LA NINA & SNOWFALL STATISTICS FOR THE INLAND NORTHWEST

So what does La Nina typically mean for the Inland Northwest? La Nina winters often bring cooler than normal temperatures and above normal precipitation. This combination brings above normal snowfall for an average La Nina. These impacts are largely due to blocking high pressure that builds in the Eastern Pacific, resulting in a cooler northwest flow with the storm track aimed at the Pacific Northwest.

The above image shows the typical jet stream orientation for both El Nino and La Nina winters.

The reason for this change in the jet stream pattern is tied to the ocean temperatures in the equatorial pacific. The slight change in ocean temperatures impacts where clouds and precipitation fall, which in turn changes the position and strength of both the pacific and polar jet streams.

The above graphic shows observed Sea Surface Temperatures on the top image and anomalies on the bottom image from October 5 th, 2011. Anomalies refer to whether or not observed readings are below normal or above normal. The blue shading in the bottom graphic shows that Sea Surface Temperatures along most of the equator were below normal.

However there are other oscillations that have an impact on the atmospheric circulation. Thus, not every La Nina is the same with a wide range of possible snowfall outcomes. For example, in Spokane, the La Nina winters of 1949/1950, 1955/1956, 1974/1975, 2007/2008, and 2008/2009 resulted in over 80 inches of snowfall for the winter season. However, only 30-32 inches of snow was observed in the La Nina winters of 1967/1968, and 1970/1971. So while above average snowfall is more likely for a La Nina winter, it is not a guarantee.

The links below this paragraph show average snowfall amounts for a La Nina winter and how this compares to the average of all winters. The top link shows a map of how La Nina Snowfall compares to average. The map shows that for the eastern Columbia Basin extending east to the Spokane/Coeur D’Alene area, Palouse, Lewiston area, Blue Mountains, Camas Prairie, and Central Panhandle Mountains snowfall for a La Nina winter is 120-140% of average. Meanwhile, along the East Slopes of the North Washington Cascades and areas along the Canadian border snowfall is typically only slightly above average. The bottom link shows the number of inches of snow for an average La Nina winter. For both links, the data is listed by site to the right of the map. Clicking on the site will bring up a chart displaying average snowfall for a Neutral, El Nino, and La Nina season. The average line is simply the average snow for all seasons.

Percent Increase
click image for map

So how did the La Nina winter of 2010/2011 compare to what the averages show for a La Nina? The table below shows average snowfall for a La Nina winter and what the winter of 2010/2011 delivered.

Data for the winters of 1949/1950 through the winter of 2010/2011 was used in this study. For sites that reported missing or erroneous snowfall data for portions of the winter season, then that year’s data was not used in the study. For some sites this meant only one or two years of missing data in the 61 year period, while for other sites that meant several years of data not used in the study. To determine whether or not each year was an El Nino year, Neutral year, or La Nina year, the Climate Prediction Center historical ENSO table was utilized. The link to the table is listed below.

http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml