Winter Outlook for 2017-2018 from the National Weather Service in Wilmington, NC

A weak La Niña pattern is expected to develop this winter across the tropical Pacific Ocean.  This natural cooling of ocean water off the coast of South America directly influences our weather by reducing the amount of tropical moisture that streams northward into our wintertime storm systems.  This typically creates drier than normal conditions across the Carolinas.  La Niña also shifts the jet stream farther north which reduces the total number of storm systems to affect our area.  With fewer clouds and less rain, our winter temperatures usually run above normal during La Niña.

We can use climate history to see what happened in previous winters when similar climate patterns occurred.  Weak La Niñas occurred during the following winters: 1954-1955, 1964-1965, 1970-1971, 1974-1975, 1983-1984, 1984-1985, 1995-1996, 2000-2001, 2005-2006, 2008-2009, and 2011-2012.  For the purposes of this outlook, Oceanic Niño Index (ONI) values of -0.5 to -1.0 in December through February constitute a weak La Niña event similar to what we expect to see this winter.  ONI values are calculated from tropical east Pacific water temperature anomalies and are available at this link.

A second climate pattern that may influence our weather this winter is the Atlantic Multidecadal Oscillation (AMO). Although best known for its impact on Atlantic hurricane activity and summertime temperatures, this approximately 60-year cycle in Atlantic Ocean temperatures also has some influence on winter temperatures.  During the positive phase of the AMO which started in the late 1990s, some portions of the eastern United States are more likely to experience warm winter temperatures.  During the negative phase of the AMO (last observed during the 1960s through 1980s) colder winter temperatures become more likely.

If we take the list of weak La Niña winters and keep only the ones that occurred during the positive phase of the AMO, we are left with these five winters: 1954-1955, 2000-2001, 2005-2006, 2008-2009, and 2011-2012.  This is the list of analog winters we will use to forecast what the upcoming winter of 2017-2018 may be like.

There are other weather and climate patterns that also influence winter temperatures and precipitation across the Carolinas.  The most significant of these, the North Atlantic Oscillation (NAO), determines the position of the jet stream from North America through Europe and has strong influences on our winter temperatures.  The Pacific-North American Oscillation (PNA) and the Madden-Julian Oscillation (MJO) also influence our winter weather, however none of these patterns are predictable far enough into the future to be useful in making a seasonal outlook. 

Average Winter Temperature (December - February)

La Niña usually brings warmer than normal temperatures to North and South Carolina.  In a weak La Niña like we are expecting for the winter of 2017-2018, temperatures typically average about half a degree above normal.  During stronger La Niñas, this departure grows to 3 or 4 degrees above normal winter temperatures.

Seasonal temperature outlooks can be useful for electrical and natural gas utilities since the demand for energy for winter heating is largely dependent on temperatures.

Coldest Winter Temperature (December - February)

Since La Niña typically brings warmer than normal winter temperatures to the area, it stands to reason that the coldest temperature we see at any point during the winter would be warmer as well.  According to temperature data collecting during prior weak La Niñas, coldest winter temperatures are typically 2 to 4 degrees warmer than average across the Carolinas.  There also appears to be a small impact from AMO on our coldest observed winter temperature.

This outlook is valuable to those who need to care for cold-sensitive animals, or those with exposed outdoor plumbing that needs to be protected or dripped on cold nights.  Farmers and backyard gardeners who grow winter greens (kale, collards, spinach, and lettuce) may experience less cold damage than normal this winter.  Backyard citrus is becoming more common across the coastal portions of the Carolinas, and these citrus trees may experience an easier winter due to warmer temperatures.

Winter Seasonal Rainfall Totals (December - February)

La Niña's most reliable weather impact in the Carolinas is a reduction in winter rainfall.  Depending on the strength of the La Niña, rainfall totals can be reduced by 25 to 50 percent compared to a typical winter.  Since this winter's La Niña is anticipated to be weak, rainfall departures are expected to be about 25 percent below normal.

This outlook is useful to forestry and fire weather users since low winter rainfall will allow for additional dry days for burning operations.  La Niña can lead to excessively dry conditions developing by spring if sufficient rainfall does not develop during March or April.  The construction industry can also benefit from this outlook of dry La Niña conditions by anticipating additional good work days during the winter.  Despite low water demand this time of year, agricultural and water resource managers may also find this outlook useful.

Winter Seasonal Snowfall (November - April)

With warmer temperatures and less precipitation expected this winter, it's easy to guess we might have less snow than normal this winter across the Carolinas.  In weak La Niña patterns like we are expecting this winter, total snowfall amounts have historically averaged about half of normal.  In Charleston, SC this weather pattern has never brought measurable snow at all!  Snow is an infrequent but high-impact event for most of coastal North and South Carolina.

Winter snowfall outlooks can be useful to state and community transportation planners, local school systems -- and to kids hoping this may be the winter they can finally build a snowman!

*New Bern, NC snowfall for 2000-2001 and 2008-2009 was missing. Trenton, NC NWS coop data was substituted.

Additional Reading:

• Official NOAA National Winter Outlook for 2017-2018 from the Climate Prediction Center
• Three-month Temperature and Precipitation Outlooks from the Climate Prediction Center
• Oceanic Nino Index (ONI) values back to 1950 from the Climate Prediction Center
• Atlantic Multidecadal Oscillation values from NOAA/ESRL
• El Niño and La Niña from Climate.gov

Research and Page Author: Tim Armstrong
Page Created: November 2, 2017
Last Updated: November 2, 2017