Monday, October 30, 2017
Last winter was a crazy one as we all know. 4 freezing rain events and 4 snow events down here in the city and it was the coldest winter in 20+ years west of the Cascades. Of course we had the biggest snowstorm in years in early January too…who can forget that evening?
It appears we have another weak “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 as last year. In fact much of this info is a repeat from what I posted last fall. 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 event. Right now the Oceanic Nino Index (or ONI) is on the edge of NEUTRAL or 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 should be into the 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. It’s 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 = plenty of mountain snow. This is probably the #2 most likely event. 10 out of 15 weak-moderate La Nina winters have brought above normal snow to ALL elevations in the Cascades. This happened last winter with well above normal up high and on the lower slopes. Somehow Government Camp ended up right AT average though. 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.
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 22 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. What I find interesting is only ONE La Nina in the last 30 years has produced a major snowfall here in Portland…last winter. 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 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. Of course the setup last winter was perfect for ice storms, the most I’ve seen in one winter in my career. So lower confidence on this one.
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. Again, as with the freezing rain, this didn’t work out last winter. It was a big east wind winter.
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 when we look back more than 20-30 years. 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. We won’t forget the snow storm from January for a long time! And 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 (again) is on “wet” and “Cascade snow”, but I’m not sold that we’ll see anything other than a minor snow and/or ice event or several of those.
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. 2017 AMS Winter Weather Conference…LOTS of graphics!