Weather Blog

Are cool, showery springs set to become the Seattle norm?

Are cool, showery springs set to become the Seattle norm?

If it seems like spring and "cool and showery" go hand in hand now in the Pacific Northwest, you might be on to something.

It began in 2008 -- I found this weather blog that I wrote on June 9 talked about "the great chill of spring 2008". A heavy snow warning had just been issued for the mountains - yes, on June 9! - for 10-12" of new snow, plus wind gusts to 40 mph in an incoming storm. They had only had one day above 60 degrees through the first week and change. The spring would finish 10th coldest March-June for average high on record.

2009 was a bit of a break -- spring ended up reasonably close to normal. That went right into one of the hottest summers on record.

If it seems like spring and "cool and showery" go hand in hand now in the Pacific Northwest, you might be on to something.

It began in 2008 -- I found this weather blog that I wrote on June 9 talked about "the great chill of spring 2008". A heavy snow warning had just been issued for the mountains - yes, on June 9! - for 10-12" of new snow, plus wind gusts to 40 mph in an incoming storm. They had only had one day above 60 degrees through the first week and change. The spring would finish 10th coldest March-June for average high on record.

2009 was a bit of a break -- spring ended up reasonably close to normal. That went right into one of the hottest summers on record.

But we're back to a three-year below-normal streak. 2010's spring had an 11-day rain streak toward late May into early June and another streak of 25 wet days out of 27. That March-June was 17th coldest overall out of 66 years of data with April and May well below normal.

2011 was the fourth-coldest March-June on record and the overall coldest April on record. You probably remember it well, so no need to recap.

Then this spring -- while scoring better points on warmth (hey, we've already hit 80!) and nice weekend weather (until lately) overall it's still lagging below normal. At the moment, our March through June average is nearly identical to 2008, but admittedly we are not even half way through June yet, although the forecast through the end of the month stays generally cool. But even ignoring June, our March through May average was the same as chilly 2010.

More Proof Your Memory Does Not Deceive You

Some other interesting stats showcasing the cool springs of 2008, 2010, 2011 and 2012, courtesy of University of Washington meteorologist Mark Albright:

Latest eight melt-out dates at Stevens Pass since 1981: (There are still 42 inches at Stevens Pass as of June 14 this year.)

8. 10 Jun 2010
(?? 14 June 2012)
7. 17 Jun 1982
6. 18 Jun 1984
5. 20 Jun 2002
4. 23 Jun 1997
3. 25 Jun 1999
2. 25 Jun 2008
1. 1 Jul 2011

Coldest average high temperature, March-June, at Sea-Tac Airport (1945-2011):

1. 56.1 (1955)
4. 57.7 (2011)
[11. 58.7 (through June 13, 2012)]
11. 58.9 (2008)
19. 59.4 (2010)
Median: 60.5

But these next graphs really tell the story of cooling springs in the Northwest, showing the past 30 year and 10 year trends of spring in Seattle:





(Data for the entire Pacific Northwest is pretty similar.)

Both show a general cooling trend around here, cooling on average about 0.68 degrees F per decade over the past 30 years, with the last 10 years quite stark -- cooling 3.68 degrees F per decade if the trend were continue.

But it's pretty localized to the Northwest. Much of the nation, it's going the other way. For example, Cleveland's 10-year chart shows warming of nearly 6 degrees a decade, while San Antonio and Minneapolis are at about 3.5 degrees warming and Atlanta is just under 3 degrees. (Just a quick aside -- it's a good illustration why you can't focus on one particular area when talking about global climate since just because your area might be hotter or colder than normal doesn't apply to the entire planet.)

So, why are we getting picked on?

One reason our springs might be trending colder recently is something that doesn't get much attention -- the "Pacific Decadal Oscillation".

This is somewhat in the realm of how the more popularly known La Nina and El Nino cycles (aka "ENSO") mess with ocean temperatures, but unlike ENSO, which affects the equatorial Pacific and runs on a 3-7 year cycle, the PDO is something that occurs in the northern and eastern Pacific and runs on a longer cycle -- sometimes as much as 30 years.



The cool phase brings colder water temperatures just off the West Coast, which can act as a bit of a drag on temperatures in general.

The PDO was in a marked cold phase in the 1950s through 1970s and temperatures around here followed suit, then went into a warm phase in the 1980s and 1990s -- amid some of the hotter years in the books here.

(Image courtesy: NOAA Northwest Fisheries Science Center)

However, we transitioned back to a general cold phase around the start of the new century and after a brief spike back to the warm phase the mid 2000s, we're back solidly cool now, and it has generally correlated with the cooling spring trend in the Northwest.

We'll see if the cold phase holds for the next few years or even decade or two, but if so, cooler than average springs could be a bit of a norm for a while.

Now, it doesn't mean we're going to be this chilly in spring from here on out -- I think La Nina has helped exacerbate the cooling the past few years and the cooling shouldn't be quite as stark  outside of La Nina years. But cooler than our long term average could be a frequent occurrence for a while.

Are summers doomed too?

Data does show that summers in Seattle have been recently below normal for temperatures as well -- although not quite to the degree that springs have.

The past 10 years have seen 3 degrees per decade cooling in Seattle, but it's nearly flat over the past 20- and 30-years charts. The chart does mimic the PDO quite well, showing cool summers from 1999 to 2003, then warming in 2004-2007 but, of course, cool from 2008-2011 with the noted exception of 103-degree-day summer of 2009 -- a great illustration that hot weather and a hot summer is not impossibe during a cold PDO phase.

But those who long for an early start to the springs and a string of hot summers in general might need to wait until the PDO trends back into the warm phase.