Winter Is Coming

Weather forecast for Bozeman, MT. Available http://www.weather.gov.

Ignore the text on the above National Weather Service forecast for Bozeman.  Focus instead on the white and gray shades arranged in six-pointed figures.  Perhaps not accurately drawn, these identical snowflakes make my heart race.  Later in the winter the percent chances of snow will approach 100%.  The forecasted accumulations will be many times tomorrow’s single inch.  The bold-red highs will read no greater than 25°F.  For this winter will be the greatest ever imagined.

In ski towns across the US–and around the Northern Hemisphere, no doubt–winter enthusiasts are racheting up their hopes for 2012-2013.  Tracking the jet stream, watching early snow reports from Siberia, and counting El Niño/La Niña cycles are apparently part of the annual pre-season ritual.  Everyone makes their predictions, and those predictions always seem to be positive.

My usual approaches to adjusting my hopes for the next ski season are admittedly unscientific.  I might come across a NY Times article citing the NWS’s forecast for an above-normal snow year for Minnesota through New York.  This assessment corroborates my boss’s suggestion that early snows in Siberia mean lots of snow for northern Minnesota, where I’ll be working and skiing this winter.  A buddy in Bozeman shared the common wisdom that last year’s La Niña should have puked snow in southwestern Montana.  Good years and bad years seem to alternate in skier consciousness, so maybe this year will make up for last year’s terrible disappointments.  Blind faith says this year can’t be as bad as last year.  It just can’t.

Understanding year-to-year climate variability provides a first step towards anchoring one’s predictions in scientific rationale.  The National Oceanic and Atmospheric Administration’s (NOAA) Climate Prediction Center (CPC) publishes monthly assessments of the El Niño/La Niña status.  La Niña periods occur when the Pacific Ocean’s surface is colder than average; El Niño periods are those when the Pacific is warmer than average.  The interannual variation in surface temperature triggers changes in associated atmospheric conditions, including convective rainfall, surface air pressure, and atmospheric circulation. Together, these cycling conditions are considered the El Niño/Southern Oscillation (ENSO).  Years in which the Pacific transitions between El Niño and La Niña, where Pacific temperatures and associated atmospheric indicators are close to average, are considered ENSO-neutral periods.

The CPC has also compiled 60 years’ worth of ENSO and snowfall data for the US to determine the relationship between El Niño/La Niña episodes and snow accumulation:

Composite of snowfall data during El Niño episodes from 1950-2010. Left map shows snow anomalies, or how much snow accumulation deviated positively (blue-purple) or negatively (tan-red) from average. Right map shows frequency of snow occurrence, or the percentage of El Niño years where snowfall was above average.

Composite of snowfall data during La Niña episodes from 1950-2010. Left map shows snow anomalies, or how much snow accumulation deviated positively (blue-purple) or negatively (tan-red) from average. Right map shows frequency of snow occurrence, or the percentage of La Niña years where snowfall was above average.

While never certain, these compiled datasets support my buddy’s suggestion that a La Niña period should give southwestern Montana higher than average snowfall, both from the anomaly and frequency perspectives.

So, what do we have in store ENSO-wise this winter?  NOAA’s CPC suggests that we are currently enjoying ENSO-neutral conditions.  The index that combines all ENSO’s contributing factors remains slightly positive, however, prompting an “El Niño Watch.”  According to the CPC, “Borderline ENSO-neutral/weak El Niño conditions are expected to continue into Northern Hemisphere winter 2012-13, possibly strengthening during the next few months.”  A weak El Niño could mean average or slightly below average snow for southwestern Montana and less snow for northern Minnesota.

The next step is to combine long-term atmospheric trends, soil moisture, and ENSO information to calculate seasonal outlooks.  Fortunately, the CPC has compiled these data into two handy maps:

The temperature seasonal outlook (left) predicts that the Western US will experience warmer than average temperatures, while southern Florida experiences colder than average temperatures.  The precipitation seasonal outlook (right) suggests that the Midwest and Pacific Northwest will both be drier than average, while the Southeast will be wetter.  Source: NOAA Climate Prediction Center/NCEP. 2012. ENSO Cycle: Recent Evolution, Current Status and Predictions. 22 October 2012. Available: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf

Before getting carried away with these predictions–warmer and drier than average!–remember that these are broad, regional predictions.  A good deal of the seasonal outlook map is still white, indicating conditions equal to average.  Other atmospheric conditions and oscillations–the Jet Stream comes immediately to mind–will also help determine how much snow falls this winter.  A more sophisticated analysis would take them into account and would thus avoid more error.  Local topography and weather patterns also play vital roles and can explain why Montana’s Bridger Bowl and Big Sky resorts often receive different amounts of snow.

Perhaps, too, if we temper our expectations with a little ENSO-informed reality, multi-day snow dumps will be that much more exciting.  Until then I will be eagerly refreshing my local NWS site, hoping for nine snow icons in a row.  Will you join me?
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2 responses to “Winter Is Coming

  1. Are you getting snow there in Bozeman today? There were winter weather road closures in Yellowstone starting yesterday afternoon around 3 PM. We’re just getting rain here in South Dakota for now, but we may see some wet snow mixed in with it over the next twenty-four hours or so. I doubt we’ll see any accumulations, though.

    • We got some overnight, which mostly melted by morning. More snow (with some thunder!) rolled through this evening around 6 pm, though, and that seems to be sticking on lawns and obscuring political yard signs. When I walked outside at 8 pm, it smelled like winter.

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