Editor’s Note: This is the fourth column in a weekly series written by Meteorologist Joel Gratz that will take you “behind the scenes” of the typical weather forecast. Joel is the founder of ColoradoPowderForecast.com, where you can sign up for his email alerts.
Boulder, CO – If you like snow, then you’ve undoubtedly read the news coverage of a weather phenomenon called La Nina in the skiing press over the last month or two. But what exactly is La Nina, and how does it affect snowfall across the United States? The answers are both simple and very complex. Let’s break it down in Q&A format.
What is La Nina?
La Nina describes cooler than average waters in the central Pacific Ocean. The water must be a certain amount below average (0.5 degrees C) for five consecutive months to be classified as a La Nina. The movie below shows a cold “tongue” of water extending westward from the South American coast into the central Pacific Ocean. This is La Nina.
 
Pacific Ocean sea surface temperatures. |
Why do we care about La Nina?
Strong La Nina (or the converse, called El Nino) episodes have strong correlations with changes in U.S. weather patterns. In general, when there is a La Nina or El Nino event occurring, we can predict with a little bit a confidence how the winter will turn out in the U.S. Without a La Nina or El Nino, there’s almost no predictability.
Why does cooler Pacific water influence snow in the U.S.?
Because the Pacific Ocean is so large, the change in water there influences air currents all over the world. Through a variety of steps (the complete linkage is not understood), La Nina pushes the Jet Stream – a fast moving river of air at around 30,000ft – a bit further west and a bit further north than normal. Since the Jet Stream governs the path of winter storms, La Nina has a direct affect on who gets more — or less — snow during the winter.
Jet stream patterns. |
How do you predict snowfall during La Nina winters?
It’s not very complex…we just look back at previous La Nina winters and see what happened. Hint: go north and west.
Much higher than average (125-150% of average:
- Alaska
- British Columbia
- Washington
- Oregon
- Idaho
- Montana
- Wyoming
Higher than average (100-125% of average):
- Colorado (from Crested Butte north)
- Vermont (northern) New Hampshire (northern)
- Maine (northern)
Average (95-105% of average)
- California (Lake Tahoe)
- Utah (Salt Lake City area)
- Vermont (central and southern)
- New Hampshire (central and southern)
- Maine (central and southern)
Joel first fell in love with weather and skiing at age four, and this passion for snow has not faded with age. After earning his Meteorology degree from Penn State in 2003 and a Masters from the University of Colorado in 2006, Joel started Colorado Powder Forecast out of Boulder, Colo., to help fellow snow lovers with accurate and entertaining weather forecasts.