Today I want to start taking a look at the oscillations and sea surface temperature patterns that may begin to point us in the direction of our Winter Snowfall Forecast. First, I would recommend reading Joel's post from today at the link below. He explains each of the patterns. I am going to expand on his post on what those patterns look like for this Winter, and what that could mean for our snowfall.
Our goal at Opensnow is always to produce easy to understand weather discussions. We don't get into all the technical meteorological terms, there are other sites for advanced scientists to read about weather. The goal of discussions like this one are to briefly brush over general terms and patterns to help understand some of our process as it relates to putting out a Winter forecast.
If you have read Joel's post like I hope most of you have, you should now have a general understanding of ENSO and PDO. One of the tricky things for the forecast this Winter is that we are not sure what the exact ENSO state will be. Earlier in the Summer some of the forecast models showed a moderate to strong La Nina developing. More recent forecasts show that we have a 55-60% chance of a weak La Nina, or 40-45% chance of ENSO neutral conditions. Here is the most recent NOAA summary.
Let's look at the current sea surface temperatures.
A few of things stand out. One is the cooler water along the equator that is giving us weak La Nina conditions currently. Another is the cooling of the water off the West coast. The PDO is currently still positive/warm, but it has been cooling this summer and the latest reading is 1.25. And finally the warm blob that is showing up again in the Northeast Pacific. The question is what happens between now and the Winter?
Here is the JAMSTEC forecast with the water warming off the West Coast and cooling North of Hawaii.
Here is the average of the CFSv2 forecasts for the Winter sea surface temps over the last 7 days. You can see that it has ENSO neutral conditions along the equator as it mixes out the coldest water, and it also warms the water off the West Coast while cooling it to the North of Hawaii.
So the models are at odds on whether or not we see colder water along the equator and La Nina conditions.
Here is a look at the same CFSv2 forecast, this time showing temperature departures for the Winter, showing a warm U.S. If the ocean temperature forecast is correct I would expect that a trough sits North of Hawaii and a ridge over the West much of the Winter, with a trough in the East. We are heading back towards the pattern we saw during the 13/14 and 14/15 seasons. I would think this map would just have colder temps in the Northeast.
Here is a look at the same model forecast for Winter precip. This time I'm a little baffled as the sea surface temperature forecast would almost seem to support a pattern opposite this. But the CFS is known to have issues with long range forecasts.
Here is a more accurate analog of weak La Nina precipitation during the Winter.
As an example of how poorly the models predict the seasonal forecast this far out, let's look at the comparison of forecast to actual for last Winter. Completely the opposite for the West Coast.
What I like to do for the Winter forecast is to look back historically at Winters that had conditions similar to what we are expecting for this upcoming Winter, and then looking at what kind of snowfall we saw. That has worked fairly well in the past. So let's look at past seasons with a weak La Nina or Neutral conditions, and a positive PDO. I also throw in the QBO (quasi biennial oscillation) and look at years coming off of an El Nino or La Nina Winter. The current QBO is positive or West. It could switch to East by Winter but currently shows no signs of that.
I have taken the snowfall records on Donner Summit and matched them up with each condition. Below is the average snowfall in inches for the years matching the conditions, and how that compares to the average snowfall for Donner Summit. You can see that a moderate La Nina during a cold PDO and with a West QBO brings the most snowfall historically. 2010-11 epic season matched those 3 conditions.
I was hoping for a stronger La Nina this year and a colder Pacific that would flip into a cold PDO by Winter. It looks like we may have to wait another year for that. We are now expecting a warm PDO and a weak La Nina or Neutral conditions in the ENSO regions.
Let's take a look at the weak La Nina scenarios. Notice that we don't have a year since 1950 that was a weak La Nina in a warm PDO with a West QBO during the Winter. That could make this Winter even more uncertain. I did throw in East QBO below that. You can see most readings are in the 85-88% of average snowfall with these conditions.
Now let's take a look at the ENSO Neutral scenarios.
Again, 85-88% of average, BUT there are 7 neutral PDO seasons during a warm PDO since 1950, and of them 4 were below average and 3 above average. QBO doesn't seem to change those numbers. Some of the seasons like 13/14 only had 47% of average snowfall while other seasons like 92/93 saw 122% of average snowfall. So ENSO neutral Winters can go either way and are hard to predict.
If you take all the weak La Nina years that match on 3-4 of the conditions you get on average 85% of average snowfall, and if you take ENSO neutral years that match on 3-4 conditions you get 92% of average snowfall. I think 85-92% of average snowfall for this upcoming season is a good starting point. What happens with the warm blob and where the ridge sits near the West Coast this Winter will also be a factor to watch going into the Winter.
Here is a map produced by Golden Gate Weather showing the % of average precipitation for CA regions during a Cool Neutral ENSO, and a weak La Nina historically. This is precip only, not the snowfall analog I was using above. Again as with the snowfall it looks like we may want an ENSO neutral season to have the best chance at average precip or snowfall historically, but remember that is coming off averages of very wet and very dry seasons. Weak La Nina seasons seem a little more consistent with 80-90% of average snowfall with a warm PDO.
How often do we see swings in the PDO? It was thought that we were heading into another cold pdo cycle starting in 2007 like the one we saw starting in 1950, but then we went warm the last 2 years. The expectation is that we flip cold again next year.
Finally, let's look at some other Winter forecasts from other weather outlets. Here is the initial Winter forecast from Accuweather...
Here is the temperature departure from average forecast from weatherbell.com...
and their snowfall forecast....
weatherbell has the ridge closer to the West Coast and more amplified as they see the warm blob off the coast.
I wish I had better news. The good news is that nobody knows for sure. We saw a few seasons with big snowfall during ENSO neutral, warm PDO seasons. We will have a better idea of what the sea surface temps will look like as we go into the Fall. We will also be putting out our official Winter forecasts during the Fall.
For now we are still trying to get a handle on what the conditions will be like this Winter and how that will play out for snowfall. Again, I think that 85-92% of average snowfall is a good starting point. But it is only August so we have a ways to go with our analysis and hypothesis.