Climate Summary

December 2010 was cold – very cold. For many locations in NC, it was the coldest December on record. Statewide, it was the 3rd coldest on record since 1895. Only 1989 and 1917 experienced colder Decembers in North Carolina.

Temperature and Precipitation by climate division
Departures from Normal for December 2010
Based on Preliminary Data

Along with cold temperatures came several winter storms and widespread frozen precipitation. Three substantial storms brought snow to much of the state, including a White Christmas for many. In parts of western NC, snowfall over the holiday lasted for several days.

Despite several storms with substantial snowfall, most of North Carolina continues to be dry. Most of the state received less than 75% of normal precipitation. These same locations did experience many inches of snowfall, but 1 inch of snowfall does not equal 1 inch of liquid precipitation. As a general rule, it takes 10 inches of snowfall to equal 1 inch of liquid rain (although the ratio can vary greatly between 5:1 and 15:1).

Below is a map of total snowfall in December 2010 measured at NWS Cooperative stations and CoCoRaHS stations across the state. NWS Cooperative observers have been a resource for local climate observations for generations. Community Collaborative Rain, Hail, and Snow (CoCoRaHS) observers are a more recent resource, and have been growing in numbers since 2007. Observations from these volunteers greatly assists with our monitoring and science efforts both in NC and nationwide. If you’d like to learn more about CoCoRaHS and become and observer, visit the CoCoRaHS section of our website:
http://www.nc-climate.ncsu.edu/monitoring/cocorahs.html

Precipitation for December 2010
Based on estimates from NWS Radar
Data courtesy NWS/NCEP

Precipitation for December 2010: Percent of Normal
Based on estimates from NWS Radar
Data courtesy NWS/NCEP

Doesn’t La Nina Mean a Warmer Winter?

A couple of months ago we stated that winter seasons during La Nina events are more likely to be warmer and drier as compared to normal. Historically, we’re much more confident in the precipitation part of that forecast than the temperature part. We continue to have a moderate to strong La Nina event that is impacting the atmospheric circulation across the globe. And while we’ve been dry, it has been consistently cold as compared to normal. Why?

Remember last winter and the brutal cold NC experienced (especially in February)? The pattern that brought us that cold is the same that is responsible for conditions in December: NAO. The North Atlantic Oscillation has been consistently negative, which means lots of cold air from Canada gets pushed into the southern and eastern United States. If the NAO index continued to stay negative, we’re likely to stay cold and dry. The SCO has a nice little explanation of NAO and how it interacts with ENSO on our Global Patterns page.

Surface Temperature Anomalies During a Negative NAO

Why Didn’t Anyone Predict This?

Unfortunately, NAO is not predictable more than a few days in advance. Unlike El Nino and La Nina events, which can be predicted 6 months in advance, the North Atlantic Oscillation changes are not yet predictable on seasonal time scales. This causes problems with seasonal forecasts in general, and in particular the winter season temperature forecasts. But researchers are working to change this, and with each year the models get better and computers are able to better simulate these types of atmospheric patterns.

Impacts to Agriculture and Water Resources

Despite the unusually high amounts of snowfall across the state in December, the lack of normal precipitation amounts are preventing recharge to rivers and streams. Many rivers in central and southeastern NC are running well-below normal, and drought continues are likely to worsen. Surface soils are quite wet, and likely to remain so, but insufficient precipitation means that not enough moisture is working its way through the soils into our groundwater and river systems.

The NC Drought Management Advisory Council continues to have weekly technical conferences to review conditions and make recommendations to the US Drought Monitor.

US Drought Monitor for North Carolina
Courtesy NC DENR Division of Water Resources