Winter Weather Forecast

December 7, 2007 3:13:00 PM PST
So what exactly is going to happen this upcoming winter and what are the key ideas that the forecast is based on?

Basically, the AccuWeather Winter Forecast was developed on the idea that you can look at past years that have had similar signals to what we expect to see this upcoming winter.

There are three big factors that have gone into this year's forecast: a strengthening La Nina, a warm Atlantic Ocean, and a warm early fall.

First, the La Nina has been strengthening and is forecasted to continue to do so until mid winter. If model indications are correct this could go down as one of the strongest La Nina's in history.

So what is La Nina? Simply put it is colder than normal ocean water temperatures across the equatorial Pacific Ocean that in turn lead to changes in the jet stream and other significant weather makers. This winter the jet stream is forecasted to stay rather zonal, meaning it will move quickly from west to east across the U.S. without many buckles that would allow cold air from Canada to be transported southward.

This trend means milder than normal conditions for most of the U.S. as the jet stream remains to our north right near the Canadian/U.S. border. From time to time there will be some small buckles in the jet stream and we will have short periods of cool weather, but overall the trend will be for above normal temperatures.

One concern with these short invasions of cold air is the threat for ice storms as cold air could get trapped near ground level while warmer air above it is moving in from the next storm system meaning precipitation falls as liquid through a large part of the atmosphere and then would become frozen right near the ground or upon impact with the ground giving us sleet or freezing rain.

Secondly, the Atlantic Ocean is in a warm cycle. This cycle is called the Atlantic Multi-Decadal Oscillation?a fancy meteorological term. This cycle, on average, lasts 25 to 40 years at a time and is especially known for contributing to increased hurricane activity during its warm cycle.

Thirdly, we have had an early warm fall. This year September was over three degrees above normal and October was the warmest on record with temperatures averaging 7 degrees above normal.

So now that we have our three key signals we now looked back in the past to see when we had a combination of these three to get an idea of what those winters were like. The five closest years we found were 1949, 1950, 1954, 1955 and 1998. Many people will only be able to remember back to 1998, but to give you a refresher here are the snowfall totals seen during these five years:

  • 1949-1950 2.0"
  • 1950-1951 4.6"
  • 1954-1955 12.1"
  • 1955-1956 23"
  • 1998-1999 12.5"

On average the winter snowfall in Philadelphia is 19.3" so you can see that we were well below normal for four out of five of these winters. Next we looked at the temperatures during these winters and the overwhelming indication was for above normal temperatures for a large part of the eastern U.S.

Here's what it all means for our area: Temperatures will average 2 to 4 degrees above normal and snowfall will be below normal. November should end below normal, but heading into the heart of winter temperatures will be well above normal with the next period of below normal temperatures not coming until March.

We calculated that snowfall would be around 60% of normal and that is how we came up with the following expected snowfall totals for the region:

Philadelphia: 11.6"
(Normal: 19.3")

Allentown: 19.4"
(Normal: 32.3")

Atlantic City: 8.1"
(Normal: 13.5")

One caveat to this winter's forecast would be if the cold shots of air line up perfectly with storms passing through. In that case we would see more in the way of snowfall, but this is an unlikely scenario as the cold outbreaks will be quick and the majority of the time it will be milder than normal.

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