Now that the season is tapering off and we know what happened, I thought it would be interesting to post this info about La Nina and the difficulty of long term forecasts from a local meteorologist. Spoiler alert--La Nina is a good predictor for the PNW but not for everywhere else.
https://powderpoobah.com/alerts/latest
Viva La Nina
I was recently asked by a skier to explain my belief in La Nina as the one and only long range forecast tool (for snow) which is reliable – and it works mainly for the Pacific NW.
La Nina was the winter forecast for the NW last fall. La Nina typically means near to above normal snowpack. Sure enough, our current snowpack is healthy and above normal, helped with the robust above normal snowfall of February. We went from near normal (January) to above normal (March 1) in one month.
Long range seasonal forecasts are a challenge. Most are unreliable. However, there is always strong interest and demand, especially for skiing and winter snowfall in the West.
I have seen long range weather forecast predictions struggle to become more reliable, but it’s a very difficult problem. While short-range (daily) numerical computer forecasts have improved greatly, they are not reliable after 6 or 7 days. They are really best in the 1-5 day range.
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. Big, strong high pressure with dry weather can be forecast (dry), 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 observations. You cannot sample every initial parcel of air, so assumptions need to be made, which introduces very minor errors, which becomes major errors as the computer projects the weather days and weeks 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.
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, which affect preferred tropical thunderstorm locations, which in turn pushes warm air high into the 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 - thanks La Nina!
Of all the phases, La Nina while not perfect, is a very reliable predictor for above snowfall for the Pacific NW. However, it is not reliable for snowfall in other areas of the West. None of the other regions of the West have a good ENSO snow signal. When La Nina is evolving, it reinforces my confidence as a snow producer for the NW, but there are no guarantees – however it’s a good bet. IMO, no other long-range indicator is a good bet. The Southwest U.S. has modest confidence with snowfall (SoCal/AZ, NM, SoCO) in the El Nino phase, but the last strong El Nino was a dud for them with a snowfall bust.
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 it’s strong it’s less likely to change to a different phase during the season. Also, in some cases the physical conditions driving the change in air circulation can be strengthened. For example, very warm equatorial ocean water will produce persistent misplaced tropical thunderstorm complexes and reinforce ENSO impacts.
It’s too early to tell what phase we will be in next year. Usually, late summer is when we have an indication of the phase. We will transition into neutral soon, but next fall and winter is uncertain. Neutral is usually good (near normal snowfall) for NW snowfall, but not as reliable as La Nina.
Larry Schick
Grand Poobah of Powder