The weekend will have variable amounts of cloudiness, but it’s very unlikely that we will see any showers out of them, although there is a small chance of a shower in the mountains north of Tahoe Sunday afternoon. Temperatures will be mild…near the 70 degree mark in the valleys…and the winds will range from light to non-existent on Saturday, with the winds picking up to brisk levels on Sunday. The pattern stays the same through Tuesday, after which a colder storm breaks away from the flow from the northwest and could bring some valley rain and mountain snow by Wednesday.
So what causes an El Niño? In a “normal” pattern, trade winds just north and south of the equator blow from the northeast and southeast respectively. That wind direction essentially pushes the surface water from the eastern Pacific toward Asia, and pulls up colder water out of the deep, which cools the equatorial waters off of South America. But periodically those trade winds weaken, or at times even reverse, which allows all the hot surface water near Asia to slosh back over to our side, causing abnormally warm water conditions in the eastern equatorial Pacific. That’s an El Niño.
But how does El Niño affect the weather? It has its greatest effect in the winter months. One main thing it does is to spin up a stronger sub-tropical jet stream coming off the Pacific Ocean, and sends it farther to the north. Since everything is connected in the atmosphere, that also tends to push the polar jet stream farther to the north. In general, this results in wetter than average conditions in southern California (and across the southern tier states), while on average the Pacific Northwest, losing the benefit of the polar jet, tends to be drier than average.
Well, that’s all fine and good, but what does it mean for us here in the Reno/Tahoe area? Therein lies the rub. We are stuck in the middle of the wet to dry transition, so in most years an El Niño really has no predictive value for us at all. We have had very wet El Niño years and very dry ones, and everything in-between.
There is some encouragement however with this winter. If it is a very strong El Niño (and this one appears to be one of the strongest ever measured), it tends to move that “wet” line farther to the north. While it is by no means a sure bet, I am cautiously optimistic this winter will be reasonable wet.