January 4, 2011 - Atmospheric Teleconnections

By: Mallory Brooke
By: Mallory Brooke

When we discussed the winter outlook in October, I mentioned that it was hard to predict the Mid-Atlantic & Northeast winter on La Nina alone. Other factors that are better predictors? Teleconnections - read about it here!

Without getting too technical, I'm going to discuss two of the teleconnections that we use to help forecast storms in the winter season. They're kind of intertwined so hopefully it will be easy to understand. Feel free to ask questions!

The first one up for discussion is the NAO - North Atlantic Oscillation. The easiest way to explain this is that there's a permanent low pressure over Iceland (Icelandic low) and a permanent high pressure over the Azores (Azores High) that control the direction and strength of the westerlies over the Atlantic.

There are negative and positive phases of the NAO. The index itself is the difference in the anomalies of the Icelandic low and the Azores high. A positive phase of the NAO means there's a large difference in the pressure between the Icelandic low and the Azores high. The high is stronger than usual and the low is deeper (lower pressure) than usual. The stronger pressure gradient results in stronger winter storms crossing the Atlantic on a more northerly track. Colder air stays in Canada and Greenland, leaving the eastern US under more mild conditions. A negative phase of the NAO has a weak subtropical high and weak Icelandic low. The smaller pressure gradient results in less powerful storms and fewer storms moving across the Atlantic, but increased cold outbreaks along the eastern seaboard - hence the increased probabilities of snow. Many times, our most intense storms occur when the NAO is switching directions (increasing after hitting a peak low or decreasing after hitting a peak high). Pattern changing storms tend to be on the strong side.

Conclusion: Negative NAO leads to colder air making it down to the Mid-Atlantic and also increases snow chances along the eastern seaboard. You can view the current NAO here: http://www.cpc.noaa.gov/products/precip/CWlink/pna/nao.shtml

Now let's talk about the AO - Arctic Oscillation. It's another index that has positive and negative phases. The positive phase of the AO involves lower than normal pressure over the polar regions and higher than normal pressure in the mid-latitudes. Storms will steer northward (similar to the positive phase of the NAO) and cold air does not extend as far south into North America. As expected, the negative phase of the AO has relatively high pressure over the polar regions and relatively low pressure over the mid-latitudes. Cold air is more likely to move into the Mid-Atlantic area with a negative phase AO.

Conclusion: Negative AO leads to colder air moving in from Canada towards our area, leading to higher snow chances when precipitation moves in. You can view the current AO here: http://www.cpc.noaa.gov/products/precip/CWlink/daily_ao_index/ao.shtml

Both the AO and NAO help us forecast for winter patterns along with the La Nina and El Nino trends. We need both cold air and the ideal storm track for the "perfect" winter storm.

As you can see from the current readings of the AO & NAO, they are both negative - indicating that temperatures will likely stay at or below our averages. We're also seeing a flip possible in the forecast NAO a few days before January 16 (from positive back to negative), so be on the lookout for some interesting weather.

I hope this sheds some light on teleconnections used in forecasting. Feel free to post any questions!

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