Mark’s Thoughts on Winter 2012-2013

October 1, 2012

Let’s get this out of the way…I DON”T MAKE WINTER FORECASTS.  The reason should be obvious; we don’t have much skill in forecasting winter weather months ahead of time.  Each Fall the Oregon Chapter AMS DOES have a What Will the Winter Be Like meeting.   This year it’ll be on Saturday, November 17th and the public is welcome.  At this meeting 2-4 forecasters get up and give their outlook for the winter.  Skill hasn’t been so hot in the past; I’d say less than 50% accuracy.  That has kept me away.

That said, some winters offer more skill than others.  So last night, while my wife was catching up on The Walking Dead, for fun I decided to see what we MIGHT see this upcoming winter.  This is not a forecast, although it’ll sure look like one.  That way if I’m right by next spring, I can edit out that last sentence and claim it was a forecast.  If it’s wrong, I’ll need to delete these two sentences.  Nice eh?

Global oceanic/atmospheric conditions play a big role.  We know that two of these have some effect on our wintertime weather.  One is the ENSO, or El Nino/Southern Oscillation.  The other is the PDO, or Pacific Decadal Oscillation. 

Let’s take ENSO first.  El Nino is a warming of the tropical Pacific waters and La Nina is a cooling.  We know El Nino winters tend to feature drier than average weather and warmer than average temperatures.  Snowpack is often lower too.  On the flip side, La Ninas tend to give us wetter than average winters and cooler than average conditions with heavy mountain snowpack.  The latter is what we have seen for the past two winters and early spring. 

This summer we’ve seen weak El Nino conditions set in, although the anomaly has recently gone back into “neutral” territory the last couple weeks.  The loop above shows sea surface temp anomaly in the tropical Pacific; only slightly above average.  Take a look at the latest “plume” of model forecasts, where +0.5 and above is considered El Nino and -0.5 and below is La Nina.  Anything in-between is considered Neutral.  Note that models generally have either weak El Nino or Neutral conditions through the winter.

And this is just the Climate Forecast System (CFS) model forecast:

It shows weak El Nino fading to neutral during the winter.  The story here is that it’s most likely going to be a very weak El Nino or neutral winter, but on th “nino-ish” side.  We are very unlikely to see even weak La Nina conditions.  This is one clue for this winter.

The 2nd factor is the PDO.  We are currently in a cool phase of the PDO and that is expected to continue through the winter or possibly go “neutral” by late winter.  In general, we tend to have cooler and wetter winters/springs in this pattern as Nate Mantua talked about in his Oregon AMS presentation a few weeks back.

I’ve always believed in the truth of the old saying, “All This Has Happened Before and Will Happen Again”, popularized most recently by BSG.  So what the frack does this mean?  Well, we tend to see certain patterns return regularly, and by looking back through history we might be able to get an idea what we might see in the future.

So, once again for fun, I decided to go back through historical records and find winters where these two conditions were matched. 

What did I look for? 

1.  ONI (Oceanic Nino Index) of 0.0 to +1.0 during at least the Fall and early Winter.  That’s neutral to weak El Nino years.  I specifically discounted neutral years on the La Nina side.  That would be 0.0 to -0.5

2. PDO below zero for the majority of the winter, or at least the first half.  These are cool PDO winters.  I did not take just “cool regime”  years, but anytime we were in a cool period of months.  Nate Mantua breaks down the winters separately, but I just stuck with monthly values.

I came up with:
–  15 neutral to weak El Nino winters
–  7 occurred in a cold PDO
The winters are: 1952-1953, 1953-1954, 1968-69, 1977-78, 1990-1991, 1994-1995, 2006-2007
– It’s important to point out that 1968-69 and 1994-1995 were both right on the border of weak-moderate El Nino, so while I’m not discounting those two winters, I use those with a lot of hesitation, especially considering the first was one of the roughest winters in the past 50 years!

Obviously not a very large sample size (one more reason not to do a winter forecast!), but this is what I found:

1.  Precipitation:

These winters were generally wetter than average, definitely not showing the usual El Nino dryness.  Take a look at the national view.  I’ve eliminated the two winters mentioned above, but the only difference adding those back in was normal conditions (instead of dry) in California:

2. Temperature:

That’s interesting, warmer than average across much of the country during those winters.  Now once again I left out the two winters.  When they are put in, our temperature ends up right around average in the Pacific Northwest.  Keep in mind that average or warmer than average doesn’t mean we don’t have an exciting winter and cold/snow.  A three week long epic cold spell in January can be negated by warmth in November and February!

3.  Snowfall & Extreme Cold:

6 of the 7 winters saw 2″ or more snowfall at PDX.  I’ve included the two winters in this case.    It looks like all except 1952-53 had at least some sort of arctic intrusion whether it was an “arctic blast” or arctic air with a brief snowstorm (Feb. 1995 &  Jan. 1954).  The snowfall is in blue below and the coldest winter temp at PDX in green.

1952-53 Trace  18   Marginal arctic airmass in late November
1953-54 10.6″  19  A brief but cold snow/ice storm (like Jan. 1998) in mid January
1968-69 34″  A crazy period from late December all through January, freezes and snowstorms 
1977-78 7.6″  23 Unusual early arctic airmass over Thanksgiving (26/35 day at PDX), again in early January
1990-1991 1.9″  12 2 arctic blasts in mid-late December
1994-1995 5″  16 Feb snowstorm and brief arctic cold.  Also a late November wet snow day with east wind
2006-2007 3.5″  19 Cold January with surprise snowstorm (find it in blog archives!)

4.  Windstorms in the North Willamette Valley and SW Washington:

This one is a big surprise; not what I was expecting.  There were no significant windstorms in any of those years except in December 2006.  We were on the edge of the Hannukah Eve storm that year.  Of course Western Washington really got nailed.  That was probably the strongest windstorm here since the January 2000 event.

5.  A Dry Fall? 

Since it’s been so dry and it appears it’ll continue, I checked into that as well.  Did any of these years feature very dry weather well into October?  Several sure did.

1952 was incredibly dry…less than 3″ of rain from July through November!
1990 was a little dry
1994 was very dry, until the last week of October when many inches fell
2006 was very dry, until the end of October and then all heck broke loose the first few days of November.

So there you go…something to have long discussions and arguments about during the boring weather this upcoming week. 

Feel free to correct anything you feel I left out too.

Chief Meteorologist Mark Nelsen