Tuesday, November 1st
Everyone keeps asking me (as they should!) what kind of winter we are going to see. The answer: I don’t know.
BUT, it appears we have our first “La Nina Winter” on the way in 5 years. 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, which means the average of the past three months is right on the edge of “La Nina conditions”. Once October is added in and July subtracted in a few days, we’ll be well into WEAK 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:
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 in Our Area:
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. Likely the #1 most noticeable event in these winters.
This actually goes with the rainfall. For obvious reasons we tend to have more flooding events in winter due to the wetter weather.
2. Mountain Snow
Lots of precipitation and cool weather systems = tons of mountain snow. This is probably the #2 most likely event. 9 out of 14 weak-moderate La Nina winters have brought above normal snow to ALL elevations in the Cascades. That’s opposed to last year’s warmer storms that brought okay snow up high, but terrible conditions again below 4,000′. Note that there CAN be a bad year, but it’s quite rare…see 2000-2001 below.
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. I live at 1,000′ east of Corbett and it’s pretty obvious the last 3 La Ninas were decent snow producers…highlighted in yellow
For you folks that live in the western Gorge…interesting to note less freezing rain in La Nina winters isn’t it? More on that below in the “Portland Snow” section.
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 21 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. Extreme Cold Snap
This is definitely not a guarantee, but we have a higher likelihood of a severe cold spell (arctic blast, like last December) during a La Nina winter. That’s because we occasionally have the flow come down from the north. BUT, the 3 most recent weak-moderate La Nina winters have NOT seen a cold blast (11-12, 07-08, 00-01).
6. 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. What I find interesting is that no La Nina in the last 30 years has produced a major snowfall here in Portland. Of course you couldn’t say the same thing about 1989 down the Valley and up into Washington, but I’m just talking about right here in the metro area. I should point out that the “cool/wet” La Nina winters (such as the last one) often 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.
7. Gorge Wind
Most La Nina winters tend to have less east wind through the Gorge during the winter season. That’s because the strong easterly flow is mainly caused by stagnant high pressure sitting east of the Cascades during slow weather periods (upper-level highs overhead or split flow patterns). During winters (like 07-08) we don’t get long periods of inversions due to frequent passage of cold fronts and low pressure centers. I remember the winter of 98-99 (or maybe it was 99-00) was real quiet out in the Gorge too. That said, when we DO get a big blast of cold air, we can get very strong easterly flow during the cold spell if a warm system approaches from the southwest.
The elephant in the living room I suppose is the fact that our winters are gradually warming, and snow in Portland is definitely more rare than it used to be. Snow each decade since the airport observations started about 1940:
And downtown records that go back to the late 1800s. The low spot in the 80s is missing some data…it should be about 15″ 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 and we all remember that event. Remember 2008-2009’s record snowiest December ever? It only went on for 10 days or so, then not much happened the rest of the winter. That’s how it works here most of the time.
We’ll see how the winter turns out…my money is on “wet” and “Cascade snow”.
Chief Meteorologist Mark Nelsen
1. List of ONI values by season (Historical La Nina & El Nino Episodes)
2. Latest MEI discussion by Klaus Wolter
3. My presentation at Oct. 2016 AMS Winter Weather Conference…LOTS of graphics!