La Nina

About this time each fall people start asking me “What this winter will be like?” or “I’ve heard it’s going to be a bad winter!“.  Actually sometimes they start asking in August!  For the record, I’ve NEVER had a person say “I’ve heard it’s going to be an easy winter“.  Apparently most of us are quite cynical and expect the worst.  Since it’s 2020, I suppose this year everyone gets a pass.

I don’t put out a “winter forecast”.  That’s because the few forecasts I see are often (not always!) wrong and seasonal/climate forecasting has a long way to go before we say we can “forecast” a winter.  So we’ll just call it “my thoughts” for the upcoming winter since we can at least glean a few ideas by looking over some weather tidbits.  I’ve been doing this for quite a few years and it seems to work.

For those of you with a short attention span, just three points:

  1. Plan on an “active” winter this year.  The last two winters were quite “boring” for the weather professionals. Which means they were “easy winters” for regular folks. I think we’ll see more changeable weather this winter; I expect to be quite a bit busier here at KPTV. A better chance for windstorms, flooding, and lowland snow.  And I doubt we’ll be locked into weather patterns for weeks/months at a time as we’ve seen the past two winters.
  2. Expect at least once we’ll see some snow or freezing rain in the metro area and lowlands west of the Cascades.  I would be surprised if we get through this entire winter without measurable snow in Portland. I’d peg the chance of “sleddable” snow at about 70% some point between November 10th and March 1st. No, we have no idea when that could happen. Any forecaster that claims so is making it up or click-baiting you…#FakeWeather.  
  3. Expect a good snow year in the Cascades. Good for both water next summer and skiing during the winter.  Go ahead and plan on a normal ski season with the usual variable ski conditions from week to week. I’d give this about a 70% chance of happening too. It is possible to get a low snow year during a La Nina winter. In fact the last one gave us terrible ski conditions through January! Then February/March were incredible.

SUMMARY OF THE LAST 2 WINTERS

Two winters back…2018-2019  Up until around February 3rd 2019, we had experienced the most boring winter I can remember in my 28 winters forecasting in the metro area.  Long periods of weak weather systems, almost no real “storms”, and mild temperatures.  This was typical for a weak “El Nino” winter.  That means the tropical Pacific Ocean was a bit warmer than average.  Things were progressing according to plan…but then all hell broke loose around February 5th.  Cold northerly flow became a common weather theme for much of the following five weeks!  Several snowstorms moved through the region, affecting different parts of the FOX12 viewing area at different times.  Who can forget the “Kale Fiasco” when some metro-area stores ran out of kale & other groceries?  A good learning time for local forecasters…

Last winter…2019-2020  So boring…this was our 2nd consecutive “El Nino” winter. There was a real lack of Pacific storms; it was as if the jet stream just didn’t want to perform last winter.

So much of the past two winter seasons have involved a lack of storminess and drier than average weather. About time for some action don’t you think?

 

WHAT’S AHEAD FOR THIS WINTER?

It appears we have a weak to moderate “La Nina Winter” on the way.  That can give us a few hints, definitely not a forecast, but what direction our winter might be “weighted” toward.  I’ve spent some time looking at past La Nina episodes and what happened here in the Pacific Northwest.  I based all my graphics/research on a weak/moderate event.   Right now the Oceanic Nino Index (or ONI) is on the edge of WEAK to MODERATE La Nina category.  

Model consensus says we’ll likely be in a WEAK-MODERATE category during this upcoming winter.  Here’s the latest plume of ocean/atmosphere models. Anything below the “-0.5” is weak La Nina, below “-1.0” is a moderate event. Strong would be “-1.5”.

Typically in these winters there are 3 effects observed to varying degrees:

  1. The north Pacific jet stream tends to be more “wavy” which means there is more of a north & south component to the jet instead of travelling straight west to east
  2. There is increased tendency for blocking somewhere in the east Pacific
  3. As a result there is sometimes more interaction of the cold Canadian air to the east and Pacific moisture with the jet stream weakening dramatically at times too.

Likely effects this winter based on a moderate La Nina event:

1.  Rainfall

I think it’s unlikely that we’ll have a drought winter; but far more likely precipitation will be above average.  La Nina winters in the Pacific Northwest are dominated by a strong jet bringing frequent disturbances across the region, interspersed with sudden ridging or northerly flow.   Then it’s back to the westerly flow.  For this reason they tend to be wet.  It’s likely the #1 most noticeable event in these winters.

1a.  Flooding

This goes with the rainfall.  For obvious reasons we tend to have more flooding events in La Nina winters due to the wetter weather. Keep in mind we haven’t seen a major regional flood in 24 years. That was 1996. Previous big flood was 1964. I wouldn’t say we are “overdue”, but one of these winters it’s going to happen again.

2.  Mountain Snow

Lots of precipitation and cool weather systems = plenty of mountain snow.  This is probably the #2 most likely event.  7 out of the last 10 La Nina winters have brought above normal snow to ALL elevations in the Cascades.  Note that there CAN be a bad year; it just happened during the last La Nina in 2017-18. Ouch! Check out the mid January snowpack during the last event…

3.  Foothill Snow

This happened in several of the past La Nina winters…significant snow to lower elevations (1,000′-2000′).  This MAY happen again if we get a succession of cold and wet systems coming in from the west and northwest.  Of course these are the same systems that give forecasters headaches because then snow it quite close to the Valley floor multiple times during the season.  News people get really excited about it too.

4.  Wind Storm

We are overdue for a regionwide major windstorm here in the Pacific Northwest.  The last BIG one was December 1995.  That’s 25 years ago!  14 years before that we had the major November 1981 storm.  It’s interesting that all those La Ninas from 1950 to the mid 70s had a wind gust of 60+ mph at PDX each time!  Not as frequent since that time though.

5.  Portland Snow/Ice

This one is tough.  Anyone who says a La Nina winter means lots of snow in Portland is mistaken.  Average snowfall in weak-moderate La Ninas DOES go up a bit, but not a dramatic increase.  Two La Ninas in the last 20 years have produced a major snowfall here in Portland…December 2008 and January 2017.  I should point out that the “cool/wet” La Nina winters sometimes produce little freezing rain because we don’t get as many inversion episodes to our east, which means less east wind in the Gorge.  We need that for a good ice storm either in the Gorge OR in Portland.

ADDITIONAL THOUGHTS:

The elephant in the living room I suppose is the fact that our winters are gradually warming, and snow in Portland is more rare than it used to be when we look back more than 50+ years.  Take a look at total snow each decade since the airport observations started about 1940. Divide by 10 to get average per winter.

And downtown records that go back to the late 1800s.  The low spot in the 80s is missing some data…it should be a bit higher…

We have always been in a marginal snow climate, but now warming temps are cutting off even more of the winter snow.  Every few winters we get 1 good snowstorm. We all remember that event and that pops up the long-term average.  It is interesting that the last 3 decades seem to have leveled out a bit at around 4″ per winter at both downtown and PDX locations.

WHAT ELSE COULD HAVE AN EFFECT ON OUR WINTER WEATHER?

  1. Warm Blob In Eastern Pacific The sea surface is much warmer than normal in the eastern Pacific once again this fall. Some would consider it another “warm blob” offshore. I can’t find a La Nina in the past 40 years that started with such a warm ocean offshore in October. Local meteorologist Charlie Phillips has a great writeup about this in his blog: http://charlie.weathertogether.net/2020/10/18/blob-the-third/

It’s possible the relatively shallow layer of warm water gets “mixed out” as fall/winter storms arrive, we will see.

2. Pacific Decadal Oscillation (PDO) This is directly related to the sea surface temps. Typically during a La Nina we get a negative or “cool phase” of the PDO at the same time. That is not the case this time, or at least ocean temperature anomaly doesn’t seem to match the current “cool phase”. Strange, although sometimes it happens. You can read up on the PDO here: https://www.climate.gov/news-features/blogs/enso/going-out-ice-cream-first-date-pacific-decadal-oscillation

This gives the general picture

3. Anthropogenic Global Warming (Climate Change) A warming globe doesn’t necessarily mean we don’t get cold air outbreaks or snow. It can also mean the usual circulations get disrupted. For example it seems to me we just aren’t getting as much storminess over the eastern Pacific the last 2-3 years. That’s just anecdotal of course. But has something shifted the past 20 years? We don’t know, although 30 years from now, we might look back and notice something did change during this period. There is still a LOT we don’t know about climate.

That wraps it up…as always we’ll see how the winter turns out…my money (again) is on “wet”, “good Cascade snow”, and at least one “snow/ice event” in the lowlands. Maybe several, but hopefully I won’t be spending too much time at the hotel right near the TV station…

Chief Meteorologist Mark Nelsen

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