EXTENDED WINTER FORECAST 2013-2014
Winter officially starts on the Winter Solstice on December 21, the shortest day of the year for the northern hemisphere while the suns rays are directly over the Tropic of Capricorn in the southern hemisphere. Every winter since 1981 Medford has received a daily snowfall of 1 inch, 2 inches every other year and 3 or more inches every 5 winters; the last time we saw 4 inches of snow was in 2008…five years ago! The west side valleys are due for a big winter snowfall this year!
Last winters’ jetstream pattern is similar to what we’ll see this Winter with a high pressure ridge dominating the area. This tends to bring quiet winter weather, though storms can break down the ridge and can be significant. Last December we saw 5 days to start the month of heavy rain and a total of 19 days of measurable rain making December 2012 the 7th wettest December on record! On the opposite of the spectrum January was record breaking dry; in fact it was the 9th driest January on record! February was also below average for precipitation and all Winter was cooler than normal.
When we do a seasonal forecast we no longer look at weather models (short-range) but rather teleconnections which are better predictors of long-range forecasts. You may have heard of some including El Nino and La Nina which are part of the El Nino Southern Oscillation or ENSO. Just like last Winter, this year we have a neutral ENSO which makes long-range forecast that much more difficult. La Nina and El Nino are the strongest indicators of what type of Winter the nation will see, and with a neutral set up you can get a “mixed bag” of weather! Just look at last Winter, we had record breaking wet AND dry months! In fact the 1962 Columbus Day storm also occurred during a neutral ENSO. Since ENSO is neutral then we have to look at other teleconnections, and the influential on the Pacific Northwest is the Pacific Decadal Oscillation or the PDO.
Just like last Winter we are in a cool phase of the PDO, which means cooler than normal sea surface temperatures in the eastern Pacific. This pattern tends to bring cooler and drier weather to the west coast during the Winter months. I looked back at the last 30 years and found years with a neutral ENSO and cool PDO to see what weather southern Oregon and northern California reported. On average December and February trended cooler and drier while January trended warmer and wetter. But remember last Winter? All months were cooler than average and December was much wetter while January was much drier, hence there is variability with this set up. My overall forecast is that we’ll be cooler and drier for the majority of the Winter, but storms will pass through and have the potential of being significant. Though, if we receive just average precipitation the drought conditions for southern Oregon and northern California will likely improve. For fire season, it is more beneficially to have a wetter end to the season to help wet the fire fuels before we head into the hot, dry Summer months.