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Under La Niña’s spell

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  • SIMA
- Posted: December 15, 2017

By Frank Lombardo

As the 2017-18 winter gets underway, there’s an unspoken excitement among contractors and the anticipation of a profitable season. Where will the big storm be? How many storms? A better question this winter might be “Will there be a big storm at all?”

The early trends of cold and snowy weather have mostly been confined to the Rockies, Upper Plains and Midwest as of this writing. But will this pattern continue? After three consecutive years of declining snowfall, is this the winter season that Chicago and parts of the Midwest finally get their “snowjo” back and dig out from bigger storms?

Global weather patterns
The general weather patterns across the United States and Canada during the 2017-18 winter season will be influenced somewhat by the development of La Niña. This fluctuation in the water temperatures in the Pacific Ocean (ENSO) has proven to have some of the best and well-established correlations to global weather patterns of all the long-term predictors.

The recent colder weather patterns in the Rockies and Midwest as well as an above-normal November in the East and Southeast is a typical response to a developing or existing La Niña. Our long-range forecasters believe that the presence of even a weak La Niña will be the prominent influence on North America’s weather this winter.

La Niña winters generally favor cooler-than-normal conditions over the north and central United States, with warmer weather generally prevalent in the Southeast. This often sets up a winter storm track across the Appalachians into New England with a pattern that favors little, if any, snow in the Southeast; and mixed events across much of the lower Ohio Valley and Mid-Atlantic states. Snow and colder weather are more common across the Great Lakes, southern Canada, New England and the northern Rockies.

Although La Niña will likely have the biggest influence on weather patterns this winter, other predictors such as the Siberian Snow Cover, Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) will also play less important roles. The correlations with these other predictors seem weak this year; but unlike La Niña, which has a correlation of months ahead, these indices and values relate to weather patterns on time scales of days and weeks and are more difficult to forecast.

The long-range predictors are offering mixed signals this year with regard to whether the Polar Vortex will move out of Canada and settle into the lower 48. This inconsistent data supports a tendency that there will be at least intermittent periods of deep cold despite an overall forecast that looks warmer than normal for most. 

Forecast highlightsCONUS_&_So_Canada_FX_map (600x435)

  • The core of the coldest air will likely be centered over the northern Rockies or Midwest for most of this winter.
  • Colder air will occasionally move Southeast, though it won’t be as persistent.
  • A Southeast Ridge (a common feature in La Niña winters) will likely keep the Southeast and Mid-Atlantic on the mild side; this will result in many events falling as rain in the southern Mid-Atlantic and more mixed precipitation in areas like the District of Columbia, Philadelphia and New York City.
  • An active storm track is likely to develop over the Ohio and Tennessee valleys into the interior Northeast and New England. The northern side of this storm track will likely see plenty of snow events, including southern Ontario and Quebec.
  • Above-average snowfall is expected in the northern Rockies.
  • Lake Effect snow will likely be above average downwind of the Great Lakes.
  • Chicago may break its trend of declining snowfalls for the last three years.
Seasonal forecasts

December: Coldest departures from normal will be in the Northern Plains and from the Midwest through southern Canada and New England. The Rockies may see a warm up after a cold start but deep cold and snow will return from Wyoming north by the end of the month. This may be the month that the Midwest becomes most prone to snow with possible record cold. The snows may spill east with mixed events from the Mid-Atlantic to New England. Overall temperatures stay near or above normal across the Tennessee Valley and Mid-Atlantic.

January and February: The greatest above-normal departures in temperature will be seen across the east with the exception of northern New England. The cold air will persist early in the Midwest with a lag before returning to the Rockies. Later in the period, the deep cold should deepen again in the Northern Rockies and Plains and could spill as far south as Texas. Storm tracks may shift west, which will bring more mixed events from the Southern Appalachians to New England. Snowfall amounts should remain near or above normal from St. Louis to Detroit and from Toronto to Montreal.

March and April: The season should come to a close with some of the winter’s colder weather finally finding a path east. The temperatures will likely remain colder than normal across the Midwest and Northeast, which will increase the threat of a late-season snow. Warmer conditions early in the Rockies may turn cold and snowy again late in April.
 
Frank Lombardo is president and an AMS Certified Consulting Meteorologist, and Ken Elliott is a Meteorologist for WeatherWorks Inc. Contact them at franklombardo@weatherworksinc.com or kenelliott@weatherworksinc.com.
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