The Weather Service’s Climate Prediction Center recently changed its home page, which normally shows a map of the US highlighting areas of drought, to display the recent history of the Arctic Oscillation. The chart is reproduced below.

The Arctic Oscillation tracks the relationship between air pressure in the Arctic and the middle latitudes (around 45 degrees North). Useful for measuring and predicting winter weather, when it’s in a positive phase, high pressure in the south and low pressure in the north tend to bring warm, dry weather to the central and southern US, while when it’s in a negative phase, the jet stream moves south, and we get colder, wetter weather.

The Arctic Oscillation is one of several relationships between pressures or temperatures called Teleconnections that can be useful in measuring and predicting weather.

As you can see, the AO has been in its negative phase since late November, just as the weather started to turn colder than normal here in Georgia. So far in December, we’re running about 5.4 degrees cooler than normal, on average, with only two days of above average temperatures. The normal temperatures for the beginning of the month are a high of 58 and a low of 39, and at the end of the month, a high of 51 and a low of 32. The normal average temperature is 44.5 for the entire month, to date, the average is 40.5 degrees.

The news isn’t good for people that enjoy moderate temperatures. Below the large historical chart of the AO is the most recent 14 day prediction for future activity, and as you can see, it’s predicted to stay negative for the period.

The Weather Service released its monthly long range forecast for January, which calls for equal chances of above or below normal temperature and precipitation in Georgia, except for South Georgia, which is predicted to have a 33% greater than normal chance of dry weather. The medium range forecast through January 1st calls for colder than normal temperatures through the period, with precipitation tending from wetter than normal the early part of Christmas week, and dryer than normal New Year’s weekend.

About halfway down my forecast page, there is a map of North Georgia, with temperatures reported by a number of official and unofficial weather stations. If you use Internet Explorer, you can put your mouse over one of the temperature readings to see additional weather information recorded at that station.

The feature is called a Mesomap, and it’s a way to see changes in the weather across an area. For example, you can see the effects of a summer afternoon thunderstorm by looking at a mesomap of the area. Locations that got rain will be cooler than the areas that did not.

Weather Underground recently added a fully interactive Mesomap feature to its website. It uses a combination of Google Maps and the information reported by official and unofficial weather stations to show temperatures, winds, humidity, dewpoint, and rainfall. For instance, if you click on the link above, you’ll see a Google map centered on Lawrenceville, along with reported temperatures and winds.

In a column to the right, you see the station names and the temperature. By mousing over a station’s name, its location is pinpointed on the map. You can also click on a point on the map, and get a balloon with the current conditions and forecast for the station point. Above the list of stations is a key to decoding the symbols on the map.

The map works just like Google Maps — you can use the vertical slider on the left to zoom in and out, the directional arrows to change the center point, and switch between map, satellite, and hybrid views. (If you haven’t tried Google Maps in the past, they are a much better alternative to Mapquest or Expedia maps). Try clicking and dragging on the map to move it around. As you zoom or move your position, the list of stations changes to conform to the area you selected.

All in all, a very cool feature.

Atlanta’s weather experienced a sudden change over the weekend, as temperatures dropped about 20 degrees compared to what we saw earlier in the month. The change was brought on by the first major upper level low pressure system driving cold air into the area.

The coldest of the temperatures for this round will be tonight and Wednesday, with temperatures in the upper 30s. Temps should stay below normal through the end of the first week of November, when we may see daytime temperatures above 70 degrees again.

The warmer than normal temperatures during the first part of the month, combined with a dry September have played havoc with the normal fall color and leaf peeping season. Last weekend I drove to Cincinnati and back. While Kentucky was beautiful, Tennessee and North Carolina, even in the Smokies, was still largely green, although clearly the leaves on the trees were thinner than they would be in midsummer. With little to no rain forecast for the next two weeks, we may see a less spectacular than normal fall season.

In case you’re wondering, the first freeze in the Atlanta area is normally around November 10th. Since 1948, the earliest freeze recorded at Hartsfield Airport was on October 18th, in 1948, and the latest freeze was on December 18th, in 1998. Of course, conditions at Hartsfield can vary a lot from what we see in Gwinnett. Last year, I recorded a freeze on November 14th, although Hartsfield didn’t see temperatures that low until December 14th.

What’s your guess for when we’ll see the first freeze of the year?

The National Weather Service recently updated the way it shows radar images on its site. The old version showed the radar superimposed on a map with county and state boundaries, major city names, and Interstate highways.

The new version displays a topographic map of the area and major rivers, in addition to the features listed above, along with the option to select which background features to display. The new format also provides a way to determine the distance between two points on the map. By clicking on the map to specify a starting point, moving the mouse elsewhere on the map shows the distance and direction from that location.

The new features mimic some of the options available on Accuweather’s radar, and overall, in my opinion, a more usable radar than what is on weather.com.

Last Monday, the British Broadcasting Corporation rolled out new weather maps based on video game technology in all of its weather broadcasts. Instead of a traditional flat map with symbols depicting sun, clouds, and rain, the new system presents a 3-D moving map that shows sunny and cloudy areas, rain falling, and even snow.

You can see some of the graphics, and even a video of the new forecast in the BBC’s article announcing the changes.

The change immediately annoyed many British citizens, whose complaint letters outnumbered compliment letters by more than 30 to 1. Complaints ranged from the brown color used to depict the British Isles in the forecast, to the fact that the 3D technology made the Northern Isles of Scotland less visible.

The decision to move to the new technology was announced last summer, and was made due to the need to replace the aging computer systems that were previously used, and because the BBC felt that people were becoming more used to 3D effects in film and TV, and would appreciate the same technology being used to present the weather.