Monday, November 7, 2011

Boring with a Hint of La Nina

Sometimes the early part of  November is the quiet before the storm;  this has been true for the last few days and will continue for the remainder of the week.  But don't forget:  the last week of November on average brings the stormiest, wettest, meanest weather to the region.  Meteorological ground zero for the Northwest.  Turkey on the barbecue.  And sometimes lowland snow--like November 22nd of last year.

Just to show you what I mean by boring, here is the National Weather Service forecast for this week:
Some clouds, some sun, a few light showers, temps reaching the 50s and dropping into the mid to lower 40s.  Enough to drive a meteorologist mad.

But there are some interesting aspects of the atmospheric situation of the next few days, some indicative of our old friend-La Nina.  Take a look at the 500 hPa upper level chart for Tuesday at 1 PM. A ridge over us (thus dry) and a high amplitude trough digging south from Alaska.

No umbrellas needed here.   During the next few days an amazing thing happens.  The trough deepens into a closed low off of California and becomes isolated from the jet stream to the north over the north Pacific and southern Canada. (remember the air flows parallel to the lines and is stronger where the lines are closer together).

There is a fancy name for this situation--a cut-off low:  the low is cut-off from the jet stream.   We are left in the dead zone...not much flow, not much action.  And you will notice a BIG ridge (High) over the central Pacific--we will get back to that. Here is the precipitation over the West Coast for the next 72h.  The action is pretty much all offshore.
But then things change a bit for Friday.   The ridge builds offshore  and the Northwest is in northwesterly flow (see below).  A weak trough is embedded in this flow..which should bring some clouds and perhaps some light rain. Not too scary, but this pattern is reminiscent of a La Nina configuration, like last year when we were stuck in cold, northwesterly flow for months.  If a trough develops with more amplitude and the ridge extends more into AK, we approach the canonical snow pattern.

On on Saturday...more of the same, but a bit stronger (see below).  Expect a low snow level and decent snows in the mountain.  The Mayor and SDOT don't have to worry...yet.

Don't forget to vote tomorrow...

5 comments:

windlover said...

I've been pretty bored with the weather so far this year. I'm ready for a good (I'm talking huge!) wind storm and a good dumping of lowland snow! Hopefully the last week of November will live up to it's reputation!

Upupaepops said...

I have a new car and this reminds me that I have to get those treads in the trunk. Last year Thanksgiving week

BAH!!!

Tony said...

Big Alaska Storm Forecast:

http://news.blogs.cnn.com/2011/11/08/alaska-faces-one-of-its-worst-storms-ever-forecasters-say/

blarsen said...

cliff can you please talk about the giant storm off of alaska right now? in the alaskan NWS forecast discussions they are calling it one of the biggest storms on record for the bering sea.

kdscatt said...

According to the NWS - LaNina probably won't kick in until just after the New Year but may gradually be noticed starting soon. An interesting factoid is that years with back to back LaNina's have had floods that close down !-5 at Chehalis in the last 2 or 3 occurrences. For this phenomenon to occur, the heavy rain event would likely occur before year's end to maintain that trend.

For the upcoming winter, snowfall expected to similar to last year in the mountains, and possibly more lowland snow with a chance of a normal summer in 2012 (?)