Historical Context of the 2007-08 Winter

So far the Winter of 2007-08 has certainly been one to remember. The graph below shows the snowfall for the season so far, as well as comparing it to an average winter.

But at this point how does it stack up to previous winters in the Inland Northwest? There are several ways to look at it. Here are just a few.

What makes the 40" somewhat more impressive is that we haven't seen anything close to this in recent Januarys. However, Spokane did receive 42.7" of snow in December 1996, so it's been about 12 years since we've seen snowfall like this.

For the winter so far, Spokane has received 63.0" of snowfall. This puts this winter 7th on the all-time list of snowiest winters through January 31st. Note that this doesn't mean that we're in the 7th snowiest winter ever. This is only through January 31st.

Since winter's not over, we'll have to wait and see how this year measures up to the snowiest winters. But even if we don't pick up any more snow for the rest of the season, 2007-08 would rank 20th on the list of snowiest winters dating back to 1881. Here's the list of the 10 snowiest winters. Note that 2 of the snowiest winters were from the 1990s.

So the question is "why has the 2007-08 winter been so snowy?" The answer is largely two-fold. First, a moderate to strong La Nina was in place this winter. La Nina is defined by the cooler-than-normal sea surface temperatures in the equatorial Pacific. The image below shows the SST anomalies this winter; blue indicates cooler-than-normal temperatures.

The affects of La Nina on Pacific Northwest weather are not always consistent, largely because there are other factors besides La Nina which govern the weather pattern. But for this winter, the weather pattern is rather similar to a composite of La Nina winters. The image below shows the typical jet stream pattern over the Pacific for El Nino (left) and La Nina (right) winters. For the La Nina, the pattern is typified by a high pressure ridge in the Gulf of Alaska. Pacific storms must either ride up and over the ridge, descending into our area from the northwest, or they need to "break under" the ridge.

While we did see one of these southerly events in early December, the predominant storm track has been from the northern Gulf of Alaska. The image below shows the average jet stream for December and January of this winter. Pacific storms this winter have typically moved into the northern Gulf of Alaska before dropping into the Pac Northwest. These storms are colder than usual and keep the precipitation all snow. In a non-La Nina winter, Pacific storms will often approach our area from the southwest. This gives us the more typical scenario of wet snow changing to rain as the warmer Pacific air moves inland.

While our temperatures haven't been much colder than normal (December was actually a bit milder than average), the temperatures aloft (around 5000' above sea level) have been colder than average. The image below shows the average temperatures for this winter (thin black lines in degress Celsius), along with shading for below (blue and purple ) or above (green and yellow). As you can see, temperatures from Alaska to the Northwest US have been below normal, contributing to the snowfall.

An additional reason for the snowy winter has been the frequency of storms. Typically in winter, the western U.S. will experience a few "blocking ridges" of high pressure. These often last for 7-14 days and steer all of the storms away from the area, leaving us with dry but foggy/cloudy weather. This winter, we haven't seen much of this weather pattern. Any breaks in the storms have been rather short-lived. For December and January, Spokane averages 28 days of precipitation. This year there were 40 days in this two month period where rain or snow fell.