It’s the last night of November and the end of meteorological autumn in the Northern Hemisphere!  As of the 29th, it was the 3rd warmest November in PDX history, at 50.2 degrees F (about 3.0F warmer than normal).  Given that today was mild yet again we will probably hold on to that 3rd place finish.  That said, we just barely edged out the next few Novembers, and are still much cooler than the two warmest on record (1995 and 2016).  Kudos to Mike Krejci for compiling a chart for November history; you can see how the different years rank here.

In recent years there has been more media chatter about the question:  When do the seasons actually begin and end?  After all, the meteorological definition differs from the astronomical reckoning by roughly 3 weeks, which is not a tiny interval in terms of actual seasonal weather change.  For example, our normal high temperature in October drops by nearly 10 degrees in a three-week period.  There are a lot of popular criteria for marking the turning point from fall to winter:  first snowfall, winter coats, nights below freezing, days below freezing, or even cultural occasions like Black Friday.  The political analysis website FiveThirtyEight recently did a poll concerning the start dates for each of the four classical seasons.  It’s an interesting read and worth a look.

As weather geeks and meteorologists, we always use December 1 as the start date for winter, at least whenever climatology or data analysis is involved.  Wintry weather in November or March is effectively categorized as a late fall or early spring cold snap.  In NW Oregon and SW Washington though, our coldest temperatures tend to come relatively early during the winter season, with daily averages bottoming out in late December and early January.  If you look at daily averages alone, then the late November – mid February period is the coldest quarter of the year.  (It’s also the three-month window when 95% of Portland and Vancouver’s snowstorms and arctic blasts occur.)  So while the calendar months of December, January and February are used for record-keeping, the “astrological months” of Sagittarius, Capricorn and Aquarius are actually a better fit for our climate region.

But while we can squabble back and forth about where to “fix” the date of a changing season, we can forget that Mother Nature doesn’t follow a fixed time table.  Ever noticed how sometimes the biggest cold and snow events come in December, while in other years they land in February?  And the iconic meteorologist’s holiday – Groundhog Day – is premised on the hope for an early spring.  How can “spring” start earlier or later if its date is fixed? At the end of the day, it makes sense to follow actual weather patterns to differentiate one season from another.

So why not just use the popular criterion, “winter begins with the first snow”?  That approach can work out pretty well in climates that see frequent snow during the winter months, but is pointless in snow-free climates like California and the Desert Southwest.  And in polar regions and very high mountains, it’s common to think of the first snow as the beginning of autumn, not winter!

Portland is not mild enough to be completely unfamiliar with snow.  But it doesn’t happen often here, and it practically never hangs out on the ground for months at a time.  In some years the first snow is in January or even February, plus a few years where snow never falls.  Using snow as a benchmark is silly for just about any location west of the Cascades that is below 2,000 feet elevation.

If we can’t use snow as a barometer, what will work for our climate?  Our normal high temperature never drops below the mid-40s even in early January.  Instead, that seems to be as good a measure of lowland winter as any:  a long stretch of days with high temps below 50 degrees F in Portland or Vancouver, or just about anywhere in the western lowlands, seems to feel like the season is turning.  Some years we get quite a lot such days in November, along with snow piling up in the Cascades.  In other years the autumnal 50s and low 60s drag on into early December, which is going to be the case this time.

Another closely related benchmark is Cascade snowfall.  While the lowlands don’t get much snow in wintertime, the mountains normally get lots of it.  Some years it piles up in November and the ski resorts are open before Thanksgiving.  Other times it takes until well into December before that happens.  Very conveniently, the two benchmarks usually coincide pretty well:  rainy 40s in the lowlands usually (though not always) mean lots of mountain snow.

Sometimes the weather gets confused.  During a strong inversion episode, the mountains may receive an extended stretch of warm autumn weather while the lowlands slip into a “fake cold” regime.  But the Cascades and I-5 corridor are two separate climate regimes entirely, so it makes sense to analyze them separately.

The 50-degree high temperature threshold works pretty well for the lowlands and explains a lot of the quirks from one winter to another:  sometimes it begins “early” in November, and sometimes we have to wait for December before the chill arrives.  In some years there are lots of mild spring-like days in February (i.e. 50s & 60s) while sometimes the late winter chill lingers deep into March.

What about this year?  I think we may have to wait another 7-10 days before we can turn the page to normal winter temperatures.  The 500mb anomalies for this Thursday, Dec. 2 still show warm air over the entire U.S.:

But a week later on the 9th, the 00z GFS shows a chilly trough diving into the Pacific Northwest, which means colder air aloft and lower snow levels:

As of right now it doesn’t look like a favorable setup for lowland snow.  We’d need northerly or easterly flow off the land for that to happen.  But the colder air moving in will drop snow levels in the Cascades, possibly to 2,000′ or lower for the first time this season:

It’s still a ways out there so hopefully the forecast doesn’t flop back to mild in the long range.  But there is increasing evidence that classic PNW-style winter weather is arriving late next week.  That means cold rain for the western valleys, and tons of snow in the mountains!

Even if it still feels like fall in the meanwhile…happy meteorological Winter!  Time to keep an eye on the models for some possible excitement coming down the pike!  -Karl