Thursday, August 13, 2015

Ocean Temperatures are Big Factor in Winter Outcomes

Good morning everyone. Since we are currently experiencing excellent weather that will continue into the weekend I thought I would share a factor that goes into my winter forecasts, the ocean.

In order to make a contrast between the ocean temperatures in cold/stormy winters vs warm/non eventful winters I ran an analysis that subtracted the 10 most snowless winters from the 10 snowiest winters over the last 60 years. Here is the result..
What is most glaring in the image above is that in the winters that produced the most snowfall, you can see a distinct area of warmer than normal sea surface temperatures off the coast of California into gulf of Alaska. There is a reason for this as warmer water off the west coast tends to support a high pressure ridge that extends up into western Canada. The result is a downstream trough over the east coast. The image below shows this using the same difference in top 10 vs bottom 10 winters.



Notice the large dark blue area south of the Aleutian Islands and the orange area extending into Alaska and western Canada. This represents a tough and a ridge in the jet stream. The downstream effects of this are a deep trough over the eastern 1/3 of the country. As a reminder troughs cause colder and stormier conditions as they represent low pressure. 

Although this is not the only factor when it comes to predicting a winter, you can see it is something that we need to keep a close eye on. Right now, here is what models are predicting for ocean temps for the winter of 2016..



For snow lovers out there you can see AT THIS TIME models are hinting that the warm water stays off the west coast. However, also notice the very warm water off the coast of South America in the tropical pacific. As I have discussed, this is EL Nino (click tab at top of page for more info) and that will also be a MAJOR factor when trying to put together a forecast. We simply do not know how all these variables will change over next 4-5 months so that is why its premature to try to make a forecast at this time. All we can do is study trends and identify the major drivers behind cold/warm winter weather. I am a firm believer ocean temps are one of them.

Stay tuned for more sneak peaks at other relevant factors. The actual Winter Forecast 2016 comes out in October. 

2 comments:

  1. Willie, I took a quick and dirty look to see what the PDO situation was like during the most recent strong El Nino seasons. I didn't double check here, so please correct where I get the data wrong. Looks like 97-98 is on the low-snow list, and had moderate positive PDO's (0.67/0.83/1.56 for Dec/Jan/Feb). 82-83, slightly less + PDO's (0.26,0.56,1.14), and I gather this was an average winter, doesn't make the top 10 lists. 72-73, is on the low-snow list, moderate negative PDO's (-0.33,-0.46,-0.61). Then we have the moderate El Nino season of 02-03 with strong PDO's (2.10/2.09/1.75) and on the high-snow list. As to mod El Nino 09-10, very slightly + PDO (0.08/0.83/0.82) and average snow, although IIRC, there was a big snow storm or two in early 2010. I see that the PDO numbers for 2015 are still fairly strong + into July, looks like the longest PDO + run in many decades (from Jan 2014, going past 18 months; usually the + runs are 6 to 9 months). If the PDO backs off late in 2015, then maybe we're talking 83 or 98; but if it stays historically high, then we're in new territory. Maybe it will be a high-amplitude version of 09-10, with overall average cold and snowfall totals, but with a few doozie storms, lots of ups and downs? Just some SWAG from the peanut gallery. Jim G

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  2. Good points Jim. I have been looking at all those years including 1957-1958. When you have a very strong el nino which this one could be it is imperititve to maintain the positive PDO which is reflected in the SSTA being warm off the west coast. In perticular I am starting to notice the Gulf of Alaska needs to stay warm. If we get any hint of a trough over GOA with a powerful sub tropical jet (from the el nino) then we are talking big blow torch for the winter. Also, it isn't so much the PDO as much as it is the overall pattern in the pacific with respect to water temperatures. 97-98 had a small sliver of above normal temps off coast but overall the pacific was not too warm. Also the el nino that year was not only very strong but the warm sst were focused more towards the east of the ENSO region. An east based el nino typcially is not favorable to a cold and stormy east. To your final point you are correct we are in new territory or territory that could be similar to 1958- a very strong el nino, positive PDO with a very large warm pool and an el nino that is more to the west of 1998. This could result in a very active winter. We will have to see how these SSTA evolve into the fall along with how strong this el nino really gets along with where in the ENSO region it falls. I can see the warm winter scenerio happening easily if the north east pacific SSTA breaks down this fall.

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