We’re in the midst of a La Nina winter in the Pacific Northwest and the weather is about as normal for a La Nina winter as one can expect. Generally, La Nina years are defined by periodic cooler sea temperatures in certain areas of the Pacific Ocean. But the effects from these cooler seas are characterized by cooler winter temperatures and more rainfall in the Pacific Northwest. Neither of these are guaranteed, but they are more likely to occur.

On temperatures, Figure 1 shows two charts. The upper chart shows the daily average temperature departures from the 30-year climatic normals at Seattle-Tacoma International Airport from 2000 to the present. The red bars (or really lines at this scale) reflect daily average temperatures which exceed the 30-year average for a particular day. The blue bars represent days where the average temperature was lower than the 30Y climatic average. The temperatures were taken and are published by the National Weather Service.

There’s a fit curve on both charts in Figure 1. The top fit curve is a 365 day smoothing curve of the daily temperature departure value. Interestingly, sometime around 2013, this average fit curve moved into the positive range and has remained above zero for about eight years now. There might be a variety of reasons for this, but we’ll save speculating and a more in-depth look for another day.

The second chart in Figure 1 shows the same data since October 1, 2020. This is unofficially the start of the water year in the Pacific Northwest, the time of year when rain storms blow in off the Pacific frequently and more consistently. It also roughly matches our ‘cold’ season.

For the most part, the cold season in Seattle continued to be a bit warmer than the 30-year average except for may to one-week cool period, one in October and one in November. But since the first of the year, we have been seeing a cooling trend, slight at first, but more pronounced beginning in late January. And this is a cooler-than-most-winters trend. There is a fit curve on this chart as well. This is what is called a LOESS fit curve of the data since October 1, 2020 and it shows the general near-term cooling trend late this winter after a warmer start.

They say the effects of La Nina years really kick in after January, and that appears to be the case this year when it comes to temperatures.

Winter precipitation amounts tend to increase from long-term normals during La Nina years in the Pacific Northwest. The top chart of Figure 2 shows clearly this is a wetter than normal winter in Seattle. It also shows this is far from the wettest and that it hews closer to the normal than to the extreme winters.

The second chart in Figure 2 shows that most of the extra rain fell in a 5-6 week period beginning in early December and ending in mid-January. Roughly 2.5-3X the amount of rain fell daily during this period this winter than the eight weeks prior or six weeks following it. We’ve had a lot of cloudy days in February, and even a lot of snow – about 12 inches a few weeks ago. But the relative amount of water falling from the sky hasn’t been that much. And with both the warmer temperatures (Figure 1) and below-average rainfall we experienced in October and November, it seemed early on La Nina was initially going to be a bust.

We move into March next week, and April 1 represents the traditional ‘end’ of the wet season. It will be interesting to see if the more moderate rains continue and if the cooler-than-normal temperatures extend into early spring. I’ve seen a few daffodils and blooming trees already, but at least in my own yard and garden, things seem to be a little slow coming in this year. We often get an early start to spring beginning in February. I haven’t noticed that to be the case so much this year.

The water year precipitation charts might be difficult to read in this application. A zoomable version can be found here.

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litterrocks, blog, weather, climate, temperature, precipitation, Seattle, SeaTac, 2021