The La Nina that U.S. government forecasters have been talking aboutsince May is at least partially responsible and will continue to influence the weather for the winter.
Current Northern Hemisphere snow cover is extensive. This includes Western Europe, where this week the earliest snowstorm since 1993 crippled travel and further strained financially strapped municipalities that needed to deal with the cleanup.
Snow also covers the region from the Pacific Northwest to the northern Plains, where potent storms last week interrupted holiday travel. In the mountains, the snow depth can be measured by feet, and even relatively low-elevation locations, such as Salt Lake City, have had plenty of early-season snowfall.
The storm diving southeastward from the northern Plains today into the Carolinas by Saturday night will leave its heaviest snow -- up to 10 inches -- from southern Minnesota to north-centralIllinois. Snow will accumulate as far south as the mountains of western North Carolina and northernTennessee, and lighter snow showers will likely make it into some cities in North Carolina, such as Raleigh and perhaps even Charlotte on Saturday night.
The ongoing La Nina, which is a cooling of sea-surface temperatures in the equatorial Pacific, has contributed to the early-season winter. But it affects some parts of the world more than others.
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During a La Nina winter, especially when the effect is fairly strong, as this season's is expected to become, storms across the northern part of the country come more frequently and are often more intense than during a non-La Nina winter. The result is typically heavier than normal snow in the Pacific Northwest and northern Rockies, along with a greater frequency of storms in the northern Plains and parts of the upper Midwest.
The effects of La Nina on Western Europe's weather are typically more subtle, with no well-defined repercussions. Therefore, the early bout of winter weather cannot be directly attributed to La Nina in that part of the world. The U.K. Met Office, which is the U.K.'s version of the National Weather Service, chalks up winter's harsh start to current Atlantic Ocean weather patterns.
It's important to understand the role that La Nina is playing now in order to help predict the rest of the winter.
Since the snow in the Pacific Northwest and northern Rockies is a reflection of La Nina, the weather pattern creating the snow is likely to continue. It might end up being a banner winter for ski resorts and a difficult year for travelers.
For Western Europe, which is similar to the northeastern United States in the sense that La Nina does not exert an obvious influence on the weather, the phenomenon creating the early-season snow and cold might break, allowing for a more normal weather pattern to take hold.