4/16/2013 2:00:00 PM Winter forecasters missed the mark
By Ron Thorkildson
The winter of 2012-2013 is now history. Weather information from hundreds of stations throughout the Pacific Northwest are being processed and combined with records collected from past years that are used to define our climate.
How well did the weather prognosticators do when they issued their forecasts last fall?
In November, weather experts speculated about the kind of winter that was in store for residents of the Pacific Northwest. The event was the Oregon Chapter of the American Meteorological Society's annual Winter Weather Forecast Conference held at the Oregon Museum of Science and Industry in Portland. Participating speakers were Clinton Rocky, National Weather Service; Kyle Dittmer, Columbia River Inter-Tribal Fish Commission; George Taylor, Applied Climate Services and former climatologist for the state of Oregon; and Jim Little, Oregon Department of Forestry.
The most important element that all the forecasters keyed on was the strength of El Niño/La Niña Southern Oscillation (ENSO) signal in the tropical Pacific Ocean. El Niño usually brings warmer and drier winters to the Pacific Northwest; La Niña cooler and wetter. But last fall the signal hovered around zero, and when neither El Niño nor La Niña hold sway the resulting weather tends to be produce near-normal values, but also with more variety.
This variety was reflected in the forecasts.
Dittmer and Taylor went with a normal November with regard to temperature and precipitation. Rocky, Dittmer and Little foresaw average temperatures in December; three of the four forecasters declared the month would be dry. Rocky and Taylor foretold of a dry January, while Rocky, Dittmer and Little said a dry February was in store. All believed average values would occur in March. Finally, three of the four thought that the region would have to deal with at least one artic air outbreak.
Here's what actually happened.
November was a shade milder and drier than normal. In December maximum daily temperatures were slightly cooler than normal, but precipitation levels at Sisters and Redmond were more than 150 percent of normal. It was during this period that the Cascades pick up most of their current snowpack.
But with the onset of January, things changed in a hurry. Fewer and weaker storms from the Pacific Ocean moved onshore, mainly in northern Washington and southern British Columbia, resulting in significantly drier-than-normal conditions throughout Oregon and Washington.
The north-to-south precipitation gradient was pronounced. The rainfall deficit was a modest -1.4 inches in Seattle and Portland, but -5.5 inches in Eugene. Sisters recorded no precipitation for the entire month! It was similarly dry in Bend, while Redmond received 39 percent of normal precipitation. Because of mostly clear skies, minimum daily temperatures were somewhat below normal.
Dry conditions continued through February as rain/snowfall was less than 10 percent of normal. Temperatures were average for the month. March was also dry, as Sisters and Redmond received just over half their normal precipitation; Bend came in at 92 percent of normal. Again, temperatures were average.
As of April 1 the northern and Central Oregon Cascade mountain snowpack (snow water equivalent) was at 83 percent of normal, according to hydrologist Marilyn Lohmann at the National Weather Service in Pendleton.
Perhaps the biggest surprises, and most significant misses made by the group of weather wizards, are the following:
â¯No one foresaw the ample precipitation levels that occurred in December.
â¯Similarly, the extremely dry January and February was completely unexpected.
â¯The Pacific Northwest was never threatened by an arctic air mass. Yes, on January 13 Sisters recorded a low temperature of 0 degrees Fahrenheit, but a true invasion of frigid air from the far north will send temperatures far below zero.
Without a strong ENSO signal upon which to anchor their forecasts, the weather prognosticators missed the mark.