Friday, December 02, 2011

Ocean Temperature Changes & Our Early Winter Pattern

One basic element of long term weather trends lies in the changing ocean surface temperatures.  The effects of these ocean temperature changes are very apparent here in North America where we are surrounded by the Atlantic, Pacific and the Arctic to the north.

My last post looked at how different the last 3 Decembers have been when comparing the different indices located over the oceans. What about the changes in the oceans comparing, say last December to the current levels?

Let's break down last December in two parts. The colors show the warmer and colder locations compared to the average. I indicated these "cool and warm" pools with a "C" and a "W". Notice how they changed from the beginning of the month to the end of the month.
Now look at this year. The Atlantic cold pool shifted more north toward Greenland. The temps off of the SE coast are a bit warmer. The warm pool from last year in the western Atlantic (driven by the strong arctic levels) shifted west into the Caribbean, Gulf of Mexico and New England waters. The cool pool in the Pacific is cooler near Alaska while the classic La Nina "cool" signature between Hawaii and Mexico is now more extensive and cooler.
How did the storm tracks differ over the US last December to the current setup?  The ripples in the flow over the US and Ohio brought small snows and repeat cold air shots throughout the month last December.

This year, the Bermuda High in the Atlantic is now situated near the Southeast Coast. The Pacific Ocean high northeast of Hawaii is also more north this year. Both of these Highs have shifted the trough out west. So any strong front (the "L" over the middle of the US) that tries to make it into Ohio gets squashed. 
The temps in Cleveland last December were very cold.  The "blue/purple" bulls-eye over the central US highlights much below temperatures. We had three separate stretches of 20 degree high temperatures. The first was 4 days, the second was 8 followed by a 5 day stretch at the end of the month. Snowdays at Progressive Field were cold and snowy. I cut down my Christmas tree in a blinding snowstorm!

The temperatures finishing out November this year were slightly above normal in Cleveland. Absent were the strong fronts necessary in driving cold air south 

All of these maps are just more illustrations of how the tiny changes in the oceans can alter the weather patterns across Ohio. Given this current pattern that features no significant drivers of arctic cold, I highly doubt we will see a long stretch of cold temperatures like last December. More details on the prospects of short-lived lake effect snow next Monday.

Wednesday, November 30, 2011

Were the last 3 Decembers All that Similar?

Since early November when our winter weather outlook came out, we emphasized how variable this winter would be. We really didn't see this winter having long stretches of arctic cold like the last 2 or 3 winters.  The variables for this winter just didn't match up like the winter of 2010-11 or 2009-10 or even 2008-09.  Similarities do exist for sure but the differences can't be ignored which I believe will be significant players.

What are those differences?  The three that top our list are the PNA INDEX (Pacific-North American Index), the AO (Arctic Oscillation), the NAO (North Atlantic Oscillation) and La Nina or El Nino.  Don't fret over the technical specifics of each index.  Just realize that each one originating from different locations (Arctic, North Atlantic and the Pacific) has the capability of influencing storm tracks and intensity over northern Ohio.  Long term trends are great but can we see day-to-day details within these indices?

Look at the daily plots for the AO, NAO and PNA for each of the last 3 Decembers: First 2008, 2009 then finally 2010. Notice the fluidity of each of the last December's daily index levels.

What about the ever-popular El Nino/La Nina which gets all of the publicity?  The daily changes are significant when you look at the changes in the pressure patterns over the tropical Pacific (the SOI Index). Never mind the geeky "SOI", just know that it changed a great deal from December to December. Each pressure variation alters the jet stream over North America and the Great Lakes. Even the La Nina of 2008 and 2010 were very different.  This year's La Nina is also very different.

How did this relate to high temperatures? Below is the daily high temperature graph for December 2008, 09 and 10. Last year was consistently cold yet 2009 and 2008 had a lot of ups and downs with only limited cold shots.

What about December snows?  Not much in the last 3 winters.

So the variability of the Arctic, North Atlantic and Pacific/La Nina/El Nino can have effects on our Cleveland weather day to day. Comparing last December to this December or 2009 to 2011 isn't as easy as it seems when you look at the how these elements change on a daily basis. Some of these elements one year can produce more snow and cold in Cuyahoga county and in other years, the frequency of cold is far less in Akron and Mansfield. A look back to the previous year doesn't always work in giving us insight on the current forecast.

Will this December be as changeable as year's past.  Will we have a December like 2010 with continuous cold?  Will this December feature cold shots with some milder breaks?  I will address that in the next post.

Meanwhile, I'll be cutting down my Christmas tree in the mud followed by stringing lights on my house in shorts!