Experimental Local Seasonal Forecasting

Weather Generators are statistical models that allow for the output from coarse resolution global climate models used for seasonal forecasting to be projected or downscaled to local scales. That is, weather generators can be used to provide more localized projections of larger scale seasonal climate forecasts. Unlike other techniques, these generators produce multiple scenarios at a daily timescale for a given season, which can be used to provide forecast guidance for precipitation and temperature, the probabilities of having above average precipitation or temperatures, or the likelihood of total precipitation in a season being more or less than a user defined threshold.

Using weather generators and the Climate Forecast System (CFS) model, the State Climate Office of North Carolina is developing an experimental local seasonal forecasting product. This product will display several parameters for precipitation and temperature, including historical averages, forecast values, and forecast departure from normal for weather stations in the southeastern U.S. Using Google Maps to display the forecast, the user will also be able to select each station and get more forecast data for each station. This option will allow the user to view both what is forecast for a season as well as what is normally observed during a season at that station. The finished product will allow the user to find the forecast for stations closest to them for several pre-defined parameters, as well as for user defined thresholds of interest (e.g. the probability of total precipitation being more than 20 inches) and compare these forecasts to historical averages.

For more information about this product in development or to provide feedback on what you would like to see included in this product, please contact us - sco@climate.ncsu.edu

Climate Summary: December Was Warm!

Temperature and Precipitation by climate division
Departures from Normal for December 2011
Based on Preliminary Data

After very cold Decembers in 2009 and 2010, December 2011 ended as the 13th warmest in North Carolina since 1895. Statewide, average temperatures were generally more than 4 degrees above normal. This is most evident in daily maximum temperatures where most reporting stations ranked December as the 10 warmest, and many (including Charlotte, Greensboro, Raleigh, Wilmington, New Bern, Elizabeth City) had maximum temperatures ranked in the top 5 warmest for all Decembers.

Precipitation patterns varied across the state in December, with above-normal precipitation in western NC and very dry conditions in eastern NC. Across the coastal plain, December 2011 ranked as the 5th driest December since 1895.

Precipitation for December 2011
Based on estimates from NWS Radar
Data courtesy NWS/NCEP

Precipitation for December 2011: Percent of Normal
Based on estimates from NWS Radar
Data courtesy NWS/NCEP

The pattern observed in December 2011 is fairly typical during La Niña events. La Niña is the name given to the cold phase of the El Niño – Southern Oscillation, when ocean temperatures in the eastern tropical Pacific Ocean are much cooler than normal. This ocean temperature change is associated with a shift in widespread thunderstorm activity across the tropical Pacific Ocean, and results in a general shift in storm tracks across North America. Typically, La Niña events are associated with drier conditions across the southern tier of the US, including central and eastern NC.

Figure Courtesy of the Wall Street Journal

While temperatures during La Niña event also tend to be above-normal across the southern US, the relationship between La Niña and temperatures isn’t as strong as the relationship between La Niña and precipitation. Indeed, temperatures last winter were quite cold, even though we had a moderate La Niña event last winter as well. Instead, atmospheric patterns such as the North Atlantic Oscillation (NAO) have a much stronger influence on the temperatures in NC. When the NAO is negative, we are more likely to see colder weather. When the NAO is positive, NC is more likely to be warm. In December 2010, the NAO was negative. In December 2011, the NAO was... you guessed it... quite positive. To learn more about ENSO, NAO, and other climate patterns interact and affect our weather, visit the Global Patterns section on our website.

Severe weather in December 2011 was limited to a single storm system that moved across the state on December 7th, producing strong winds and causing damage. Later in the month on December 27th, a strong winter storm caused sustained high winds primarily in central and eastern NC.

Local Storm Reports for December 2011
Preliminary Count of LSRs courtesy National Weather Service
http://www.nc-climate.ncsu.edu/lsrdb/index.php

Impacts to Agriculture and Water Resources

While drought isn't a concern for most agricultural sectors in NC in December, water supply recharge is. The colder months are when North Carolina generally recharges its groundwater, river, and reservoir supplies. While the cooler half of the year doesn’t necessarily produce more precipitation across the state, the nature of that rainfall is different. Widespread storms help to recharge water supplies more effectively than localized thunderstorms. The other big factor for winter recharge is the low evaporation and plant transpiration rates. During the cold seasons, nearly every drop that falls goes to recharge. In contrast summer evaporation rates are so high that much of what may fall from a single thunderstorm is returned to the atmosphere within days.

The lack of rainfall in eastern NC has resulted in drought impacts to river and groundwater supplies in those areas. If dry conditions persist, it may also start to impact winter grain production for growers.

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