The number one question from viewers this winter has been “Where’s winter??” Typically, we should be in the middle of the wet season- days of rain and mountain snow with ferocious winds along the coast. Not this year. In fact, last month almost set the all-time driest December on record. Until Dec. 27th, we had only received 0.26″ of rain for the entire month! The last week however, we made up nearly 4.50″ of rain in about three days. And most of that was rain all the way up in the mountains as well, leaving ski resorts pulling out their hair as ski lifts sat in pools of slushy water. So far, through 9 days of January, we’ve only seen a little more than 0.10″ of rain.
So what gives? We’re technically in a “La Nina” pattern which generally for the Pacific Northwest means a cooler and wetter than average winter forecast. We saw that last year with ski resorts seeing record snowfall and being able to stay open, in some cases, almost to the beginning of summer. This year? Not so much. An important thing to remember is that even though we’re in a La Nina pattern, each pattern (year) can shape up differently than the previous ones. It can be difficult to say because we were so wet & snowy last year, it’ll repeat this year.
So what else can we use to try and peek into any potential future frosty forecast? One very useful tool is the Arctic Oscillation (AO):
Roughly put- it is a measure of pressure tendencies in the Arctic. Pressure changes in the Arctic can have an effect on weather changes thousands of miles away, even here in the Northwest. When the AO is trending positive (like in December), the cold Arctic air is locked up in the highest latitudes, which generally means we don’t see really cold and dry air. But when it trends negative, like the red lines are doing in graphic to the left, it creates sharp rises and falls in the jet stream and allows the bitterly cold North Pole air to plunge southward into much of North America. A great example of this is last winter when both Washington DC and New York were blasted with over 20 inches of snow on two separate storms. Both of those storms came during a strongly negative AO index.
The graphic above shows the AO index from Sept. 2010 to now. As you can see, the closer the red lines are together in unison, the more the model runs are in agreement, and thus the more reliable that data is to be accurate. So if we test it against another long-range model, here’s a snapshot of the EURO 8-10 day ensemble:
The EURO (on the left) backs up the idea of a big High pressure center setting up in the Arctic circle. This would buckle the jet stream and send the cold, dry air southward. The model on the right (GFS) shows that cold air blasting into the Midwest and Northeast. While the other slams it into us. IF and this is all still speculation to some degree, we see that airmass headed for us, then we can expect a sharp drop in temperatures and any storms that hit us will bring plenty of mountain snow and likely chances for very low elevation snow and ice. Interesting to see if it pans out, so far, I’m betting that it will. Someone over the next two weeks is gonna freeze, let’s see who wins out …
-Chief Meteorologist, Justin Stapleton