Published: 2012-02-06 07:57:55
Updated: 2012-02-06 07:57:55
Posted February 6, 2012
By Mike Moss
Now that we've wrapped up the first two months of the winter season (in terms of climate statistics, anyway), how do the numbers look at RDU for the December-January period? It will not surprise you to hear that for that two-month span, the average temperature of 47.1 degrees was more than 5 degrees above the long-term average, making it the 3rd warmest out of 68 seasons on record for that site. In terms of precipitation, it was fairly dry with 4.01" of rain (2.6 inches below normal), continuing a trend of somewhat suppressed rainfall that began back around November. With warm temperatures and a fairly dry pattern, we received no snowfall at the airport for December-January, compared to a normal for the period of 3.5 inches.
It's always interesting, but difficult, to consider the question of why the weather in any given year might run unusually warm or dry or whatever the departure in question happens to be. This season there has been a tendency to frequently have cutoff upper level systems park for a while over the western U.S., leading to a compensating ridge of high pressure aloft over the eastern parts of the country, and associated with the ridge warmer temperatures and less than typical precipitation. It's always tempting to simply ascribe such a result to a large-scale influence like La Nina (cooler-than-normal equatorial Pacific sea surface temperatures), which in a long-term statistical sense has been shown to correlate weakly to warmer and drier-than-average winter seasons in our area.
Unfortunately, that isn't a terribly satisfying answer, because of all the other more variable influences that come into play, which means that the presence of a La Nina pattern isn't all that reliable in predicting, or explaining, a given season's results. The past year is a pretty good indicator of that, since we can consider the fact that this year's Dec/Jan temperature at RDU (with a weak-moderate La Nina in place) was the third warmest on record there, while last year during the same time frame, with a moderate La Nina underway, we had the third coldest Dec/Jan period at the airport. Both seasons did result in total precipitation amounts that were close to 3 inches below normal, but the difference in temperature mean this year we only got rain, while last year we had more than 8 inches of snow.
That same variability in results is evident in a look back at previous La Nina Dec/Jan periods. I scanned through the last 10 such seasons prior to this year, which goes back to the winter of 1974-75, and found that in those ten years, we had 2 with temperatures very close to normal, 4 that had above normal temperatures and 4 that were below normal. For total precipitation, the results were similarly mixed, as 4 seasons had precipitation that was at least an inch below normal, 3 were at least an inch above, and 3 were close to normal. Snowfall in those seasons ran above normal by at least an inch 4 times, and below normal by at least an inch on the other 6 occasions.
We'll be watching to see if the rest of the winter follows the trends we've seen so far, keeping in mind that patterns can change with rather short notice, and that we have had some notable snowfalls in the past as late as March, and some measurable snow as late as mid-April!