So why does La Nina impact the Inland NW? Colder water in the Pacific Ocean along the equator from the west coast of South America to the dateline suppresses thunderstorms while over Indonesia warmer water results in more thunderstorms. The wind and convection patterns in these areas impact the jet stream in the mid latitudes. How is the jet stream different? See map below
Typically, a common feature of a La Nina is amplified high pressure in the eastern Pacific resulting in a colder northerly flow over western Canada into the northwest. Meanwhile the pacific jet stream brings in storms from the west. Colder air plus the moisture equals increased odds for above normal snowfall in the lower elevations. The NMME (National Multi-Model Ensemble) that was issued in September agrees with this:
|NMME Temperature anomaly forecast for Dec 2017-Feb 2018|
|NMME Precipitation anomaly forecast for Dec 2017-Feb 2018|
|Average snowfall for La Nina (inches)|
If you are interested in more snowfall maps related to La Nina and how these numbers compare to normal, check out our La Nina briefing page found here.
So should we expect 59" in Spokane, 29" in Wenatchee, and 22" in Lewiston this winter? While these are the averages, it doesn't show the variability that occurs with each La Nina. Let's examine 3 cases of La Nina since 2005 and see what happened
2016-2017 La Nina
Many remember last winter. It was a cold and brutal winter, and snowy with several episodes of freezing rain for many. It was the 6th coldest winter on record for Ephrata, WA. Blowing snow on January 10th created very high snow drifts for portions of the Waterville Plateau into Grant County in the Columbia Basin leading to road closures. See picture below.
|Photo courtesy of Grant County Sheriff Tom Jones|
2008-2009 La Nina
|Downtown Spokane - Photo courtesy of Kerry Jones|
|Photo courtesy of Spokesman Review|
2005-2006 La Nina
This La Nina was slightly warmer than normal (about 1 degree) but was very wet with precipitation 130-150% of average for most. With the milder temperatures this meant less snow for most lower elevations. Wenatchee only got 6.9" for the winter with Spokane at 27.3".
Since this may be the second La Nina winter following a strong El Nino winter, we went back and looked at five analog years since 1950 where this occurred. What happened in these cases with the second La Nina?
As you can see, a tendency for cooler and wetter conditions, typical of La Nina.
Finally, we'll leave you with the latest winter outlook from the Climate Prediction Center
|CPC winter outlook for December 2017-February 2018 issued September 21st, 2017|
Equal chances for temperatures and slightly elevated odds for wetter than normal conditions.