We could see some cooler and unsettled weather in the last 10 days of September. We have an El Nino season setting up and we will start to look at what that could mean for CA weather.
Short Term Forecast
The long-range models are suggesting some troughing over the West and West Coast starting around the 21st of September and possibly sticking around for several days.
The long-range models show some storms dropping into the West during the period, with a few trying to bring a storm or two through northern CA.
It's still early so we will keep an eye on the pattern. Late September is always a time of year when we can start to see some weaker systems move through that can bring some light snowfall to the upper elevations. Just enough to start getting us excited for the winter season!
It's still a little early to be putting out any official winter forecasts, but we are starting to look at the current and forecast atmospheric and ocean conditions that can affect our winter patterns.
Historically, El Nino conditions, when water along the equator is warmer than average, bring an average pattern that has high pressure along with warmer and drier weather to the Pacific NW, and the jet stream aimed at southern CA with a wetter pattern.
El Nino seasons are typically warmer than La Nina seasons, which means we tend to deal with snow-level issues more often in the Sierra. We look at other conditions such as sea surface temperatures in the northern Pacific as well as wind patterns.
Looking at the current sea surface temps, you can see that there is a lot of warm water along the equator as well as in the North Pacific.
Interestingly, the latest PDO reading, which measures the northern Pacific Ocean temps compared to average, still shows a negative reading. There has also been cooling in the northeast Pacific recently.
We currently have a 3-month average in the ENSO region of 1.1 degrees above average for June-August. That would be moderate strength El Nino conditions, but El Nino is not official until we see that average be above 0.5 for 5 straight months.
The forecast models suggest continued warming with some showing the 3-month ONI average reaching up to 2 degrees above average.
That would put us into a Strong El Nino category and conditions this upcoming winter season. The model spread is between moderate-strong El Nino conditions. The last time we saw this was during the 2015-16 season, which had near-average precipitation and 96% of average snowfall at the Snow Lab near Donner Summit. Not the big season you would think of during El Nino.
There is a bit of a misconception about El Nino seasons thanks to really wet ones for CA like 1997-98. They are not all really wet seasons. During Moderate and Strong El Nino seasons CA sees above-average precipitation about half of the time.
Moderate El Nino Seasons:
Strong El Nino Seasons:
You can see that sometimes the Pacific NW is actually the wettest area during El Nino seasons instead of southern CA, like during the 2015-16 season. Either way, Tahoe tends to me in the middle and can go either way. So there are no guarantees of a big winter just based on El Nino.
Throwing in the QBO wind pattern and the PDO sea surface anomalies, we can try to separate out a few winters historically that may line up similarly to the forecast for this upcoming season. The 4 closest seasons I'm coming up with currently are 1951-52, 1965-66, 1972-73, and 2009-10.
The least snowy of those was 65-66 with 86% of average snowfall. The other 3 seasons saw above-average snowfall, with the biggest being 51-52 which saw 205% of average. If this ends up as a strong El Nino then we would be closer to 65/66 and 72/73 which saw an average of 99% of average snowfall.
Less snow can also be from rain and higher snow levels during warmer storms associated with the southern storm track in El Nino seasons. That would put us close to what the map below shows for the historical snowfall averages during a mix of Moderate-Strong El Nino seasons.
Can strong El Nino seasons be very wet and snowy? Yes. Moreso over southern CA? Yes. Sometimes a reverse into the Pacific NW? Yes. With Tahoe typically going either way by being on the northern and southern edge of the storm track? Yes.
So that doesn't give us much confidence in a forecast. The southern branch of the jet stream should be enhanced by the El Nino conditions and we shouldn't see a well below average and dry seasons. The odds lean towards seeing at least a near-average season with the possibility of slightly above or below average, and an outside shot at another big season.
Snow can be below average while precipitation is above average during El Nino seasons with higher snow levels typically on average. If we get a really wet season we tend to still see above-average snowfall overall even with the warmer storms.
That is a taste of what we are looking at for now. More to come through the fall.