Warm winter wrap-up
Posted March 13, 2012
For those of you who enjoy keeping up with the statistics of our seasons, the numbers are in for the winter of 2011-12, and they certainly confirm our perception that it was a mild winter. For a broad overview, I included a pair of maps from the Climate Prediction Center that show the mean temperature for the 90 days ending February 29th - they show that most of our state averaged between about 42 and 48 degrees for the winter (top map), and that those temperatures were significantly above normal, by about 3-6 degrees for most of the state. It's also apparent that we had a lot of company across the country, with only portions of the desert southwest and the Pacific northwest falling short of normal.
Zooming that down to some more specific numbers for the Raleigh-Durham airport, we find that our mean winter temperature there was 47.0 degrees F (based on monthly means of 48.5 for December, 45.7 for January and 46.9 for February), which makes it the 3rd warmest of the 68 winter seasons in the RDU period of record and placing it in the warmest 5% of winters there. The only two winters that were warmer were 1948-49 (averaging 47.5 degrees) and 1949-50 (47.1). All of these temperatures are well above the 68-year winter average of 41.9 degrees.
Winds during the winter overall averaged 5.8 mph at the airport, a little below the normal winter average of 7.0 mph. Just the same, we had some windy episodes, with gusts as high as 44 mph in December, 41 in January and 46 in February.
When it comes to precipitation, it was a rather dry winter overall, with RDU recording 5.95 inches. This compares to a period of record average of 9.79 inches, and was the 5th driest winter so far for the airport, and among the driest 10% of winters. The driest winter on record there was 1985-86, when only 4.34 inches was observed.
Only a small fraction of this year's winter precipitation was "wintry" in nature, as we ended up with 9 tenths of an inch of snow there, compared to a 68-year average of 5.5 inches. There have been a number of years with too little snow to measure, of course, and on the high end we've had as much as 25.8, which fell in 1999-00.
These results are consistent with the overall trends for a La Nina winter, although it is worth noting that while La Nina tilts the odds toward drier and warmer than normal winters here, it does not serve as a guarantee of that outcome. Incidentally, the La Nina that is currently in place has been weakening in recent weeks, and appears as if it will end by April, as hinted at in the second image, which shows a group of computer model projections for the departure from normal of sea surface temperatures over the equatorial Pacific. There is a spread of solutions, but the general trend is for warming, and the Pacific is considered to be "neutral" when the anomaly is between +/- .5 degree C, while La Nina conditions are represented by values lower than -.5 and El Nino conditions are represented by values above +.5 degree. Most of the models in the graph show neutral conditions from April or May through the year, although there are a few warm outliers that indicate a possible summer/fall El Nino development.
Apart from lower heating bills, some of you may have experienced other effects of the mild season. Here at WRAL, Tim Grissom, the person in charge of managing the plant cover that keeps our station campus, and our transmitter site, looking nice, reports that there have been numerous effects from the warmer weather, including problems with weeds that would normally not exist in January and February, along with a requirement to continue mowing fescue lawns that normally would have become dormant for about two months.
He's seen some pines and oaks that are on the verge of blooming two to three weeks ahead of schedule, and noted that this could be an indicator of an early pollen season to come. Also, with many species running early with their progress, there is the concern that a late turn to colder weather, and perhaps a couple of late season season hard freezes, could be more damaging than usual. We have no way of being certain as yet whether that will happen or not, though for the moment model projections are leaning toward warmer than normal temperatures through at least the next ten or twelve days.