Forecasting a season is no small task for meteorologists. In terms of weather prediction, getting a forecast accurate a week or two in advance is a tremendous challenge, let alone 3 months in advance. However, while it is impossible to predict daily conditions for this winter, we can get a good idea about the overall trend of the weather-like whether it will be warmer or colder than average or wetter or drier than average, etc.
First, I'll explain some of the tricks of the trade. One of the most well-known and obvious trends to look at is El Nino and La Nina. More than likely, you have heard of El Nino, but you may not know what it is.
El Nino/La Nina
The water around the equator in the Pacific Ocean will oscillate from average to warmer than average to colder than average (not necessarily in that order). When the water is warmer than average, it is in an El Nino phase. When the water is colder than average, it is a La Nina phase. This warming and cooling affects and changes weather patterns across the world. El Nino is still a relatively new concept and scientists are still learning about why this happens. What we do know, is how these two phases tend to change our weather here in the US.
For example, when we are in a strong El Nino phase, the southern jet stream is stronger and more active. We tend to see more storms across the Deep South. The northern part of the country tends to be drier than average and also milder.
When La Nina takes over, the weather patterns shift. The northwest is colder and snowier than average and the south tends to be milder and a bit drier than average. The northeastern US will get bursts of cold air, but they tend to be short-lived.
Here in the Virginias, we don't tend to see dramatic changes from El Nino to La Nina. We do get trends in our weather, but it's not a guaranteed forecast for a colder, milder, wetter, or drier winter. After looking at some of the snowiest, wettest and coldest winters on record in Bluefield and Beckley, I've come up with a few conclusions:
~In an El Nino year (strong or weak), we tend to have cooler than average temperatures. In a La Nina year, winter temps tend to be warmer. However, there doesn't seem to be a correlation between the strength of El Nino/La Nina and extreme cold and warm.
~When it comes to precip, not much changes from an El Nino year to La Nina year, although in La Nina we are expecting to see a wetter than average winter.
~In terms of snowfall, La Nina and El Nino don't play too much of a roll. However, another feature meteorologists study plays a huge roll in the amount of snow we get. This feature is called the North Atlantic Oscillation or NAO.
The NAO is basically the relationship or oscillation of air pressure between the Polar Low and the Subtropical High (see map). When the central pressure of both the low and high are extreme, this is considered the positive phase. In the positive phase, the jet stream tends to be mainly west to east and quick. This gives us a zonal flow across the eastern US, which will push cold air and storms out quickly. When the NAO is positive, we typically see milder and quieter weather in the 2 Virginias.
When the NAO goes negative, meaning the central pressure of the low and high are weak, our weather can change dramatically. In the negative phase, high pressure tends to get stuck around Greenland (called a blocking high) and will cause more dips in the jet stream. These dips allow for bursts of colder air and storms tend to linger. In the negative phase of the NAO, we usually get much snowier weather here in the 2 Virginias.
So, it would seem easy to predict a snowy winter-we just need the negative phase of the NAO. Here's the problem, while El Nino and La Nina can last up to a year or more, the NAO goes positive and negative often. It's also very unpredictable. Scientists can only predict the NAO about 2 weeks in advance.
This winter's forecast:
Winter 2012-2013 began early for us this year due to Sandy-which was a very unusual storm. I wouldn't consider Sandy a foreteller of what this winter will bring us though. This winter looked as if El Nino was going to begin and stay weak. So, early winter forecasts called for a more active winter season in the East. However, that didn't occur and ocean temps in the equatorial Pacific are right around average. When there is no El Nino or La Nina, the term coined is "La Nada"-which is what we get this winter. For us here in the 2 Virginias, that means our winter should be close to average. The last 3 winters in our area have been extreme in one way or another, so this may be a welcomed change. This winter is not expected to be record-breaking for us, but we will see a mix of everything-from warm spells to cold spells to snow storms to rain storms. Rain and snow amounts for the season should be around average and our temps will go back and forth a lot.