Why is Northwest weather worst in November?

We are now entering the period of the year that is meteorological "ground zero" for the Northwest:  mid to late November.   By any measure, it is the windiest, wettest, and most difficult time of the year.  Storms return in vengeance, summer growth and remaining leaves result in more power outages, and the sudden arrival of clouds and darkness shocks even those accustomed to our NW climate.  Thanksgiving is expected to be a blustery wet time around here, with cooking turkeys on the grill a familiar act of desperation. Strangely enough, the statistics show that by early December the worst is over and there are subtle signs of improvement.  I know you don't believe that, but I will prove it in this blog.

Why is November so bad?   Why do things decline so quickly?  Why does our weather become less severe in December on average? 

First, some proof about November.  A fairly wet day is one that brings a quarter-inch of rain.   Here is the frequency of those events across the year at Seattle-Tacoma Airport (other locations in the area would have a similar variation).   The highest probability (about 35%) is during the third week of November.  Very rapid ramp-up between roughly October 15 and November 15th.   The switch is turned on.  And there is something startling:  the frequency of such wet days DECLINES in December. 

So why is mid to late November so bad?  The atmospheric fire hose, the jet stream, is headed directly into us!

The basic meteorological explanation of Northwest weather in November.

The jet stream is a strong current of winds, centered in the upper troposphere (roughly 25,000 to 35,000 feet above sea level), that is produced by the large difference in temperature between the subtropics and polar regions.  The temperature difference is concentrated in a relatively narrow region of the midlatitudes (a few hundred miles wide) and this concentration produces strong winds.  The one sentence explanation for this?  A big temperature gradient causes a large pressure gradient that results in strong winds.  All the rest is detail.

The jet stream weakens and moves northward during the summer, but during the fall the opposite happens:  it revs up and begins to move southward.   The jet stream is the conduit or pathway for major storms and such storms derive their energy from the strength of the jet stream (and associated temperature changes).  

 A graphic illustration of November

During August and September the jet stream is relatively weak and north of Washington and Oregon, but during November it strengthens and moves southward.  You guessed it, RIGHT OVER US.
During December and January the jet actually moves south of us on average, thus the fire hose is not over us at much and the weather improves.   Don't believe it?  Let me show you.  I am going to present the monthly mean wind speeds at 250 hPa--about 35,000 ft above sea level-averaged for the period from 1980-2010.    Colors indicate wind speeds (meters per second). Reds are the strongest winds.

In August the jet stream is north of us., with relatively light winds above our area.   Nice weather and few storms.

By October, things are different.  The jet, particularly over the Pacific, is much stronger and its extension over the West Coast has strengthened and moved south over southern British Columbia. This is when I should play the JAWS theme music.

 

But then November comes and the hose is directly over us.  Not was strong as in the central and western Pacific, but strong enough.

But look at December.  The jet over us has weakened (colors went from yellow to green) and has moved south of Seattle.

And by January the trend is clear.  The fire hose has moved south.

 By February, things are really different.  The jet is weaker and far south of the Northwest.  Our winter is over.

Not let me be clear.  These are mean charts for the month.  The jet stream on a particular day can be very different, bringing us a big storm in March or any other time.  But the typical variation of the strength and position of the jet stream explains why things get so bad here during November and why I am going to fill the propane tank for my grill today.  I am going to have warm turkey on Thanksgiving, power or not.

 The result of a strong jet stream