Saturday, December 7, 2019

Why so Dry? Will it Continue?


Announcement:  I will be teaching Atmospheric Sciences 101 during winter quarter

If anyone is interested, either a UW student or outside folks using the ACCESS program, I will be teaching this general introduction to weather in Kane Hall (210) at 12:30 PM.

During the past week, there have been a number commenters asking the big W question.   Why have we been so dry?   Yes,  they understand that persistent high pressure (or ridging) over the Northwest and the northeast Pacific have been the proximate cause, but why is there such ridging?   And is it a random event or is there some underlying cause, such as global warming? 

Let's examine this issue here.

First, the proximate cause--the ridging or persistent high pressure.  Meteorologists often evaluate such ridging by looking a level in the mid-troposphere:  500 hPa--about 18,000 ft above the surface.

Below is a nice graphic produced by the NOAA Climate Prediction Center, showing heights at that level (analogous to pressure), with red/orange colors showing you anomalies (differences) from normal.  October and November is shown.   You can clearly see the wavelike nature of the flow aloft with alternating ridges and troughs (lower than normal heights).   A ridge sets up in the NE Pacific in late October and holds through November.     That is why we were dry, since ridges are associated with sinking motion and the lack of midlatitude cyclones and fronts.

During the past week, the ridge has shifted westward and an area of low pressure/heights has developed west of California.  This pattern has drenched California with the Northwest getting the meteorological "leftovers".   A bit of light rain, but not much of significance.  So far this month, Sea-Tac has only received .05 inches, over an inch below normal.

Now the first question many have is if there is a trend for such high pressure over the northeast Pacific.  If so, we would need to consider global warming as a contributor.   We can examine this issue easily by plotting the heights over a box encompassing the region for an extended period (see below)
Here are the results, plotting the 500 hPa heights over that box for November from 1948 to 2019.  Heights were record high in November, tying the extreme value observed in 1956.  But there is no clear evidence of a trend.  One way you can see this is by simply covering the last year in the plot.

Without a trend, there is little likelihood that there is a climate change origin to this feature.

So what is the cause?   First, it is possible that this persistent pattern is the result of chance, of the essentially chaotic nature of the atmosphere.  The atmosphere does not necessarily need a "cause" to produce persistent configurations.

But sometimes there is a physical origin of persistent patterns.  And one such mechanism deals with El Nino or La Nina.

El Nino is associated with persistent warm water anomalies in the central and eastern Pacific.   Such warm anomalies can shift the locations of thunderstorm areas in the tropical Pacific and in turn alter the upper level wave pattern in the Pacific.  La Nina is the opposite.

There are some interesting changes in the water temperatures and resulting thunderstorms/convection over the tropical Pacific.   But since this blog is getting long, my discussion of those anomalies will have to wait for a future blog.

But I will show you the prediction of the upper level pattern for the next week--the average 500 hPa heights and anomalies from normal over the north Pacific based on the European Center model.  Yellow/orange is above normal (ridging) and blue is below normal (troughing).  The ridging has moved back over us, which suggests below normal precipitation.   But substantial troughing is offshore--very reminiscent of a classic El Nino pattern.   But enough for now...

Wednesday, December 4, 2019

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.

Monday, December 2, 2019

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.