Friday, October 12, 2012

"But Your Kids Are Gonna Love It."
- Marty McFly in Robert Zemeckis' 1985 film Back To The Future


9:15 AM EDT 10/12/12 (Forecaster Jason Isaacs - Atlanta, GA) In Back To The Future, Part 1, Marty McFly's closing stage act during the high school dance not only alters history, but shows the shocked audience a glimpse into the future of their music. (3:27 min YouTube clip). 

For those looking to get a glimpse into the future of snow this winter, you might not be ready to find out that one of the biggest indicators doesn't even have any UNITS. It's just a few little numbers that make a big different in the outcome of a winter storm. 

We're talking about the little-known climate indicator called the "North Atlantic Oscillation" or affectionately known in weather circles as "the NAO."

Recent cold snaps throughout the Mid West and East have been prompting the same question over and over again from my students in the Gwinnett County Schools of north Georgia: 

"It is going to SNOW AT ALL this winter?” 

Recently, our Winter Storm Team released a lead story on the effects of a virtually neutral El Nino thus far this Fall. As alluded to in the article, there are many additional factors that go into any "flux capacitor" of forecasting if one wants to predict the future of weather this winter. The NAO is one of those key factors, so if you want to get back to the future of big snows, study up with us on this one. The State Climate Office at North Carolina State University has an excellent backgrounder website on both the NAO and it's important sibling, the Arctic Oscillation.

THE NORTH ATLANTIC OSCILLATION. The NAO is a measure of large scale air mass movement between the persistent Atlantic high pressure ridge and the Icelandic low pressure trough. This "teleconnection" of air mass is so called because it connects and controls strength and direction of westerly winds and storm tracks across the North Atlantic. This in turn has a major "upwind" influence on how low pressure systems move across North American, and ultimately, on the fate of winter storms for the U.S. and Canada. 

Much like adjusting the destination date in your DeLorean Time Machine, one slight change of even the day can set you down in a totally different timeline. With the NAO, minute changes in this one "climate indicator" as we call it, can be either positive or negative and can greatly affect the possibility for snow in much of the U.S., especially the eastern U.S. 

(Images above from the NCSU State Climate Office showing NAO differences in positive vs. negative phases)

WHERE WE'RE GOING, WE DON'T NEED ROADS. Over the last several months, the NAO has been showing a negative trend for the upcoming winter. The problem recently has been the unusual increase into positive territory over the past couple of days. However, even with this sharp increase, the NAO still has a negative average for the month of October. As of October 10th, the NAO for October is -0.31. This is considered a weak negative NAO. Other impacts a negative NAO can produce on the U.S.

  • Cold East, Warm West, Story Middle. An imbalance of eastern cold vs. western warm, as shown in the current U.S. weather setup. The severe weather outbreak in progress through the Great Plains is in part due to influence NAO has had on the large scale pattern in recent days. 
  • Jet Stream Changes. A negative NAO can often redirect the northern Jet stream further further south, where it can interact with the southern stream tapping Gulf moisture and increasing the potential for wintry weather.  
  • Storm Tracks. In what will be welcome news for Powderhounds, when the NAO goes "strongly negative" such as what was observed in December 2009, once the NAO begins to slowly rise, it can influence the track of coastal storms. The result can be that storms which forecasters initially project to stay farther south, to end up affecting areas much farther north. This was the case for the December 19-20, 2009 or Jan 28-30, 2010 storms 

COMPARING LAST WINTER TO CURRENT TRENDS. Last winter’s pattern was extremely different that what is being observed presently. In October 2011, the NAO ended moderately positive, with the index averaging 0.94 for the month. (Remember, there are no "units" with the NAO or other teleconnections.) The image below from NCSU shows the general temperature trend in a positive NAO, which ended up being the pattern for much of the 2011-12 winter which resulted in much-below normal snowfall for the eastern U.S.

This allowed for the warmer temperatures to stay on the eastern coast and the cold air to remain in Canada. This winter looks to be much different with the possibility that the NAO may spend more time in negative territory for portions of the season. Thus far, the 14-day NOAA Climate Prediction Center forecast forward from October 10 projects a negative NAO trend for much of October. If you're hoping to see "White In The Winter Night"... this is a good sign.

WHOA DOC, THAT WAS HEAVY. If this meteorologically-heavy science lesson was too heavy for a Friday, like Marty McFly said after that crazy guitar act, "I guess you guys weren't ready for that..." But, if we see the NAO continue on a negative trend into the Fall,  then "your kids are gonna love it." 
Students in New Orleans enjoying a rare December 2008 snow from our 12/24/2008 article "And Winter Came."  Image credit: The Times-Picayune

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