The Block

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This is turning into a lame winter and the upcoming weekend and early next week should leave us completely high and dry.  Why?   A huge area of high pressure will form over the eastern Pacific, a pattern often termed an "omega block" because the flow situation looks like the Greek letter omega and the ridge "blocks" weather from reaching the high (or ridge) area.  As shown by the schematic below, the upper level flow associated with the omega block has a central high and two lows or troughs on the east and west sides.  Weather systems follow the jet stream (indicated by the arrows on the diagram) and thus the West Coast will be left high and dry.


This pattern tends to be self-reinforcing and can be stable for days.  The current model forecasts shows this pattern setting up on Friday and lasting for quite a while.  Let's take a look at the upper level (500 hPa, about 18000 ft) for several times, starting with the forecast for 10 PM on Friday.  The upper ridge has begun to push northward over the eastern Pacific.

Twenty four hours later (Saturday at 10 PM) the ridge and the block have intensified.  No precipitation in this pattern!

Or Sunday night:
Monday night, still there:


 and even late Tuesday, it is holding on!


 You get the picture...an example of an amazing persistent blocking pattern.  No precipitation over the West Coast, and yes the threat of persistent low clouds and fog over interior valleys (including Puget Sound and the Willamette valley)

Based on this pattern, the National Weather Service's Climate Prediction Center is going for below normal precipitation over both the 6-10 day (left image), and 8-14 day (right image) periods.  We get stuck in a dry situation, the eastern U.S. in a wet one.















It is good we have a decent snow pack right now....there will be little additional accumulation starting Friday.  
We are now running out of the time for real winter weather.  Generally, it is rare to get major windstorms, precipitation events (atmospheric rivers), snow, or anything else after the third week of February around here.  We know nothing is in store for the first two weeks of the month.  Not much time after that.
Finally, the meteorological honor of Seattle citizens is at stake.  Atlantic Monthly has called us "weather wussies" because we are sensitive to a little snow. Let them check out our hills or the ice that tends to develop after light snow.  Consider that East Coast types, such as the Atlantic editorial staff,  give names like  "Storm of the Century"  and "Perfect Storm"  to events that would invoke a tepid shrug from a Northwesterner.  We know who the real weather wimps are.