A La Nina pattern across the Pacific Ocean typically means a cooler and wetter winter across much of the Pacific Northwest.
We've been hearing a lot lately about the term "La Nina" but all it basically is colder than normal temperatures in the Pacific Ocean along the equator.
Typically temperatures here are very warm, even as we enter the winter months, but during a La Nina year, temperatures here are in the 60s and 70s, which is much colder than normal.
How does that cold water actually form? We have very strong trade winds that blow from Peru to Darwin, Australia. That causes the ocean to actually move. The top layer of warm water is shifted to the west and that is where all of the warm water ends up located. With that warm water pushed aside, that allows for deep cold water to be pushed to the surface, cooling the surface temperatures and allowing for the warmest water to stay well to our west.
The atmosphere and the ocean have a very close interaction with one another, so this changes the overall weather pattern that we see across North America during the winter months when this La Nina signature is present.
This allows for a very large system of high pressure to form over the northern Pacific Ocean. That pushed the polar jet stream well to the north. With the jet stream coming directly out of the Northwest, it ushers in very cold, polar air across northern Alaska and parts of western Canada as well as the Pacific Northwest.
With that blocking high pressure, it also causes the meandering Pacific jet stream to kind of push toward the Pacific Northwest, also driving in tropical, moist air. The moist air interacts with the cold air in place and most of the precipitation will fall as a cold rain or snow. We also see colder than normal and wetter than normal. We are currently entering a La Nina winter so there is a very good possibility that our pattern this winter will look very similar to what you are seeing right here.
Snow levels vary greatly around Southern Oregon and Northern California. Despite its close proximity to the Pacific, we usually only see about an inch of snow yearly out on the coast.
Here are the snow levels for the interior valleys including the Rogue. Because Mount Ashland intercepts a lot of Pacific storms, we see about 11 inches of snow in the city of Ashland itself. About 7 inches is our yearly average in Medford and around 5 inches is our average for Grants Pass.
For the areas east of the Cascades, Klamath Falls averages about 38 inches, Bend averages 33 and Lakeview which is further to the east and much drier, averages around 14 inches.
For Northern California, we see very close amounts for Yreka and Weed...between 18 and 20 inches, but Mount Shasta City, just below Mount Shasta, sees a lot of snow throughout the year, just over 100 inches.
For the mountains, Mount Shasta averages about 275 inches. We see about 300 inches on Mount Ashland, and Mount bachelor tops out at about 370 inches.
Chemult holds the record in Oregon for the most snow in a 24-hour period. In 1959, Mount Shasta received a record 189 inches from a single storm over a period of roughly 5 days. Mount Shasta holds the record for most snow from a single snowstorm in the United States. Crater Lake's biggest year ever between 1928 and 1929 when it received 839 inches.
So the question is if this is a La Nina year, what impact that would have on the snowfall for Southern Oregon and Northern California mountains. The answer is that it depends. We are right in between the stereotype. The stereotype is that La Nina will bring wetter than average precipitation in the pacific northwest and it will be drier in California, while an El Nino year is the opposite, drier in the pacific northwest and wetter in California. We happen to live in between.
The average precipitation is 533 inches at Crater Lake in a year. Out of 4 strong La Nina years, two of those years featured below average snowfall. So what about El Nino years when it is supposed to be drier? We had above average snowfall during 2 of those years and 1 year in 1991 when precipitation was well below average.
What does this mean? There is not a really good correlation between El Nino and La Nina and below and above average snowfall totals.
So what should we expect during a La Nina episode? Well, it depends!
