Wednesday, November 23, 2011

Comparing NOV/EARLY DEC 2009 vs 2010 vs 2011

Comparisons are always tricky. Go ahead and pick the data and sure enough, you can find some correlation or causal relationship to the current weather situation.  The operative word here is "CAUSAL". Is the information or data giving you an accurate snapshot of what is driving the current weather? In other words, is there a direct cause and effect relationship?

The index we look at as a direct measure of the strength of the arctic air to the north is the ARCTIC OSCILLATION.  Pressure differences over the arctic and temperatures high in the atmosphere can make the arctic air more or less changeable for us living around the Great Lakes. If the AO index is negative, the arctic air becomes unstable and has a tendency to drive southward. If its positive, its stays locked up in the higher latitudes of the arctic.

Look at 2009 from the fall into early December.  Notice how the AO was strongly negative into late November and early December.  We had 20 of the 31 days in December with highs in the 20s and 30s!!!
Now look at 2010, same ultra-low negative AO index reading. We had 28 of 31 days in December with highs in the 20s and 30s!!!

Look at the early December snow from last year:

Why is this year so different? Check out the AO index recently.  Its been strongly POSITIVE with no big signals that it will drop sharply negative. While this isn't the ONLY measure of a potential cold air shifts into Northern Ohio, it is a strong one.  So if you are going out to any one of the many Christmas farms in Medina County to cut down your tree, the chances of their being a prolonged snowfall are much less than in the last 2 winters. 

Monday, November 21, 2011

Colder Air Still Coming after Thanksgiving? You Bet!

I love the science of weather and the patterns that govern it. While some of my posts can get a bit scientifically technical, my aim is to describe in the most simplistic terms what we look at in formulating our forecasts both in the short and long term. Knowing the forecast is one thing.  Yet developing a basic understanding in how and why we come up with our forecasts is quite another beast. That's why I like to describe elements of the weather on the air. Its fun for me and hopefully informative for you. I liken it to a great meal: Eating the savory food is great. Being able to understand the recipe and recreate it later is even better!  Remember that I not only give you the meat and potatoes, I give you the recipe too!

Which brings me to the pattern changes in the upcoming week. Last Wednesday, I hinted at some colder air and snow for the Sunday after Thanksgiving. Here's what I said.

It still seems like that is a good possibility. But the colder pattern is slowing in its evolution. In other words, the low doesn't seem to want to establish itself over the Great Lakes as fast as it did last week. Here is Sunday evening...the following map is Monday

The squeezing of the LOW from the west and the east has me concerned that it might either, weaken or hold off longer until Tuesday. Either way, this is the ingredient for cold air and waves of either rain and/or snow!!!

That was the "meat and potatoes". Now the recipe:  Why the change in the overall pattern?

Normally we would look to the Arctic's behavior to see potential cold air outbreaks. In this early winter season (2011-12), the arctic shows no signs of heading into what we call "negative territory".  So we have to find another driver of this pattern shift.  That driver might be the highly variable Pacific Ocean/Northern Hemisphere pressure patterns. The area we look at is here:
 When this area is "negative", the snow and cold stay out west and the eastern US stays milder and snow free. When it turns "positive", the cold and snow shift east and the milder air shifts west as seen below.
The pattern has been strongly "negative" which has driven the milder air northward into northern Ohio. Temps in Westlake made it close to 70 a week ago. Norwalk hit a high of 65. Even in Akron, temps were well into the upper 60s. My kids were outside in t-shirts!  It happens in November.

It look like this pattern might shift "positive" in early December. Does this mean lots of cold air? Does this mean more snow? I think a little of both. IF this pattern gets "locked in" for a week or two, we can surely expect a handful of lake effect snow events and perhaps a general snow with temps below normal.

We are still holding on to the notion that this winter will have breaks in any cold period perhaps more than in the last several years. So when it gets cold, we don't anticipate it lasting weeks on end!

Hopefully for skiers, Brandywine will open before Christmas and the hills at Virginia Kendall Park in the Cuyahoga Valley will be primed for sledding.