The snowfall in the country this week is limited to light snowfall in the Rockies and heavier snow in areas that don’t have any ski resorts. The theme is the same as we have looked at the weather for each ski region this week. A large area of high pressure off the coast is sending the jetstream and storms up into Alaska.
In a La Nina season like we are going into the jetstream normally slams the Pacific NW and things shouldn’t be any different this season. We have been off to a good start already in November and this break should be brief. As the jetstream gets stronger going into the Winter it will be harder for a ridge to build as far North as it currently is. Winter will soon return to the Pacific NW and it should be a hot topic again this Winter.
Here is a map of the current pattern showing the ridge out around 45n 130w in the Pacific.
The position of the ridge is far enough offshore to allow the cold trough dropping down across the Rockies to scrape the Pacfic NW with some cold air. That will continue this weekend as another cold trough comes down to the East and brings a little more cold air in. What we need though is for the ridge to shift West out into the Pacific, idealy past 150w.
There are several factors causing the ridge to sit where it is including sea surface temperatures in the Pacific, the position of air masses globally, and several teleconnection patterns. But one of the main things we watch is the PNA (Pacific North American) teleconnection. In the postive phase which we are currently in favors a ridge off the coast and in the negative phase which we hd in November it favors a trough off the West coast. The forecast over the next 2 weeks is a trend back towards negative.
The other thing we can watch is the MJO (Madden Julian Oscillation). It is currently moving through the Indian Ocean towards the Maritime Continent. As it move from 70E in the Indian Ocean towards 120E, or phase 5 on the chart below, it tends to support a trough along the West Coast this time of year. The forecast puts it there two weeks from now. Beyond that it can support a ridge but it looks like it could stall and die in phase 5.
The forecast models can wait until only a week out to pick up on some of these signals, but let’s look at what they are showing a week out around the 7th. This is the GFS model showing the ridge shifting out toward 140w.
That will allow the cold troughs coming dow from Canada to dig further West into the Pacific NW bringing a more direct shot of cold and a better chance at some light snowfall. Now let’s look at 2 weeks out from now. Notice how the models now suggest the ridge has shifted out to 150w in the Pacific and a bit South to 40N.
That is a continued trend of the ridge retrograding Westward and allowing the troughs to dig even further offshore and to pick up some moisture off the ocean. Now finally let’s look at the first sign of a more significant storm around the 16th.
Of course this is looking really far into the future and it could change, but as we are in a break from the snow we have to look at any possibility of a pattern change back to storms and snow. I will be watching the trends in the pattern and will continue to update over the next couple of weeks. BA