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LA Nina and the NW winter

Hello skiers,

No doubt you can see and feel the changes in the weather. Expect some minor snow (trace 2) on the highest slopes this week, with Whistler seeing much more. The snow will fall on Tuesday and again next weekend, with two separate but weak storms.

I am optimistic the snow season will get off to a good start and will be reasonably consistent with La Nina helping to aim storm track in our direction.

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What is La Nina and how does it help the NW snowfall?

Long range seasonal forecasts are a challenge and most dont pan out. However, there is always a strong interest and demand, especially for skiing and winter snowfall in the West, so I'll give my best. Fortunately, La Nina is evolving and that helps in the long-range forecast for our winter ahead

We all know short-range (daily) forecasts have improved greatly, but they are not reliable after 6 or 7 days. They are really best in the 15 day range.

I have seen long range weather forecast predictions struggle to become more reliable over the years. But its a very difficult problem because after a week or two the atmospheric is chaotic.

Forecast accuracy depends on the complexity of the weather situation and what weather element (timing, duration, location, rain, snow, wind, temp etc.) you are trying to predict.

A big, strong high pressure with dry weather can be forecast, even beyond 7 days sometimes! Theoretically daily weather will never be reliable after 14-16 days, due to the chaotic nature of weather with inadequate initial weather observations. You cannot sample every initial parcel of air, so assumptions need to be made, which introduces very minor error, which becomes major errors as the computer projects the weather days ahead. Plus, you cannot model every nook and cranny in complex mountain terrain like the Cascades, which can also introduce errors. Our mountains accentuate and often change airflow patterns, compounding computer forecast errors.

Enter ENSO.

The seasonal forecast phases which have at least some limited forecast reliabilities are La Nina, Neutral and El Nino (collectively called ENSO El Nino Southern Oscillation). But even those different phases can be unreliable considering their specific weather impacts and where they impact.

ENSO phases are connected to changing equatorial ocean temperatures in the Pacific, which affect the preferred tropical thunderstorm locations, which in turn pushes warm air high into tropical atmosphere where the disturbed airflow and temperatures reaches northward and affects the winter storm track in our mid-latitudes, and aims the winter storm track in our direction. Tons of snowfall in the Cascades and great skiing is the result, if all goes well but there are no guarantees.

Of all the phases, La Nina while not perfect, is a very reliable predictor for above snowfall for the Pacific NW (WA, OR, ID, MT, BC). However, it is not reliable for snowfall in other areas of the West. When La Nina is evolving, it reinforces my confidence as a snow producer for the NW.

Also, to give a forecaster stronger confidence, it matters how strong the magnitude of the ENSO phase is. The stronger the phase the more confident the forecast. The reason is, if its strong its less likely to change to a different phase during the season. This years La Nina is of similar strength to last year, but it doesnt mean an exact repeat. Snow storms tend to come in cycles which last a few days or more than a week. Also, the weather today does not remember what happened a month ago. So, a snowy or a dry month does not telegraph ahead what will happen. The lesson is: when the snow is good GO!

As the snow season unfolds, La Nina will not dictate the weather, but rather tend to nudge the storm track our direction more times than not. That will pile up the snow in the mountains of the NW making for a fun ski season. Get ready. I expect an early season start near the Thanksgiving holiday weekend, maybe earlier.

Larry Schick - meteorologist
Grand Poobah of Powder

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