A month ago, I posted on the relationship of the Arctic Oscillation teleconnection to the colder weather we experienced in December. Essentially, when this indicator goes negative, you can expect colder winter weather in the Southeast, while when it’s positive, the weather is more likely to be warm and dry.

There is another teleconnection that can shed some light on likely winter weather patterns, the North Atlantic Oscillation. The NAO, as it’s called, measures the differences in unusual air pressure in the northern and southern Atlantic. When the index is positive, the eastern United States tends to have milder and wetter winter weather, while when the index is negative, the east coast tends to be colder, with more snow.

The graph below shows recent measurements of the NAO, along with the forecast for the next two weeks:

As you can see, the NAO has been in a positive phase since late December, corresponding to the milder weather we’ve seen so far in January. However, since the second week of the month, its effects have been getting weaker, and if you look at the second graph, which shows the 14 day predicted position, it should go into a negative phase soon.

The third chart shows the Arctic Oscillation prediction. After being in a positive phase in early January, it too is predicted to go negative.

While not as accurate as your typical three day forecast, these two teleconnections provide some indication that we may very well see more normal winter weather for February. The CPC’s 6 to 19 and 8 to 14 day outlooks through the first of the month still call for warmer and wetter conditions than normal through the first of the month, although the warm area shrinks in the longer-range forecast.

The weather service will issue its long range forecast for February tomorrow. It will be interesting to see what they predict.

The Climate Prediction Center today issued the National Weather Service’s first winter outlook. In summary, the prediction is for warmer than normal weather for much of the country, with equal chances of above normal or below normal snowfall.

For Georgia, the northwest corner of the state is predicted to have a better than 50% chance of above normal temperatures, while the entire state has equal chances of above or below normal precipitation. The image below, courtesy of the weather service, shows the extent of the predicted warmer than normal weather in the nation’s midsection.

The interesting thing to note about the winter outlook is that the weather service doesn’t expect much of an effect from El Nino or La Nina this year. These two phenomena refer to the Pacific Ocean temperatures compared to normal. Because most of our weather patterns flow from west to east, higher or lower than normal Pacific Ocean temperatures affect our weather.

Here on the East coast, and particularly in Atlanta, we are affected by a weather trend called the North Atlantic Oscillation. Essentially, the NAO, as it’s called, measures the position of the jetstream over the Atlantic: if the jet stream is further north, the NAO is positive, while if it’s further south, it’s negative. Unlike El Nino, this pattern changes frequently, and can’t be predicted more than about two weeks in advance.

Because here in the Atlanta area we are usually fairly close to the lowest position of the jet stream, it seems to me that we either have weather that is wet and temperatures too warm to freeze when the jet stream is to our north, or clear dry weather that is cold when the jet stream approaches the area. We rarely get situations that bring snow, when temperatures are cold and a low pressure system moves through.

Overall, the winter forecast closely resembles the December through January forecast issued by the weather service last month. We’ll just have to wait to see what the season actually brings.

Meanwhile, recent weather, providing over a foot of snow in Colorado and the eastern Rockies and torrential rainfall over New England, shows that the inevitable change in the weather pattern from summer to winter is well under way.

With the start of the new month, it’s time to take a look back at February, and a look ahead to March. Here in Lawrenceville, we had a low of 26.9 degrees on the 11th, and a high of 73.2 on the 22nd, with a mean temperature of 47.4 degrees, about two degrees warmer than normal.

For precipitation, we recorded 5.3 inches of rain, or about half an inch more than normal, when you figure in averages for Atlanta and Athens. About two inches of that came in on President’s day, when the year’s first severe thunderstorms and hail pounded the area. Overall, though, we’re still about an inch and a half below normal precipitation, due to the extremely dry weather in January.

I dutifully looked at the National Weather Service’s updated monthly forecast this morning, and saw what looked a lot like the maps I posted back on February 17th. The main difference is that it’s now predicted to be colder than normal out through Arkansas and Missouri, where before it was only supposed to be colder in Louisiana and southern Mississippi.

What I found more interesting was this graphic of predicted 6-10 day temperatures:

The entire eastern half of the country will be colder than normal, and the western half will be warmer than normal.

This unusual situation is being caused by what the forecasters say is “a high PNA combined with a negative NAO and a blocking ridge over Iceland”. Translated into English, that means that high pressure aloft over the Pacific Northwest forces the cold, Arctic air down over the Great Plains until it lands at the East coast, where it is blocked by the high pressure over the North Atlantic. Right now, there is no end to this pattern in sight - we should continue through mid-March with highs only reaching the low 60s, and the reason we’ll be that warm is that the sun is getting fairly high in the sky, and providing daytime heating. At night, it will still go down to around freezing.

So, what about precipitation? Forecasts for the month call for equal chances of above or below normal precipitation north Georgia for the month. A storm later this week is likely to pass to our south and spoil spring break in Florida, and models show another chance of wet weather on the 8th, the 12th, and again around St. Patrick’s day. Keep in mind, though, that models that far out can change considerably within a few days, so this is by no means definite. I still think that we have a chance to see some more wintry weather, although it may turn out to be no more than this morning’s flurries.