Monday, December 19, 2011

Why the Mild Early Winter? Updated January Outlook

Most of everyone were anticipating a sharp and cold start to the winter. That hasn't happened.  This winter is sure not shaping up like the winters of the last two years. Here are the reasons why we missed it and what we can learn from it. I often like to post the reasons we come up with the forecast so that you can understand our thinking. Forgive me for being technical in the post. Its wordy but it illustrates the "why" in our weather fairly well.

The lack of snow with almost 7 inches of rain since November 1st is the result of underestimating a few factors:  The Pacific Ocean temperatures, The stable Arctic and the resurging La Nina in the tropical Pacific Ocean.

First the Pacific Ocean Temperatures.

I indicated in my previous blogs how the Pacific Ocean temperatures are cooler than last year along the west coast/Gulf of Alaska driven by the warm pool in the central north Pacific sliding east. The ridge east of the warm pool kept the trough out west and the eastern US ridge ridge over Ohio has kept temps above normal with little snow making fronts.

The Stable Arctic: In this blog post a few weeks back, I talked about the "Arctic Oscillation" which measure the stability of the arctic cold. A positive number means the arctic air is locked up north and very stable. A negative number means an unstable arctic cold air mass meaning it has a much better chance of slipping south.  Last year, the arctic was VERY unstable. So far this month, the arctic oscillation is strongly positive. The strongest positive December number since 2006; it might be the highest since 1950 by the end of the month!

Lastly La Nina:  All measures of the La Nina (colder than normal temperatures in the tropical Pacific) show this second La Nina to be far weaker.  While most La Nina feature "Alberta Clipper" type snows (2-4 inches at a time), this La Nina so far has featured the southern Jet Stream as the main driver.The main reason we've had more than 7 inches of rain since November 1st.

Now that we've learned our early season lesson of not underestimating these factors, how can we apply these newly found points into formulating an updated January Outlook?

1) Currently, the computer projections indicate that the ridge/mild weather over the east will break down a bit late this month and early January.  Using the current levels for DECEMBER and the final levels for NOVEMBER which measure the PACIFIC and the STABILITY of the ARCTIC, we come up with only one matching month and year:  

January of 1995That January we received 23.4" of snow.

Since one year isn't a good, unbiased indicator (January 1995 was an El Nino month) Are there any January years that match up a bit better?  I think there are.

First, let's assume that the western trough slides east EVEN SLIGHTLY.  That said, there are only 12 January years where the PACIFIC INDEX going slightly positive and allows the trough to push into the Great Lakes while the stability of the arctic drops only slightly negative

These year are: 2010, 2001, 1998, 1987, 1985, 1977, 1970, 1963, 1961, 1958, 1953

 If you count the years where the North Atlantic Index becomes slightly negative allowing for more Alberta Clipper Snows, you get:

2010, 1987, 1985, 1977, 1970, 1963, 1958

Since this year's Pacific ocean (cool temps) is a bigger player than in some of the past years on this list. I eliminated 1970 & 1977 (milder Pacific Ocean temps), 1958 (El Nino Year) and 1985 (Atlantic Ocean temps different) added 1950 and 1955 because these two years featured very cool Pacific Ocean temps similar to this year. 

This La Nina is weaker than last year so I eliminated 2010.

The final list of year used in formulating January's outlook are:

1987, 1963, 1955, 1950

I weighted 1987 and 1963 higher. I also weighted 1950 and 1955 but not as high.

This gives us: 14 inches of snow for January for Cleveland. Slightly more in the snowbelt!

The result is this:  Notice the shades of blue indicating the trough out west sliding a bit east over Ohio. Compare this with November's map where the shades of blue were mainly out west.
How about temps? They should trend slightly below normal but not by much

Where will storms originate? Southern panhandle systems will still be a problem. Clipper snows will be more frequent.
The bottom line is that analysis is the key to weather patterns. There is never a best fit year or month to the current setup especially this year.  I still don't see any large scale cold air drivers. Cold air in spurts will continue. I have a feeling that the kids will be using one of their snowdays in January!

1 comment:

WeatherInfo said...

Excellent Analysis.
Is globaly warming the reason for positive numbers for Arctic Air or colder La Nina?
what does this mean for the overall winter season? milder or close to normal?