Strong December Storm to Strike Pacific Northwest
10:30pm WINDSTORM UPDATE —- The 0z models this evening are stronger yet with tomorrow nights storm. The GFS now brings a 971mb low to the NW tip of Washington overnight tomorrow night. There is a forecasted 20mb pressure gradient from Eugene to Olympia. If this trend continues, areas from Salem north to Seattle will see the biggest windstorm since December 14th 2006. Winds could gust to 55mph in Portland after 10pm Sunday evening. With all the rainfall as of late, there will likely be power outages with falling trees. More details tomorrow morning.
Holiday travelers take note. After a “snow tease” this morning across Portland, a strong December storm just now developing in the Pacific will bring with it the potential for strong winds, followed by more low elevation snowfall across the Pacific Northwest beginning Sunday evening and lasting through Tuesday. Computer models are now coming into better agreement on the path and strength of a system slated to arrive Sunday evening. This system is currently developing well off the Washington and Oregon coast. The path of this system will ultimately determine the outcome of who will see the strongest winds and the best chances for low elevation snowfall Monday night after the storm passes. This will be the strongest system to strike the Pacific Northwest this fall season.
This storm system is currently forecast to take a similar path to that of the December 14th 2006 Hanukkah Eve storm which tracked into extreme southern Vancouver Island. The depth of the December 2006 system was approximately 970mb at its peak, which is nearly identical to what is forecast yet again tomorrow night. The December 2006 storm brought with it winds as high as 50mph in Portland and 75mph across some locations near Seattle. The system was then followed by much colder air aloft and some low elevation snowfall across the region. When comparing these two systems, there are some similarities as well as some differences as of Saturday afternoon. At this point there are still a few more model runs to look at when assessing this storms potential. If the track remains the same as currently forecast and the storm develops deeper than anticipated, the concern for a more widespread damaging wind event similar to December 2006 is possible. The next run of models will come out approximately 9pm PT this evening and will better help clarify the exact forecast track and strength of this system.
Regardless of the potential for strong winds, the system will pass by Monday morning, followed by much colder air and lowering snowfall levels. Anything that falls from the sky will be scattered in nature and not likely to cause any travel issues at the lowest elevations. The air mass that follows this system is forecast to be as cold if not colder than what we see over the area today. So, again I expect snow to fly (at least in the air) at all elevations Monday night and Tuesday morning. Hills above 750-1,000ft will likely see some sticking snow. Tis the season, right? The Cascade mountains will see several feet of new snow between now and Wednesday. This will cause holiday travel problems over the mountain passes, just as schools let out for the winter break. Longer range models continue to show colder than normal temperatures across the Pacific Northwest with more storm systems on tap next week. Snow levels will also remain below pass levels. Below is the path of the December 2006 storm (courtesy, Wolf Read), followed by forecast track of tomorrow night’s storm from the WRF-GFS and MM5-NAM models. As you can see, there is quite a bit of agreement on the path and depth of the storm system.
At the present time I do not expect damaging winds in the Portland area as the tightest pressure gradient remains offshore or just to our north, but that could change with this evenings models. In 2006 I recorded a 21mb pressure difference between Eugene and Olympia. That netted Portland wind gusts of about 50-55mph. This time it looks like 15mb or so as of the latest model runs. If the old Jim Little theory still works it goes something like this — PI x Max Eugene to Olympia gradient = max wind gust potential in Portland. With that in mind, Portland is looking at maybe 45mph right now (3.14 x 15mb = 47mph). Seattle is another story. The pressure gradient will be tighter up there. Could see some 60mph+ gusts up there if things develop just right. I would like to see a more northerly track to this system for strong winds in Portland. These west-to-east traveling systems rarely bring Portland anything exciting. Then there is the old Steve Pierce theory. When my right knee hurts, we are in for a storm. Right now my knee feels fine. So as I always say, “stay tuned!” If tonight’s models show a 965mb monster taking the same path, that will get my attention fast.
Steve Pierce, President
Oregon Chapter of the American Meteorological Society (AMS)