February 20, 2020

Why is Meteorological Spring Earlier on the West Coast Than For Much of the Nation?

Spring starts on March 20 or 21st, everyone knows that!

But that is the beginning of astronomical spring, also known as the vernal equinox--the day when the sun crosses the equator and when daylight lasts roughly twelve hours everywhere on the planet.

But astronomical spring is not the same as meteorological spring, and I would suggest that meteorological spring arrives quite early in the Northwest, generally during the third week of February.   Well before astronomical spring begins.  Furthermore, meteorological spring comes much later in the central and eastern portions of the country, where cold and snow can last well into March.

But why is this so?

Before I get into that, let me describe my definition of meteorological spring.   I would suggest that meteorological spring occurs when:

1.  The chances of a major cold wave declines profoundly.
2.  The frequency of major winter storms (midlatitude cyclones) plummets and the chance of a big storm is very low.
3.  The chance of getting a major atmospheric river and heavy rain declines precipitously.
4.  When the amount of sun increases substantially and cloudiness is often broken by sunnier periods.
5.  Leaves start growing on some trees and bushes, the number of birds increases noticeably, forsythia and crocuses begin to flower, and some insects become evident.

There is a lot of objective evidence that our spring generally starts well before March 20/21.  For example, the big snows in Seattle are in January, with amounts declining greatly in February and are very infrequent after the first week of March.

Or looking at the record low temperatures at the Seattle forecast office (blue colors), the major cold waves are in December, with some good chills extending two thirds in February, where they peter out.  After February 26th, there are no major cold excursions (lows below the upper 20s).

Multi-days of thick continuous clouds become infrequent in late February, with a major surge in solar warmth, something illustrated by the solar radiation reaching Seattle during the previous two years (see below).

So why does spring start relatively early here in western Washington/Oregon, particularly compared to the northeastern U.S.? 

Thank our mountains and the Pacific Ocean.

By the time we get into February, the sun is getting much stronger and the day is longer.  Our air is generally coming off the Pacific Ocean, which is relatively mild (45-50F) all year round.  So mild air coming in and protection from the cold air in the interior by two mountain ranges (the Cascades and Rockies), allows the stronger sun to do its job.  In addition, the jet stream begins to weaken and move northward, resulting in weaker and less frequent storms.

But the folks in the central and eastern U.S. are not so lucky!  Residual cold air from Canada can make its way down into the central and eastern U.S. well into March, providing the continued potential for cold waves and snow.  And the interaction of the cold air with warm air from the south can fuel some great coastal storms (Nor'easters) anytime in March.   In fact, some of the greatest eastern U.S. snow storms on record have occurred in March, such as the "Storm of the Century" on March 12,  1993 (see picture below).

Want to see a good recent example of cold-ridden conditions in the eastern U.S., while the West basks in warmth?  You bet you do!

Here is the forecast of heights (like pressure) and temperature around 5000 ft for next Thursday (Feb 27th). Blue is cold.  Wow   A major storm is moving up the coast, with cold, subfreezing air extending over much of the eastern U.S.  Nice spring weather for us though!

Finally, there is another sign of meteorological spring, one that is much more definitive than the weather variables I have discussed above.  Clearly, the wise folks in many supermarkets seem to know that winter weather is over....


  1. Very interesting, Cliff!

    I didn't realize our spring arrives arrives earlier here, but it makes sense now. I like everything about it except for the pollen. Then, I want to see a good rain to clean out the air regularly.

  2. Cliff,

    Nice image up top for your blog!

  3. How i adore the SUUUNNNNNNN.... :D

  4. Hey Cliff, I have to quibble with you a bit about the two equinoxes meaning 12 hours of daylight everywhere on the planet. Daylight ranges from a minimum of 12:05 at the equator, to 12:15 in the mid latitudes, to 13 hours in the high Arctic, to 24 hours at the poles.

    I think equinox is actually a misnomer, as no place on Earth has exactly 12 hours of light that day. Maybe omnilux better describes the concept, as one of the distinguishing features is that all locations receive daylight.

  5. Cliff, Thank you for the sunny outlook. I love you for this. Spring is my favorite season and I have been stopped dead in my tracks by the birds several times already this year. Hope to see you again at Ivars or something. That was a beautiful dinner a few years ago.

  6. I just got up to see the sunrise, and it was mostly clear. It wasn't even that cold! I think it was maybe 35F but still, c'mon...
    That's spring for you!

  7. Great post. But my Seattle daffodils are blooming three weeks earlier than usual, which needs further explanation. Long stretch without the ground freezing?

  8. I am a student at the University of Wyoming in Laramie and we seem to only have two seasons here: Winter, which lasts for 9 months, and then Summer, for 3 months. I miss the PNW seasons!

  9. I know when it's spring because that's when my neighbor finally takes his Christmas lights down

  10. Coming from the Midwest, I agree at how early Springtime comes in the PNW. It still surprises me when the migratory birds begin to arrive, and February still has a week to go!

  11. This must have been the first truly sunny and warm Friday in ages. To say that most everyone was "checked out" is an understatement.

  12. Down in western Oregon they call this sunny spell in February a "filbert spring" because during this time the filbert trees drop their catkins (male flowers) and shed pollen. If this sunny dry period fails to occur, pollination also fails and the price of filberts goes through the roof.

    You can see our local filberts (actually hazelnut trees) do the same thing. They are easily recognized - small woody shrubs with arching branches loaded with long yellowish catkins growing in the woods along roadsides. Shake their branches and watch clouds of pollen erupt. It's great fun if you are a plant geek like me.

  13. I will be happy when the lows are above 26°. This week down here in Olympia we have had a low of 23°, low of 24°, and, today, a low of 26°.

  14. Very interesting! I went to the Oregon Truffle Festival in Newberg Oregon last weekend; it was Spring all over the place!

  15. I've known this unknowingly for a long time, always seem to start mowing the grass the third week of February. Although it was the second week this year!

  16. thanks for this, Cliff, as I have argued my whole life against the "First Day of Spring" misrepresentation. as you say, that's an astronomical phenomenon, NOT meteorological.

  17. Unfortunately, and early spring does not bring an early summer. From here we have an unbearably long slow crawl before a truly sunny summer arrives after Independence Day. You can see this in any 12-month graph of temperature. Worse yet, once we achieve that good weather, the days are already getting shorter, and by mid-September we don't have a high probability of "warm" or clear weather on any given day. Finally, in October the temps plummet, and what took us 5 months to gain we lose in 2.5 months! Ugh.


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