Heatwaves in the Northwest: Are Extreme Heat Events Increasing Rapidly?

From the Cliff Mass Weather Blog

Cliff Mass

My Friday podcast on extreme heat events in the Northwest stirred up a lot of comments and some controversy, so perhaps I need a detailed blog that puts the details in front of you.

My basic point is that the peak temperature, duration, and frequency of extreme heat events are not rapidly rising in the Northwest.

And that mean/average temperatures or minimum temperatures are rising faster than the extremes.

Let me explain.

First, you have to be careful to use reliable observing sites

The Seattle Times and some climate activists like to look at temperatures at Seattle Tacoma Airport, but SeaTac is the LAST station you want to use for climate studies.  Why?  Because of the profound growth of the airport, with the addition of a third runway, many additional buildings, and lots more concrete.  Plus, massive urbanization around the airport.  Plus, the observing record only goes back to the late 1940s and the sensor location has been moved.

SeaTac Airport.  The weather sensors are at the black dotBased on the recommendation of Mr. Mark Albright, past Washington State Climatologist, let’s consider Olympia Airport instead. No additional runways, a much less developed environment (see below) and the record goes back to 1941.

Below is a plot of the highest temperatures in July and August for the entire record at Olympia.  

Do you see much of a trend in the extreme high temperatures?  I don’t.

There are more very cool years in the earliest part of the record, so if you calculate a linear trend line, you get a slight upward tilt (about 1.2F over the period).  Again this is not driven by the temperature extemes but by more unusually cool years early in the record.

Now let’s look at the trend in the daily average temperatures for those months.  This is the average of the daily highs and lows.   There is more of an upward trend: about 2F over the entire period.

Next, let’s examine a different site…in this case Lind 3NE, which is found in a totally rural area in eastern Washington (see a picture of the surroundings below).  This station goes back to 1931. No urbanization here.  An area of natural conditions or dryland farming.

Here is a plot of the annual maximum temperatures at Lind.  Mama Mia!  The extreme highs are GOING DOWN.  Many of the warmest years are early in the record.

On the other hand, the daily average temperature is going up (by 2.4F over the entire period)

Other well-exposed, mainly rural, observing sites show similar behavior.  In contrast, highly urbanized or disturbed sites (like SeaTac), particularly those with sensor exposure issues, show much more of an upward temperature trend. Obviously, these observations are not suggesting an existential threat…just a small upward trend of mean temperatures and less upward trend in extreme warmth.

Six Day Heat Waves

A lot has been made in the press and by certain climate activists about SeaTac achieving six days above 90F for the first time.  This kind of frequency above a threshold can be very deceiving, since very small warming (say .1 F) can cause one to cross the threshold.

It is far more meaningful to look at the actual temperatures during the warmest six days for each year.

So let’s do that!

For Seattle, the six warm days last week came in third for the period of record.  Note that many other years were right behind.   And remember all the recent artificial warming due to the third runway and more.  Without all that development, SeaTac temperatures last week would have been much further down the list.

For Olympia, last week falls to number five, and was over two degrees cooler than the “winner” in 1981.  1941 was also warmer.

It was mighty warm in eastern Washington last week, so what about Lind (see below)?  The heatwave last week did not even make the top ten.

Were we unusually warm for an extended period last week?  You bet.  Did global warming contribute to it?  Quite possibly by a few degrees.   Was this the record six-day warm period at any of the observing sites in the region?  No.There are some folks that get concerned when I put our summer heat waves into historical perspective, but it is critical to do so.  Heat waves are nothing new in the Northwest.  Although our mean temperatures are slowly increasing, the extremes are rising more slowly.  And when it comes to impacts, the extremes are really what count.

Why does global warming affect the means more than the extremes?

This is a topic I will cover in a future blog in some detail, but there are many reasons why global warming influences mean temperatures much more than extreme high temperatures.  

This is not surprising.  Radiative effects are stronger for minimum than maximum temperatures, some local wind circulations supporting heat weaken under global warming, irrigation causes cooling during the day, and many reasons will be discussed.