May 08, 2018

Roller Coaster Weather Season in the Northwest

The temperature variations this time of the year are often like a roller coaster, with a steady temperature increase over a few days, followed by an abrupt cooling over a few hours.

Only half-jokingly I call the period from middle spring into early summer, the roller coaster temperature season here in the Pacific Northwest.

During the last month or so we have experienced a number of roller-coaster temperature changes (see the temperatures at Seattle-Tacoma Airport below).  An earlier major event around March 13 had a rise to about 73F, followed by a drop to 45F.  Or the recent one around April 25th, with a rise to about 80F, followed by a drop to around 50F.   Plus, several more minor declines.

The biggest one-day temperature changes in our area are NOT in the middle of winter when fronts and storms are strong, but during spring.

Don't believe me?  Here is the proof from a paper I did a number of years ago.  The figure shows the average number of days per month with one-day temperature drops of certain magnitudes.

For the biggest one-day declines (10°C or more), May is the biggest month by far.  For somewhat lesser drops (7.2-9.4°C), June takes the lead.   For moderate temperatures drops, the summer is tops, with August taking first place.

 Temperature drops in winter are small in comparison.

How does one explain this bizarre state of affairs?  Why do temperatures drop more when it gets warm around here?

As we will see, the big issue is the vast Pacific Ocean and the seasonally changing temperatures differences between land and water.

During winter, we are dominated by onshore flow off of a cool (roughly 50F) and vast Pacific Ocean.  Air temperatures over land (west of the Cascades) are just a minor tweak of the ocean temperatures.

Weather fronts coming across the vast Pacific Ocean are heavily modified by their long traverse across water, with temperatures changes at low levels greatly weakened.    A strong front coming off of Asia, with large temperatures differences at low levels, barely produces a few degree change when it hits are shores.

Yes, the Pacific Northwest suffers from wimpy fronts at low levels.  Something I try not to admit to outsiders.

But in spring as the sun warms up and clouds abate, something changes.  The land starts warming up, particularly east of the Cascade crest--and  yes, even on our side to a lesser degree.  But the eastern Pacific Ocean sea surface temperatures hardly change.     Thus, a large contrast between ocean and land temperatures can develop over the Pacific Northwest.  And there lies the answer.

When we have days with onshore (westerly) flow off the ocean, our temperatures are controlled by the ocean temperatures, and thus remain cool.  By when we get periods of offshore (easterly) flow, our temperatures zoom up.

Cool, onshore flow is the default.   But in spring, there are still upper level weather systems moving through, which can result in high pressure building east of us, resulting in offshore flow...and thus we can get periods of warming.

Here is the plot of wind direction at Hoquiam on the Washington coast for the past four weeks.  Some major swings of direction as weather systems move by.  But look closely around April 24th when we got warm...the winds were easterly (or offshore), pushing the cool, ocean influence out to sea.

The warming tends to take time as offshore flow brings warm air down into western Washington, but the influx of cool air (often associated with an approaching trough) comes in fast...and we have a name for it:  the onshore or marine push.

Why are the largest temperature changes in May, rather than in early August when the temperature contrasts between ocean and land are greatest?  Because weather systems that cause the offshore flow and incite rapid onshore flow tend to weaken during the summer over the midlatitudes.

Why?  Because north-south temperature differences--the drivers of midlatitude disturbances--are less during midsummer.

Anyway, being on a roller coaster can be fun, particularly if you understand how it works.


  1. Hi Cliff,

    Posting here so a better chance of you seeing it, but perhaps more applicable to yesterday’s Yakima thermometer discussion - the UW Atmos website has been showing the temperature at the UW Horticulture at approx 85 C (C not F) for some time. Can this be fixed?

    Thank you.


    P.S. I really enjoyed the NW Weather seminar!

  2. Cliff,
    Could you post on the May 1999 cold wave that hit the area? Kinda curious what caused that one.

    1. Looking back on my notes,it was a NW flow around an offshore ridge that directed a series of cold upper level troughs into this area.1999 was the coldest May since 1962 at Sea-Tac.

  3. I'll take a roller coaster than a solid month of broiling temperatures. Please no repeat of 2015-2017. Please God no.

    1. I don't think the NW had ever seen broiling temperatures... 🤣

  4. Speaking of roller coasters... how about this afternoon's rather sudden thunderstorm? (If you consider two flashes of lightning a thunderstorm, that is). Didn't see it in the forecast but I definitely saw it in the clouds beforehand!

  5. May is probably the month that is most similar here and in my old home in New England. And remember, it was just a week ago last year that we had the big thunderstorms- May 3.

  6. Curious about forest fire/wild fire predictions for this year and smoke events in the NW. Any data?

  7. The thunderstorms of 2017 were actually on 4 May. I was surprised by the clap of thunder and lightning yesterday. I was riding my bike home from work to Issaquah (Highlands) from Bellevue (Eastgate), and saw two brilliant flashes of lighting. It was reminiscent of my time living in the Midwest, except much less illuminating.

    I've been using the weather to my advantage - it has been great to get out and ride my bike (though I'm by no means fair weather, I have almost 3,000 miles down so far this calendar year). I am hoping that this holds out for some great hikes (but with enough precipitation this time around to keep the forest fires tame!)

  8. Wait so the Pacific Ocean, the largest thermal mass on earth is a majority force in determining our weather and climate? But I thought it was all humans and carbon dioxide?

    1. Seriously. Do you get tired of dragging that dead horse around from one post to the next?

  9. Somewhat off topic, but relevant to the climatology discussions here:

    The National Association of Scholars has now weighed in on the crisis afflicting the broader scientific community, regarding the lack of irreproducibility in many scientific studies over the past years. This includes the fields of medicine as well as many others, including climatology. Now that journals like Nature and Science have made their own concerns known as well, it's well past time for the scientific fields to either make their findings transparent and legitimate, or admit that they're carrying water for their funding masters. In like manner, the MSM needs to admit their obvious complicity in this scandal as well. Until the public begins to see that legitimate inquiry is being reestablished and that contrarian voices are included, many of the scientific "boy who cried wolf" denizens will eventually be tuned out altogether. A bad omen for our society.


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