May was a cool and wet month as we saw a series of low pressures drop into the trough that sat over the West Coast much of the month. It was the pattern we begged for during the Winter. Squaw was the last resort to close and they saw fresh snowfall for their last weekend. We saw even more snow the 2nd half of May even to lake level after all the resorts had closed.
Total precip amounts ended up at 188% of average for the Tahoe Basin and 216% for the Truckee River Basin. The Truckee River Basin starts on the North end of the lake and Northward. That brings the totals for the water year to 65% for the Tahoe Basin and 72% for the Truckee River Basin. The Northern Sierra as a whole is at 74% of average precip for the water year.
Plenty of talk about El Nino already. More so this Spring because we are seeing El Nino conditions strengthen rapidly, especially in the Eastern equatorial Pacific. During the Winter we saw weak El Nino conditions only centered in the Central equatorial Pacific region. You can see regions 1+2 and 3 were below average Sea Surface Temps SST's last Winter and now have warmed drastically.
Currently we are seeing moderate El Nino conditions with 7 month running average of a weak El Nino state. The hype is over the forecast models showing a very strong El Nino by the Fall. But we have to remember they showed the same thing last Summer and showed El Nino conditions coming 2 Summers ago, and we never saw either. The difference this time is that we actually have moderate strength currently, it's not a forecast.
The other difference is that the conditions that contribute to the development of El Nino continue to favor further development into the Fall. The problem though is that eventually the conditions produced by an El Nino contribute to a reversal of the pattern and a decline in SST's over time. It tends to flip faster after a strong El Nino and many times we flip to a La Nina the following year. Most strong El Ninos don't last more than a year. Right now most computer models have the peak around November just breaking the threshold for a strong El Nino state and then declining to moderate by the heart of next Winter. So a strong El Nino in Summer may be too early to hold on through next Winter.
Here are a couple of good resources on how the El Nino effects the West Coast weather, and how it eventually causes its own demise.
So as of right now I don't think that anyone can predict a super strong El Nino through the Winter of 2015/2016. NOAA is currently saying an 80% chance of just El Nino through the end of the year. We have to remember the failure of the models the past couple of years predicting ENSO events. However, coditions are favorable to continue at least a moderate strenght El Nino into the Fall. That tends to bring some early snowfall to the Sierra in the Fall.
For next Winter it is too early to tell how strong the El Nino will be, and there are a lot of other factors to look at as well. We will look at those later in the Summer and into the Fall. Also, a moderate or strong El Nino doesn't necessarily mean big snow for the Northern Sierra. Historically moderate El Nino events during the Winter have only produced above average snowfall on Donner Summit 50% of the time since 1950. Averaged together the average is 469 inches because of some big winters like 68-69 and 94-45.
If we get lucky enough for a Strong El Nino event to last through the Winter, historically 4 of the last 5 had above average snowfall similar to strong La Nina Winters. The average snowfall for strong event Winters is 513 inches on Donner Summit to an average of 409 inches. Of more interest to me currently is the chance of at least moderate El Nino state during the Fall. In moderate to strong events during the Fall we average 131 inches on Donner Summit Oct-Dec. That is at 7000 feet so it would be more than that on the mountain tops.
So we will see, but it's too early to get excited about next Winter. Models have the forecast at moderate strength by next January with only 50% of the Winters with moderate El Ninos producing good snow. There are those years with only 235 inches like 1986-87.
More to come as we get closer to Fall.....
We will also have a trough digging into the West for the first 2 weeks of June it looks like right now. That could mean more below average temps and precipitation to start the month.