October 27th, 2021
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.
I don’t put out a “winter forecast”. That’s because 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:
- Plan on an “active” winter this year. The last 3 winters were quite “boring” (most of the time) for the weather professionals. Which means they were “easy winters” for regular folks (most of the time). Of course we all remember the 4 day stretch of snow/ice around Valentine’s Day right? But most of last winter was quiet weather-wise except for that event plus some flooding in mid January. The odds are tilted toward more changeable weather this winter; I expect it to be busier here at KPTV. A better chance for a windstorm, flooding, and lowland snow. And I doubt we’ll be locked into weather patterns for weeks/months at a time.
- 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 until at least 7-10 days ahead of time
- 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 two La Ninas back in time we saw terrible ski conditions through January! Then February/March were incredible.
SUMMARY OF THE LAST 2 WINTERS
Two winters back…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.
Last winter…2020-2021 It was a surprising La Nina winter because of the mild temperatures, plus almost nothing interesting happened weather-wise until mid January! Some widespread (light-moderate) flooding showed up at that time, but then all was quiet until a 4 day blast of cold air arrived starting February 11th. That cold air was overrun by plenty of Pacific moisture which produced a snowstorm from Portland north/east and a severe ice storm from south metro down to around Albany in the Willamette Valley. Then temperatures warmed and typical (wet) winter weather resumed. We dried out dramatically in March and April, but with plenty of cool temps.
You can check out the rest of my winter recap presentation here: https://oregonams.files.wordpress.com/2021/10/mark-nelsen-winter-weather-recap-2020-2021.pdf
So much of the past 3 winter seasons have involved a lack of storminess and drier than average weather. About time for some action don’t you think? But that’s what I thought would happen this past winter too…there’s so much we still don’t know about our climate.
WHAT’S AHEAD FOR THIS WINTER?
We have entered weak/moderate “La Nina conditions” once again this fall. Models tell us most likely it’ll be a weak to moderate event through the winter. So this will be our 2nd La Nina winter. 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 in the WEAK 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” or lower.
Typically in these winters there are 3 effects observed to varying degrees:
- 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
- There is increased tendency for blocking somewhere in the east Pacific
- 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:
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. And the chance for “wet” is much higher in northern Oregon than the southern half of the state. Although the current weather pattern this week suggests otherwise with an incredibly wet northern California very early in the season.
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 winter 2017-18. Ouch! Check out the mid January snowpack during 2001 & 2018…
3. Foothill Snow
This happens in some 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. It didn’t happen last winter, but colder than normal ocean water is poised to the west/northwest of the PACNW.
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 26 years ago! 14 years before that we had the major November 1981 storm. It’s interesting that all the 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. Three La Ninas in the last 20 years have produced a major snowfall here in Portland…December 2008, January 2017, & February 2021. 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.
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 70+ years. Although the past couple of decades total winter snowfall seems to have stopped it’s downward plunge. 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 a 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?
- Cooler Water In Eastern Pacific There is no blob of warm water in the northeast Pacific like last fall/winter.
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. Last year that was not the case. But this year we seem to be in the cool phase (right side figure below). Could that make this La Nina winter behave significantly different (cooler/wetter) compared to last year? 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 3-4 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…
WHAT OTHERS THINK
Each autumn the Oregon chapter of the American Meteorological Society puts on a “Winter Weather Conference”. It’s my favorite meeting of the year! I have been a part of this chapter my entire career. Speakers present their thoughts/outlook/forecast for the upcoming winter. The public is always invited. Last year and just this past week we gathered virtually due to the pandemic. Interested in watching? Check out my recap, plus 5 different outlooks on our Oregon AMS web page. I was especially impressed by the passion/talent the younger folks are bringing to our chapter! There is some really good information in each of these presentations. Enjoy!
Chief Meteorologist Mark Nelsen