Saturday, November 9, 2013

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



10 comments:

Upupaepops said...

grilled TDay turkey is a family standard many years

simply because it is so delish.

I remember the first one from childhood in the 60's I suspect mom had most everything done the day before, the Turkey spent all day on a BBQ grill during rain and lightning storm.

I work at a company that has hired many new people in the last year, most from outside the region. The last few weeks have been a shock to their systems.

Kenna Wickman said...

This November is nowhere nearly as bad as that one in which it rained every day of November, a few years back during La Nina.

And in California last weekend it was downright lovely.

Rod said...

Hi Cliff,

It is not the November, December, January weather I hate so much, it is the fact that I feel forced to see "family" members that I do not want to be around. I hate the holidays. Hate 'em.

I can hardly wait for spring and summer. As always.

Roger said...

I have followed your blog for about 4 years and have learned a lot and always enjoyed your insights, such as in this post about the jetstream. Now I live in Boise and I'm curious if you know of any online intro meteorology courses so I could get a broad based intro to the field, such as an entry level undergrad would get. Thanks for keeping your blog running!

Buddy said...

All of my skiing buddies were excited to see several shades of blue and green in the extended outlook a week ago but it didn't really pan out correctly. You can see why in the current models...

It'll be interesting how long the high will persist way out in the Pacific because one thing I look for is the jet stream you mentioned undercutting the high while energy from the north merges with it over the PNW. Brings great snow to us. Perhaps by Thanksgiving!?!

Carl said...

I believe! I'm always telling people that November has the worst weather. Usually they just look at me like I'm crazy. I'm going to save this post so I can tell them, "See, I'm right!"

Thanks for backing me up on this one! :)

RLL said...

Great post - You have written about this before. An implication which somehow avoided me is that mid-November is a great time for retired folks to visit the Southwest or Hawaii etc. I shall so plan vacations next year.

ps - February is so soggy here in the country I like to go somewhere sunnier. May is maximum daylight and nice for some trips. Early September as well as May are nice for us retired and lighten the load for those with kids if we stay home June thru August.

Ignado said...

working for the fire department for the last 40 years in this area I can agree with this, outside of a few major storms in other months November is always a month for chasing down lines and trees.

OrcasMom said...

Is there a trend for heavy precipitation in January? It seems like the really big snowstorms often hit right after Christmas break (sometimes extending it, to the delight of school kids). The Pineapple Express seems strongest then. Except, of course, for the Thanksgiving storm several years ago, which caught a lot of vacationers without provisions here on Orcas.

Unknown said...

I've lived in the Northwest for 68 years and have hated November for all the reasons you describe. It is the most rotten month of the year, both for absolute and relative reasons. I can't begin to recall how many Thanksgiving weeks turned out to be the worst week of the year. I used to go to Hawaii during Thanksgiving week, but always worried I would return home to find everything had been trashed by Mother Nature.