La Nina: This was not as big a factor as it was last year. Last year at this time, we were in a moderate to strong La Nina. This year we are close to a neutral signal in the central Pacific and we may end up in a weak La Nina through most of the winter. La Nina is a cooling of sea surface waters near the Equator off the west coast of South America and extending westward. The magnitude and placement of the cooler water impact the upper level winds which in turn impact our winter weather patterns. Neutral to weak signals in the central Pacific cause more variable weather across the United States than years when we have a strong signal either with a La Nina or El Nino.
Snow cover in Canada and in the northern part of the U.S.: The snow pack is more than normal in the northern areas and this is something I take into consideration. Air masses that come our way from Canada will not have as much time to modify as they will be moving over cold snow covered ground. This means the time we are under the northwest flow aloft, we will have colder air arriving here compared to if the ground northwest of here did not have as much snow pack.
North Atlantic Oscillation (NAO): This is an index that when negative, we tend to see colder weather in the eastern and especially northeastern part of the country. A positive NAO tends to bring us warmer weather. It has to do with pressure differences in the north Atlantic. I think it will run mainly neutral to negative this winter. When it's negative, we will have our colder temperatures such will be the case as we start December with colder than normal temperatures.
Sea Surface Temperatures (SST) Across the World: I make comparisons to other years in which SST's are similar along with other factors. Analogs: This is where you take certain factors from other years and find years that may have been similar. Some key years I used were: '85-'86, '89-'90, '95-'96, '00-'01, '05-'06. I weigh these years with different values based on how good of a comparison I believe each is.
The above are just a few of the things I looked at, others included: trends, solar cycles, volcano activity worldwide, fall weather patterns, etc.
WALKER'S WINTER OUTLOOK 2008/2009
1. A cold start to winter
2. Early first snow
3. First measurable snow by December 10th (December 12 is average)
4. There WILL be a white Christmas this year.(A white Christmas means having at least 1" of snow on the ground Christmas morning)
5. Colder than normal December ( could be the coldest since 1989! )
6. Some record cold possible in December
7. There will be a nice warm up in mid January
8. There will be six sub-zero days ( average is 4 )
9. Many windy days
10. The first one half of winter will be the coldest
11. Most of our snow will come as "smaller" snow events and not bigger snow storms
12. Large temperature swings within the winter ( 65 degrees or greater from lowest temperature to the highest )
13. Late start to spring ( Cold and snow deep into March )
14. Temperature forecast: ABOVE normal
* Forecast -- 32.8 degrees
* Normal -- 31.0 degrees
15. Precipitation forecast: Slightly BELOW normal
* Forecast -- 7.25"
* Normal -- 7.93"
16. Snow forecast: Slightly BELOW normal
* Forecast -- 17.0"
* Normal -- 19.0"