By Bryan Allegretto, Forecaster Posted 1 year ago November 2, 2022
Northern Sierra 2022-2023 Winter Analog Forecast
Every season I take a look at the analogs comparing the current and expected conditions for this winter and how that compares to past seasons' snowfall. I usually put that out by the beginning of November, so here you go.
Last year my guess at the seasonal snowfall was 90-100 percent of average. It was a strange season with early and late season snowfall before and after the ski areas were reporting. So they missed some of the snow in their totals. The ski resorts as a whole around the Tahoe region averaged 87% of average snowfall. The official report from the Central Sierra Snow Lab northwest of the lake near Donner Summit (actually located in the northern Sierra) was 104% of average snowfall.
So the ski areas were 3% below my forecast and the Snow Lab was 4% above. Take the middle and we saw 95% of average, right in the middle of my forecast range? LOL, not sure how to score it with the weird season. But I dubbed it "the worst above-average ski season" as we saw barely any snow from January - March. The season before we only saw 71% of average, but I dubbed that season "the best below average season" as the snow fell evenly through the season and it was a cold season with the snow not melting much between storms.
Unfortunately, we are headed into a 3rd weak/moderate La Nina season that could be just as weird as the last two. We can split either way with above/below average precipitation and snowfall as the storm track is usually very active just to our north in the Pacific NW. We do tend to see colder storms from the north/northwest during the La Nina seasons and also a few warmer AR events.
As an example, last season the Snow Lab only recorded 97% of average total precipitation but 104% of average snowfall because of the colder storms and lower snow levels than average. We've seen that the last 2 seasons with the La Ninas. I've been keeping track of the snowfall compared to average vs the total precip. The positive seasons saw more snow as a % of average than precip and the negative seasons were the opposite.
The overall trend has been downward over the last 50 years with several seasons with snow levels averaging higher than the historical average. The other thing we tend to see during La Nina seasons, and usually more so with warmer water off the coast, is more rain & snow early and late season with below-average amounts in the heart of the season like last year. We look to start off wet this November, but the Canadian seasonal model suggests we dry out mid-winter again.
The European seasonal model is a bit more optimistic with near to above average precip reaching northern CA. I would tend to lean towards the Canadian above with the warm water off of the coast again this fall. If that cools and we have a true cold PDO pattern by mid-winter, I may be more optimistic as well like the Euro.
Of course, a few big AR events can push us over the top even with mid-winter droughts, as we saw last season with big storms in December and then April into May. During La Nina seasons true cold PDO (Pacific Decadal Oscillation) patterns we average106% of average snowfall vs 99% in warm PDO patterns. We are technically in a cold PDO based on the latest readings but much of the northern Pacific is warm still.
Looking at the QBO (Quasi Bienniel Oscillation) pattern, it looks like we are back into a western phase like 2020-21 vs the eastern phase we saw last season. In La Nina seasons during a QBO east pattern, we have averaged 109% over average snowfall vs 71% in QBO west pattern. But going back 70+ years the only season we saw a La Nina with a QBO west pattern was 2020-21. This would only be the 2nd.
Overall all La Nina seasons averaged together give us above-average snowfall at 108% of average. It decreases slightly based on the strength of the La Nina, but all strengths still average slightly above average based on 70+ years of snowfall data from the Snow Lab for those seasons. But that average comes from a low of 58% of average (1975-76) and a high of 164% of average (2010-11).
So even though a La Nina seems to analog with near to above-average snowfall, like I mentioned earlier, they can be extreme in either direction. The QBO phase and the warm eastern Pacific historically and in my opinion lean us toward a drier forecast outlook once into the heart of the winter. The closest matching season currently looks to be 2020-21.
With all of that in mind, my seasonal forecast guestimate for snowfall I'm going to go with is 75-85% of average this season. If the eastern Pacific cools by Winter from the early season storms, then maybe I'd bump that forecast up 10 points. Hopefully, I'm wrong. Don't worry, I've been way wrong in the past like in 2016 when I forecast 85-95% and we saw 155%, and then in 2018 when I forecast 85-95% and we saw 134% of average. Those were weak La Nina/El Nino seasons with a warm PDO and west QBO pattern.
Overall my average variance of the last decade is 7% more snowfall than what I forecasted. I tend to be conservative with all forecasting. I try to be as accurate as possible and err on the conservative side so you aren't disappointed. Of course, I've erred the other way as well, but I get just as upset or more than you when that happens.
Either way, here were are, heading together into season 17 of northern Sierra storm chasing and forecasting. I hope you've enjoyed the ride thus far as much as I have. Looking forward to another fun, frustrating at times, and hopefully snowy season. Any day on the snow is better than the office...