What’s happening with Sisters’ weather?

Except for a couple of light dustings of snow that quickly came and went, Sisters Country has been remarkably free of winter-like conditions even as we advance deeper into the heart of the cold season.

Last fall weather prognosticators advertised a La Niña was on the way that would likely result in a colder and wetter than normal winter. Not only was her arrival right on time, but she’s been gaining strength and is now classified as a strong La Niña. As mid-winter approaches, let’s see if we can sort out what has gone right, and wrong, with those outlooks.

The broadscale weather picture during a La Niña usually features a healthy jet stream that directs Pacific storms into the Northwest, as well as chilly air that overspreads western Canada. A team of forecasters at AccuWeather foresaw an early start to winter, with “…mountain snow and stormy conditions (that) may arrive in late fall for the Northwest, Northern California and northern Rockies… even the I-5 corridor from Medford through Seattle will have several opportunities for accumulating snowfall before 2020 draws to a close.”

Clearly, this did not happen. Western Canada hasn’t seen any cold temperatures yet.

Although La Niñas do sometimes delay the onset of winter weather compared to El Niño or neutral ENSO (El Niño Southern Oscillation) signals, this is way beyond what was expected. At this writing, the snow line in the Cascades is at about 5,000 feet and the west side of Black Butte is snow-free. According to the Pendleton office of the National Weather Service, the current snowpack in the Central Oregon Cascades stands at about 70 to 80 percent of normal.

The Sisters Ranger District Office in Sisters stopped collecting weather data after March of last year and its doors remain closed due to COVID concerns. In November 2020, temperatures were slightly warmer than normal in both Bend and Redmond, drier in Bend, wetter in Redmond. Last month it was warmer and much drier than normal at both stations.

Through much of December and the first half of January, a strong jet stream has indeed roared across the Pacific carrying with it a whole string of storms. But too often just before these disturbances make landfall, the jet stream weakens or splits, sometimes shunting the energy to the north or south of us.

The fact that the jet stream has been approaching from a slightly more southerly latitude is yet another abnormality. And this pattern has persisted for seven solid weeks! So, are we doomed to spend the rest of the winter with a malfunctioning La Niña? If there’s enough truth to new information that’s beginning to show up, perhaps not.

Medium-range forecasting models are starting to show a fundamental change in the broadscale weather pattern. A ridge aloft that has lingered over or near the Pacific Northwest for weeks is predicted to retrograde westward into the eastern Pacific and amplify. If this does occur and the alignment is right, this would open the door to much colder air from the far north.

Currently, the forecast is for a cooling trend to begin by about Thursday, January 21, that may continue through the weekend.

Even if this cold snap does materialize and lays down a good bit of lower elevation snow, can we count on this pattern to redevelop in February and March, or will it snap back to its early winter configuration? Of course, nobody knows the answer, but our end-of-season mountain snowpack and other water resources heading into summer may depend on it.

While scientists aren’t quite sure why this La Niña is behaving oddly, many of them suspect climatic teleconnections, the interaction of several different atmospheric circulations with varying time scales. Climate change may also be a player.

Looking ahead from now through the end of March, NOAA’s Climate Prediction Center is forecasting below normal temperatures and above normal precipitation levels for our area.