La Nina Digs In…

September 21, 2010

I briefly talked about La Nina last night on the blog (not so briefly on-air), and tonight’s 10pm weathercast details my thoughts on Cascade snow, and snow/cold here in the lowlands.  Let’s wrap it up…here’s what I see for this upcoming winter:

Moderate-Strong La Nina is here

Most likely it’ll be the lowest ONI (Oceanic Nino Index) since the strong La Nina of 1988-1989.  So I based all my graphics/research on a moderate-strong event.  All models keep it going through at least spring…great, another chilly spring???  But let’s not get too far ahead…

Likely Effects This Winter:

1.  Rainfall

Very unlikely that we’ll have a drought winter; 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.

1a.  Flooding

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.  11 of 12 moderate-strong La Nina events since 1954 have produced above normal snow at Government Camp.  Seems like a pretty good snow signal doesn’t it?  The last La Nina winter (2007-2008) was a massive snow year in the Cascades…definitely at the extreme upper end.  Get a season pass!!!  No, I haven’t accepted a free ski pass in years, so you can be assured I’m not biased.  I told folks last year that it would probably be a bad ski season with El Nino and that’s pretty much what happened.

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 major windstorm here in the Portland Metro area.  The last BIG one was December 1995.  That’s fifteen years back!  14 years before that we had the major November 1981 storm.  Then if you take a look at previous moderate-strong La Ninas?  Whew!  A large majority had at least one big wind event.  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.  Of course during the last La Nina in 2007 the north Oregon Coast saw it’s strongest storm since the Columbus Day Storm…the Great Coastal Gale of 2007  But we missed out in the interior.  Of course I use the term “missed out” loosely.  By the way, when I say “major windstorm” here in the Portland Metro Area, I’d call it that when I see widespread 60-70+ mph gusts at the major airport weather sensors.

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, two of the last three La Nina winters have NOT seen a blast (07-08, 99-00). 

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 moderate-strong 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.  As for ice (freezing rain), records for that are harder to come by, but it seems that we have a better likelihood for freezing rain in some La Nina winters.  This is totally anecdotal evidence from my experience.  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

For the hometown folks in the Gorge here…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.

ADDITIONAL THOUGHTS:

I didn’t mention two other possible effects…PDO and Solar Minimum.  We ARE in the cold phase of the Pacific Decadal Oscillation, which some theorize that combined with a La Nina means a better chance for snow and cold.  That did not occur in the last 3 La Nina episodes (late 90s and 07-08).  So I’m not totally sold on that.  Very interesting and something to watch though for sure.  Then we have the unknown of the solar minimum.  This is well out of my area, but the sun has been VERY quiet and some theorize that we are entering a “grand minimum” or something similar to the Dalton/Maunder Minimum.  Both of these were associated with much cooler than normal global weather.  Also very interesting, but I didn’t factor that into the winter outlook either.

Now, let’s see how the winter turns out…my money is on “unusually wet” and “windstorm” and “Cascade snow”.

Chief Meteorologist Mark Nelsen