Merry Christmas; Stormy End to 2011

December 25, 2011

It’s back to work for me this evening.  A nice slow time to get all those important Christmas Day items in order; mainly making 2012 vacation requests (a priority!), clean up emails, and get back into the meteorological groove.

Most of our region hasn’t seen more than a few hundredths of an inch of rainfall for about a month now.  In fact it was Sunday 4 weeks ago the last time we had more than .04″ here in Portland.  Today was a pitiful .05″ officially at PDX.  Ignore the .06″ below at PDX…it miscounted .01″ this morning when it was dry:

There is Good News and Bad News in this week’s forecast:  the dry spell is going to come to a quick end this week would be the good news.  And for storm lovers it appears we’ll see two very strong systems coming inland.  The bad news?  The biggest week of the ski season for the ski areas is going to feature heavy rain for about a 24 hour period, even at the higher resorts, from later Tuesday through Wednesday night.

IF YOU PLAN ON SKIING THIS WEEKChoose Monday, Thursday, or Saturday/Sunday.  Forget about Wednesday and Friday, and Tuesday will be a wet snow at best, more likely rain below 5,000′.

The big picture shows a strong westerly jet stream, the first in a month, punching right into the Pacific Northwest.  Several strong waves and their associated deep surface lows move by to the north.  The strongest is later Tuesday and Wednesday, but the low is so far north that we end up in the wet and warm sector south of the low.  We’ll be in the 50s during this period (regardless of the time of day/night) with gusty southerly wind west of the Cascades.  The low pressure center is far enough north that we’ll see gusts stay below 45 mph here in the western valleys of Oregon/Washington.  The snow level goes way up during the day Tuesday, at least up to 6,000′ by sunset, then even higher Wednesday.  Then we get a break Thursday.

The Friday surface low is deeper and tracks closer to us…00z GFS show it hitting the central or southern part of Vancouver Island.  It’ll be very interesting to see what the ECMWF does with the system.  Any closer to us or deeper and we’d see southerly wind gusts over 50 mph here in the metro area.  Once again, as the warmest air mass passes overhead with this storm, snow levels will jump up to at least 5,000′ sometime between late Thursday and late Friday.  This is the system I’m watching most closely this week.  10:30pm update:  Ohhh…that 00z ECMWF is even deeper, fast moving with big pressure rises behind, and makes landfall just off the NW tip of Washington…the plot thickens a bit.  This would be a signficant windstorm north of Portland or possibly here as well.  Two items grab my attention here:  6 hours later the pressure has jumped 20 millibars over Portland; a big surge behind the low from the south/southwest.  And 16-17 millibars surface pressure gradient from Eugene to Olympia.  A common guess for peak south wind gusts in the Portland metro area is 3.15 x EUG-OLM gradient.  That’s 50-60 mph here. 

Looking farther ahead…who would have thought that in a La Nina winter, we’d have a month-long dry spell followed by a week or 10 days of warmer than normal systems moving through.  Upper-level heights remain quite high through the first few days of January, so after a break over New Year’s Weekend, more mild storms move inland next week.

Check out the 00z GFS Ensemble forecast of 850mb temps.  Note the ensemble average (red line) is for warmer than normal 850mb temps most of the time through the next 7-10 days, then a bit cooler after that time.  What I don’t see is a big blast of cold air or low elevation snow as we head into early January.

Chief Meteorologist Mark Nelsen