NOAA has just released its official winter weather outlook for the 2012-2013 season. Alex has a great blog including Devon's interview of the deputy director of NOAA's Climate Prediction Center.
But what are the factors and tools that go into these winter outlooks that many of we meteorologists (Doug and Bob will give theirs in a few weeks) make or have already made? NOAA does put a lot of stock into winter pattern correlating with the status of El Nino (ENSO) . But we can also look at hints from current snowfall in Eurasia.
First and foremost, it doesn't appear El Nino will be strong this year. Currently, the ONI index, basically a measurement of the strength of La Nina or El Nino is 0.3. Anything above 0.5 is considered an El Nino and above 1.0 is a strong El Nino. Conversely, a -0.5 is La Nina and -1.0 or below is a strong La Nina. Therefore, -0.5 to 0.5 is a neutral phase.
Forecast models show it will likely increase to around 0.6-0.8 this winter (December, January and February).
What is El Nino? The warming of the Equatorial Pacific water which has been found to change weather patterns across the globe. Note below how the Equatorial Pacific has warmed since January (circled area in each image).
Analog winters in which a weak El Nino (0.6-1.0 ONI index) was in place for DJF include: 1953-1954, 1958-1959, 1969-1970, 1976-1977, 1977-1978, 1994-1995, 2004-2005.
These winters tended to bring cooler than average temperatures but below average precipitation.
The second biggest factor worth looking at (I do consider myself) is Eurasia snowfall. Why look at snowfall in northern Asia as an indicator of our weather back home? Well, studies have shown that October snowfall in Eurasia is a driving factor for another very important variable that has a significant impact on temperatures; the North Atlantic Oscillation during our winter months.
What’s the North Atlantic Oscillation? It’s basically the fluctuation in the jet stream that brings cold air from the north and warm air from the south and based on the atmospheric pressure between low pressure that is usually firmly established near Iceland and high pressure that tends to reside over the Azores.
Essentially, when the jet stream dips south in the East and allows cold air to funnel into the Nation’s Capital, the NAO, as it’s called, gets into a negative phase. The opposite happens when the jet stream retreats to the north and warm air invades from the south.
A recent study has linked Eurasia snowfall with the NAO, especially in October. One report suggests if snowfall is ABOVE average in Eurasia in October, the NAO will tend to be negative during the winter and if snowfall is BELOW average in Eurasia in October then the NAO will tend to be positive during the upcoming winter.
Eurasia snowfall has been below average for the past several months and continues that trend through mid-October. Take a look at the connection between Eurasia snowfall anomolies and winter temperatures in Boston below. When snowfall is above average, temperatures tend to below average and vice-versa.
Given these two major factors, (right now mid-October, remember) the odds or probabilities suggest this for the winter: The winter will start out warmer than average and then trend towards near normal for temperatures by February. Precipitation will be slightly below average.
Will we get a “biggie” snowstorm? Well all it takes is one storm even if the winter is otherwise an "easy" winter and predicting one storm months in advance is impossible. With the current ENSO outlook and the lack of widespread snow cover in the far north, the odds (remember right now, Doug will have more data for his outlook on the 5PM News Monday November 12) of major snowstorms are I think, less than 40%. NOAA's outlook is sure a bit fuzzy with "equal chances" of above or below average temperature and precipitation (in winter that means snow or ice)
What is the typical winter like in D.C.? The average winter temperature is 38.2 degrees and average snowfall is 15.4 inches. In 8 of the last nine winters the D.C. snowfall has been below average. But there was that one winter of 2009-2010 that not only was above average with 56.1" of snow but was a record breaker. The odds of something like that again this winter? Want to bet you win the next Powerball jackpot?
Typically, the Shenandoah Valley and high areas west and north of D.C. averages a bit more with 20-25 inches from Winchester, Va., to Hagerstown. Be sure to keep watching and reading. We'll all be updating anything to do with winter weather and if any storm does threaten, the largest team of broadcast meteorologists in Washington will sure be on top of everything. Before we know it, the first crocuses of spring will be out.