Cohen, the scientist who has come up with a index for predicting the AO, said as much in our post about his work yesterday.)

The forecast for the Arctic Oscillation is positive through December (NOAA)

For a more in depth discussion of the AO and its impacts on the weather globally, see the University of Washington AO page.

Pressure (height) patterns at 500 mb (about 18,000 feet) during snowy and not so snowy Decembers in the Baltimore/Washington region. Blues/purples represent lower pressure whereas greens/oranges represent higher pressures.

Notice the differences across Canada eastward into Greenland. The snowy years had above normal pressures across that region while the non snowy months had below normal pressures (and no blocking to help force cold air south).

The other area where you see major differences (between the snow/no snow cases) is across the Pacific where the no snow cases have a ridge (warm colors) north of the Hawaiian Islands while the snowy months had a trough (cold shades) with their ridging along the west coast and over western Canada.

Left - pressure (height) pattern at 500 mb (18,000 feet) forecast for December 9. Right - pressure (height) pattern at 500 mb (18,000 feet) forecast for December 16. Blues indicate lower pressures (heights), oranges indicate higher pressures. (NOAA)

Note on the forecast valid at 7:00 a.m. December 9 (left panel above) how the ridge position (warm colors) on the left hand panel is very similar to that shown on the La Nina schematic that was shown earlier. The model does pull down Arctic Air but it initially is aimed at the northern Rockies. Conversely, the East Coast is under mild southwesterly flow aloft. If you compare that map to the snowy composite, notice that there is no trough (blue area) over the Southeast, instead, pressures are above normal. Also, no ridging is present across Canada or along our West Coast (which you’d like to see if you want a trough and cold air to support snow in the Eastern U.S.).

The ensemble mean forecast valid at 7:00 a.m. December 16 (right panel above) loses most of the amplitude to the various features because it averages out different ensemble solutions that vary greatly at such a long range projection. By that time, model runs have little skill in predicting the day-to-day weather but do sometimes have some skill in signaling the general pattern. The December 16 forecast retains the La Nina ridge signature in the Pacific albeit flatter. Such a ridge position is common for the no snow Decembers that makeup the composite. Just as significant, the run still shows a large area of blue across the higher latitudes signaling that the AO is still strongly positive. A strongly positive AO is also common during the no snow Decembers in Washington. The forecast pattern is a snow lovers’ nightmare.

Does that guarantee we get no snow? Of course not. The models can be wrong even in forecasting the broader scale features. I could be wrong about the AO and how predictable I think it is this December. Also, snow sometimes happens when the overall pattern is not that favorable, especially the lighter events. That said, my guess is that snow lovers are going to be disappointed this December.

This is not a pattern that favors getting a major snowstorm. It is a pattern than favors below normal snowfall unless the pattern unexpectedly changes later in the month.

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