Raleigh December Climate Summary

Winter in the southeastern United States is usually mild, but this past December was far from normal. December 2015 ranked as the hottest December on record average maximum, mean, and average minimum temperature for the Raleigh weather station. Across the entire state, almost every station recorded its warmest December on record for mean temperature and a few others recorded their second warmest. Along with warm temperatures, total precipitation was about 3 inches above normal for Raleigh.

Temperature

The mean average temperature was 56.1F, which is 12.7F above normal! December 2015 ranked as the hottest December ever recorded for Raleigh. Raleigh experienced a few cold days, but overall the temperature was much above normal (Fig. 1). The warmest day was Christmas Eve as the temperature topped at 77F. The average maximum temperature was 65.1F, which is 11.6F above normal. The average minimum temperature was 47.1F, which is 13.8F above normal. 

Figure 1. The average mean temperature during December for Raleigh

Precipitation

The first half of December was relatively dry with only a few light rain events around 0.04". The second half of December was active which caused the total precipitation to surpass the normal amount (Fig. 2). Raleigh received 6.07" of rain, which is 3.00" above the norm. Overall, December 2015 ended with the 11th wettest December on record. 

Figure 2. The total precipitation amount during December for Raleigh

Why was it so warm?

There are many natural climate oscillations that have a big impact on our climate. Studies show that the El Nino-Southern Oscillation influences our climate during the winter months. An El Nino enhances the subtropical jet, which directs storms along the southern states. This pattern normally causes above normal precipitation anomalies along the southern states (much like we observed during December). Another climate oscillation that greatly influenced this past December is the Arctic Oscillation (AO). Studies have shown that the AO impacts our wintertime temperatures. A positive AO indicates that the pressure system is strong, which traps the Arctic air in the north. A negative AO means that the pressure system is weaker, which allows the frigid air to move equatorward. Figure 3 shows that the AO was positive for much of December. 

Figure 3. Arctic Oscillation observations and forecasts. Data source: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.sprd2.gif 

Additionally, the surface and upper air pattern directed the warm, moist air from the equator up towards the midlatitudes (Fig. 4). Following a record warm autumn, December was far from normal, too. 

Figure 4. Upper air map at 500 mb. Black lines are lines of constant pressure (isobars), dashed red lines are lines of constant temperature (isotherms), and blue flags indicate wind speed and direction in knots. Source: http://www.spc.noaa.gov/cgi-bin-spc/getuadata.pl?MyDate1=151224&Time1=12&MyDate2=&Time2=12&align=V&Levels=500

The remainder of winter

A weather pattern shift occurred in the beginning of January causing Raleigh to experience below normal conditions for both temperature and precipitation thus far. The AO shifted negative, which caused the temperature pattern to be below/around normal. The remainder of January should be below normal for temperature and precipitation. If the storm tracks follow along the southern states and into the Gulf of Mexico, we could see a winter storm. As for February and March, the climate models indicate that the temperature could swing either direction (above or below normal) and the precipitation should be above normal (which is indicative of an El Nino pattern).