The Most Boring Late Fall in Years

What does a meteorologist fear most?   Severe thunderstorms, heavy snow, a powerful tropical cyclone?   Or perhaps a powerful midlatitude cyclone?

Nope.   What we really fear is boring, pedestrian weather, and November and December of this year is proving to be  terrifyingly dull.

The stormiest weather of the year in our neighborhood should be the last two weeks of November.  Not this year---little rain and lots of sun.  And the forecasts for the rest of the month are not promising.

As described in my previous blog, November 2019 was one for the record books--the driest November in 43 years in Seattle and most of Washington State.

Winds?  We got to 19 mph one day, here in Seattle, and the big potential storm headed to northern California.

Snowpack?   Right now, it is well below normal-- roughly one-third of what we usually have this time of the year (see plot).  As a result, the North Cascades highway is still open.

OK...this is depressing.  Surely, we will be getting some active weather soon, right?  Well, a modest low pressure system will move south of us on Friday and Saturday morning (see surface map at 8 AM Saturday), but that will dump most of its precipitation over California (see 24-h total ending 5 PM Saturday).  California gets the winds and precipitation...we get the leftover.   Thanksgiving is way past--I am tired of leftovers.

But then the unthinkable happens:  a series of ridges (high pressure) will build over the northeast Pacific providing generally dry conditions through mid-month.  Looking at the highly skillful European Center ensemble system, its predicted  precipitation anomaly shows drier than normal conditions over us).

Major storms?  Intense atmospheric rivers?  Forget it.  The origin of this dull situation?  Persistent high pressure over the northeast Pacific and the our region.    You want interesting weather?  Head to California.

The Driest November in 43 Years

November was an extraordinarily dry and sunny month over Washington State.

In Seattle, the monthly total was only 1.71 inches, 4.86 inches less than the normal 6.57 inches (26% of normal).    More like our typical rainfall in September or May.

Spokane received .68 inches, only 30% of the normal 2.30 inches.

How unusual is this?   The last time Seattle was drier  in Novmeber was in the extraordinary winter of 1976, 43 years ago.

For Spokane, you would have to go back to 1993 to tie this November and 1976 to beat it.

Yes, 1976 was a startlingly dry year with a ridge of high pressure dominating much of the winter.

To get a spatial perspective on November precipitation, here is the difference from normal for the month.  Western Washington and Oregon were quite dry, with precipitation amounts that were 4 to 16 inches below normal.

But if you want to be impressed, here is the percentage of normal for the month.  Much of the Northwest received less than 25% of normal precipitation in November.

What makes this dryness particularly notable is that this should be the wettest period of the year.

The origin of the dry conditions? 

High pressure over the northeast Pacific.  To see this, here is the different from normal (the anomaly) of 500 hPa (about 18,000 ft) heights.  Substantially HIGHER than normal heights over the northeast Pacific.  Equivalently, think of unusually high pressure to the northeast  of Washington.  Such high pressure results in a much drier than normal conditions over the Northwest.  Cooler as well.

What about the future?

Amazingly, more of the same.  Take a look at the latest European Center precipitation prediction for the next 10 days (the anomaly, or difference from normal, is shown).  Dry over the Northwest and very wet over central California, the result of the jet stream heading south into the Bay Area.

Is the new normal expected under global warming?   Probably not.  In fact, our climate models suggest the opposite:  warmer and wetter conditions in November for our region as the planet warms.