Nov 30,2007 00:00
For Oregon agriculture, this winter might bring the kind of weather that leads to ample water supplies next growing season. For all Oregonians, the long-term advice over the next several months is to dress warmly and try to stay dry. A three-month seasonal climate outlook has been released by Oregon Department of Agriculture Meteorologist Pete Parsons as part of a new service to farmers and ranchers who may need the data to do some initial planning for 2008. That same service provides an educated prediction on what the winter weather may bring.
"This is not like the daily weather report or the seven-day forecast you see on television," says Parsons, himself an experienced TV forecaster in Portland now with ODA. "This is a climate forecast that compares the next three months with what is normally expected for weather that period of time. In the last 20 years, we've improved our ability to predict seasonally whether we'll have above or below temperatures, precipitation, and other factors with some degree of accuracy that is now much better than a fifty-fifty chance."
Specific short-range forecasts may pinpoint these types of events, but the ODA seasonal climate forecast is designed more to give producers a heads up on what generally might happen based largely on historical records that show what happened when conditions in the tropical Pacific Ocean were similar to current conditions. There is enough data from the past to provide a good idea of what the immediate future may hold- in this case, the winter of 2007-08.
The accuracy of climate forecasts increases during what is called "El Niño", the name given to the periodic warming of sea-surface temperatures in the tropical Pacific Ocean, and "La Niña", which describes the periodic cooling of those same sea-surface temperatures. Both phenomena occur about every three to seven years. El Niño is usually associated with warmer and drier than normal winters for the Pacific Northwest with any cold spells typically seen early in the winter- from Thanksgiving through New Year's Day. La Niña usually brings colder and wetter than normal winters to the area with an increased chance for above normal mountain snowpack and valley snow in Western Oregon.
"During La Niña events, our mountain snowpack has a tendency to start building later in the winter, but it ends up being much greater than during an El Niño," says Parsons.
The higher snowpack and expected rainfall this winter bodes well for water use next summer. Oregon does not receive the summer rainfall that helps agriculture in the Midwest and other parts of the country. Oregon farmers and ranchers must rely on irrigation which, in turn, feeds off streams and reservoirs. A good buildup of snow in the mountains over the winter usually means ample water for those who will need it the following summer. Many parts of Eastern Oregon suffered water shortages earlier this year.
Oregon is dramatically impacted by changes in the tropical Pacific. La Niña's impacts are well documented and relatively clear. Oregon only sees a La Niña this strong every 15 to 20 years.
"The other thing that tends to happen with a La Niña event is that we get at least one Arctic outbreak of significance in Western Oregon with above average snowfall on the Willamette Valley floor for the season," he says. "And, we'll tend to get all or most of that snowfall during the Arctic event. I would expect us to get something on the order of six inches or more of snow in the valley, and the most probable time for it to happen is sometime after the middle of December up through the first few days of February."
The coldest weather in history for Western Oregon took place in February 1950 during one of the strongest La Niñas of the century.
With Oregon so diverse in topography, it is hard to paint the state with one weather brush when it comes to forecasting the winter. While statewide snowpack could look good this winter and Western Oregon could get an Arctic blast, Parsons believes Central and Eastern Oregon will generally have warmer and wetter weather than usual. Obviously, any Arctic air impacting the western side of the state will affect the east side as well. But the usual cold winter temperatures east of the Cascades may moderate more than normal this year.
The seasonal climate forecast also contains a couple of cautionary possibilities that could have adverse impacts on agriculture and all of Oregon.
"There is the potential for flooding in Western Oregon because of rain that would come right after a snow-producing Arctic outbreak," says Parsons. "History shows that during La Niña, Oregon comes out of an Arctic event with a warm, mild, rainy period. Also, looking at past La Niña years similar to this year, there is a higher than normal tendency for a wind storm in Western Oregon with wind speeds in excess of 80 miles per hour along the coast and 60 miles per hour in the Willamette Valley."
Parsons says the severe floods of 1996 and 1964 occurred during La Niña.