Throughout the continental 48 states, the 2011-12 winter has been oddly mild. While past winters in Pennsylvania and other hardy winter regions have delivered a few warm, sunny days each year like previews of spring, they usually have been surrounded by more seasonally appropriate temperatures, freezing and below. This year it seemed that very cold days were the minority, and most states had little snow.
Paul Knight is a senior lecturer in Penn State\’s Department of Meteorology, Pennsylvania state climatologist and producer, co-host and on-camera meteorologist of \”Weather World,\” a 15-minute weeknight weather magazine show broadcast on the Pennsylvania Cable Network (PCN) and WPSU-TV. He also researches long-range prediction techniques and the use of artificial intelligence in forecasting significant weather events. He talks about the warmer weather this season.
Would you say that a mild winter like the one we\’ve had in Pennsylvania happens every few years, or is it fairly rare? And is it rare for the lower 48 states to all have a warmer winter, or is it usually more contained to one or two areas of the country?
Knight: The persistence of the mild weather this winter has been rare; that is, many parts of Pennsylvania measured 75 of the 91 days from Dec. 1 to Feb. 29 with above-normal temperatures. Overall, both in Pennsylvania and coincidentally nationally, the winter of 2011-2012 (December-January-February) was ranked fourth-warmest. The widespread nature of the warmth — most of the lower 48 averaging milder than normal — is unusual, though not truly rare.
When was the most recent winter in Pennsylvania\’s weather history that is comparable to this year\’s weather? Can you see many parallels between that season\’s data and this year\’s?
Knight: The winter of 2002 had about the same amount of snow as 2012 (21.6 inches versus 19 inches) and the following spring was wet with normal temperatures and the summer was rather warm and quite dry. However, analogs have limited utility when comparing a point location anomaly for a season’s departure from normal for an area.
Are forecasters able to look at weather patterns from the past and apply them to present-day weather to help with forecasting? If so, how? How accurate do past weather cycles help forecast future cycles?
Knight: The skill in predicting monthly and seasonal weather has been slowly improving during the past decade. The more reliable method has been to \”match patterns\” (analogs) of temperature and precipitation anomalies (departures from normal) or air flow regimes (shape and intensity of the jet stream) currently being experienced with historical data. Many of the matches are somewhat subjective and therefore are imprecise, but offer a viable alternate solution. The skill of computer models of the atmosphere (like the ones we use to make tomorrow\’s forecast) in predicting the next month\’s or season\’s temperature or precipitation is growing, and there is considerably more skill in predicting the correct sign of the temperature anomaly than there is for precipitation a month or two in advance.
Can you forecast how this summer, fall and maybe even winter will be based on previous weather patterns? If so, what might we expect for the rest of 2012?
Knight: Forecasts of temperature and precipitation anomalies are made for the entire United States for the next 13 months and updated each month by NOAA\’s Climate Prediction Center. These and other experimental model forecasts, along with analog prediction schemes, complete the suite of forecasts that are made each month for the next few months to a year in advance. Most are offered free, since they are developmental, and skill varies considerably by model and season. You can see an example at: http://www.cpc.ncep.noaa.gov/products/predictions/30day/.
This summer should be noticeably less hot than the previous two years, though may still average above normal. The winter of 2012-13 should see an El Niño develop, which would produce a bit cooler and wetter conditions than the past winter in Pennsylvania.
Some areas of the country, like Washington D.C., saw flowers and plants blooming earlier than expected, in mid-February. Is there still a chance these areas could have a frost?
Knight: The last freezes in Washington, D.C., usually do not occur until late March, so it is quite possible that the District will still have a cold snap that could damage new plants, especially in the suburbs. There are no indications of this occurring in the next 10 days, so if it does happen, it would probably not be until the last week of the month.
Can meteorologists point to any natural or predictable causes for these kinds of extended weather abnormalities?
Knight: There is a definite connection between an ocean index (La Niña) and a warm winter as well as an atmospheric index (the Arctic Oscillation), which was persistently positive, and a very mild winter. Both of these are natural causes with La Niña showing some predictability but not the Arctic Oscillation.
Are weather abnormalities occurring more frequently than in the past?
Knight: It somewhat depends on what scale — if you are measuring abnormalities by \”billion-dollar storms,\” then in 2011 they were much more frequent. But so far in 2012, there have been none.
Do you believe we should be concerned about any possible longer-term effects of this abnormal weather on things such as crop yields and water levels, or might future weather patterns this year leverage those effects?
Knight: The current warm spell, which should last until March 21-22, will make flowers and perhaps fruit trees vulnerable to a normal late freeze. This could reduce the availability of some local fruits this summer and raise the price of some annuals this year. While January and February were somewhat dry, it is really the spring that sets the trend for growing seasons and most indicators point to an average to wetter than normal spring.