The Long Range Outlook

December 29, 2011

Lots of moisture surging inland again this evening for one last bout of valley rain and heavy mountain rain.  The snow level has climbed back up to around 7,000′, forcing Timberline to shutdown night skiing for a 3rd night in a row.  Somehow Mt. Hood Meadows and SkiBowl are stubbornly hanging on to a very wet night.

The good news is the big dump of snow that will be here in just about 8 hours.  Much colder air behind tonight’s cold front moves into the Cascades around daybreak, lowering the sticking snow level down to around 3,000′.  Models are showing strong westerly wind at ridgetop elevations all day tomorrow along with plenty of moisture for heavy snowfall.  Maybe a foot to 18″.  It’ll be a real powdery snow too by tomorrow evening as temps fall.

The snow level eventually drops to around 1,500′ or even a bit lower late Friday evening as the coldest air pours inland, but by then the showers will probably be all gone (sounds familiar doesn’t it?).  Nevertheless, some of you in the hills above 1,000′ will probably wake up to a dusting Saturday morning.

We get some gusty east wind New Year’s Eve and Day.  Beyond that, the weather still looks real dead through most or all of the first week of 2012.  The reason is the stubborn upper-level ridging re-asserting itself over the west coast of North America once again.  This weakens Pacific storms and gives us warmer than average temperatures (at least in higher elevations).  What little precipitation we get next week may fall as rain once again in the Cascade passes.

Beyond next week, there are signs of a change.  All models show the ridge shifting to the west within a few days of January 10th.  Yes, it’s 2 weeks away, but we’ve seen several model runs now showing SOME sort of change after 10 days or so.  Check out these maps (click for a better view):

First the 12z GFS 850mb temp ensemble chart. It goes out to around 16 days.. 

The 12z ECMWF…

Note they both show a downward trend in the latter half of the period. Today the 4 week ECMWF came out as well. It’s run twice a week out to 4 weeks. Here are the weekly 500 millibar height anomalies. Orange/red areas are higher than average heights (ridging), blue areas lower than average (troughing).

 


Notice the current ridging over the west coast shifts well to the west from the two week mark onward. This doesn’t give us any detail of course, it’s just the weekly average. But having a trough nearby would spell much cooler weather than we’ve been seeing. This would also be far more typical of a La Nina winter.

Chief Meteorologist Mark Nelsen