I’m dreaming of a white Christmas,
just like the ones I used to know.
Where the treetops glisten and children listen
to hear sleigh bells in the snow.
The holidays are here, which means thousands of people here in the U.S. are commuting, flying, or driving out to their local stores to purchase their last-minute gifts. Many people are singing along to Christmas songs that are played at the stores, on the radio, and at home. Among those songs are “White Christmas” and “Let it Snow.” So here’s the big question for everyone: Will it snow? A White Christmas is defined as having at least an inch of snow on the ground on Christmas Day. However, most of the U.S. is experiencing above average temperatures. Those in the southwestern United States and higher elevations such as mountain ranges probably have the best shot at a White Christmas in 2011. Who’s to blame? The positive AO and the NAO, also known as the Arctic Oscillation and North Atlantic Oscillation.
Here are the average probabilities to see a white Christmas across the United States:
At this time last year, the southeastern United States was watching a storm system developing along the Gulf of Mexico coast that brought a rare Christmas snow across the area. The weather pattern we were in contained a negative NAO, which typically influences troughs and colder air across the eastern United States. In 2010, nearly 50 percent of the country had a white Christmas. Temperatures dropped into the teens and 20s across the U.S. Southeast in 2010. Yesterday, many areas had low temperatures in the upper 50s and lower 60s! In fact, many areas broke their record high minimum temperatures, including the cities of Athens, Columbus, and Macon in the state of Georgia.
Look at the snowfall across the United States in 2010:
Now, take a look at snowfall across the United States on December 23, 2011. Less than 30 percent of the country is covered in snow:
So why are we seeing less snowfall this year compared to last year? For colder air to push into the eastern United States, the AO and the NAO need to dip into the negative category. When they are both positive, cold air and even storm systems invade portions of the southwest and central United States. The overall pattern does not favor snow across areas east of the Mississippi River. In the AO, or Arctic Oscillation, we mainly focus on the cooling and warming of the stratosphere in the North Pole as it influences the overall pattern. In a positive AO phase, the stratosphere is colder and pushes storm systems into Europe. Cold air is typically trapped further north, and it typically has a difficult time pushing south. As you can guess, a negative AO is the opposite and it brings colder air further south to the east coast and parts of Europe.
The NAO typically fluctuates more than the AO, and typically influences weather patterns in the short term range. When the NAO is positive, a stronger than usual subtropical high pressure center and a deeper than normal Icelandic low occurs. In this phase, troughs typically do not dig into the eastern United States. Instead, the eastern United States and Europe experience warmer and wetter conditions. In a negative NAO, the phase shows a weak subtropical high and a weak Icelandic low. In this phase, Greenland experiences warmer winters while colder air invades the eastern United States and Europe.
Models are hinting at the NAO becoming neutral towards the end of December 2011, but many models show this being short-lived as the NAO becomes positive again by the beginning of January. As of now, there is not any sign that the AO or NAO will become negative in the upcoming weeks. Perhaps by late January 2012, it might actually feel like winter across the eastern United States. I am sure people across New Mexico, Texas, Colorado, and Kansas have had enough of winter. It seems like storm after storm has affected the region, providing up to a foot of snow in many areas.
In summary, only 29.7 percent of the United States is covered in snow, and that will likely be the case come Christmas morning. A positive AO and NAO are the main reasons why December has been lackluster in the snow department. If you want to see snow for Christmas, you will likely have to search “Let it snow” in a Google search. If you plan on commuting next week, it looks like stormy weather will occur across the northwest United States in areas such as Washington and Oregon. A new storm might develop around New Year’s Day across the central portions of the United States. With the NAO dropping by that time, it’s possible that colder air could invade the eastern United States. However, it is a pattern that will likely change and will only be temporary if it occurs. I want to wish everyone happy holidays and a happy New Year! I leave you with “Deck The Halls” brought to you by the National Weather Service’s official “voice” behind the NOAA radio. Absolutely hilarious!
Matt Daniel is weekend Meteorologist for 13WMAZ (CBS) in Macon, Georgia, and founder of the blog Athens GA Weather. He's a self-described "big weather and music geek" and has produced weather content for CNN, MSN Weather and EarthSky. He has a passion for helping to keep people safe when severe weather strikes and says if you don't have a NOAA Weather Radio ... you should get one.