PORTLAND — Meteorologists at the annual winter weather forecast conference earlier this month came to near agreement on three things: the 2016-17 winter was unusually nasty, weather this coming winter will likely be influenced by a neutral to weak La Niña, as it was last year, and there will be near normal snowfall or more at higher elevations in the northern Cascade Mountains in Oregon.
What they didn’t agree on was the strength of the La Niña nor did they agree on what that could mean this year following a very dry summer, as well as the amount of snow that could fall at low elevations in the Willamette Valley and Portland.
Every year for 25 years, members of the Oregon Chapter of the American Meteorological Society come together to compare notes on upcoming weather prediction at the Winter Weather Forecast Conference at the Oregon Museum of Science and Industry in Portland.
Last winter’s La Niña lived up to its reputation, said Fox 12 meteorologist Mark Nelson as he reviewed the winter of 2016-17. “This was a crazy year,” he said, “and the most active winter since the 1995-96 winter.”
For much of the Northwest there were three consecutive cold months and it was the coldest in 24 years (1992-93 winter). For the Portland area it was the coldest since the 1979-80 winter, he said. With four snow events and four ice storms, the official snow total in Portland was 11.2 inches, while the downtown area got 15.5 inches.
The area set a record for the warmest November, followed by a record for the coldest December, and below average temperatures in January and February.
Precipitation was above average for much of the Columbia River basin December through February. Snow depth above 5,500 feet elevation was an average of 56 inches December 6, the most in 10 years, Nelson said.
NWS: Weak La Niña
Evan Bentley of the National Weather Service in Portland is predicting a weak La Niña with cooler than normal sea surface temperatures this winter (2017-18), after last year’s weak to neutral La Niña. No weather models, he said, are predicting any kind of El Niño with warmer than normal sea surface temperatures, but some are predicting neutral conditions.
Deviations in ocean temperature that are 0.4 to -0.4 degrees are considered neutral conditions, while -0.5 to -0.9 degrees are considered a weak La Niña, he said. The sea surface temperature at this time is identical to the 2016 temperature, signaling a weak La Niña. However, that could change to neutral in January or February, he said.
Bentley concluded saying there is a 55 to 65 percent chance that a weak but potentially short-lived La Niña will occur this year. Last year the chance was 70 percent.
Over most of the Columbia River basin, there is an equal chance of normal temperatures, with a greater than 33 percent chance of lower than normal temperatures in the northern areas.
For most of the basin, average precipitation is predicted, except in Idaho where there is a greater than 33 percent chance that precipitation will be above normal and in Montana where the above normal chance is over 40 percent.
Snow in the mountains looks good, he said. Generally, with a La Niña comes greater than normal snowpack in the Cascade Mountains.
Odds favor drier year
Rod Hill, meteorologist with KGW TV in Portland, was the contrarian. He predicts lower than normal precipitation in the Portland area, which measured in 2016 51.4 inches. An average water year in Portland is 36.5 inches. He said he wouldn’t be surprised if the La Niña didn’t develop and instead stayed neutral.
“What happens after a hot summer (24 days of 90 degrees or more)? No winters go on to develop a La Niña and the odds favor a dry year,” he said, predicting precipitation in Portland will be just 32 inches, four inches below normal.
Hill said that looking back at five dry summers, none were followed up by a La Niña, weak or strong. In fact, three of the five were dry winters. He predicts a quiet winter, below normal precipitation and little Willamette Valley snowfall.
He also predicted a normal November, but a December that will be 2 to 3 degrees below normal, followed by a warm January and a February similar to December. Snowpack on Mt. Hood will be 105 percent of normal, he said, but that lower elevation ski areas, such as SkiBowl at the Mt. Hood pass has a 33 percent chance of being too warm.
Or similar to last year?
November 2016 in Portland had 123 percent of normal precipitation and temperatures were 7 degrees higher than normal, said Kyle Dittmer, hydrologist/meteorologist at the Columbia River InterTribal Fish Commission.
December, however, turned around with 83 percent of normal precipitation and temperatures 3.6 degrees lower than normal. January 2017 was 7.6 degrees cooler than normal and 89 percent of normal precipitation; February was 3.3 degrees cooler than normal with 260 percent of normal precipitation; and March was 0.5 degrees cooler with 203 percent normal precipitation.
Overall, the winter in Portland was 1.6 degrees cooler than normal with 152 percent of normal precipitation, Dittmer said.
Last winter’s water supply forecasted by Dittmer at The Dalles Dam (January through July) was 111 million acre feet or 109 percent of normal (issued October 2016) and 101 MAF (issued April 2017), but actual water supply was 137 MAF, he said. This year, Dittmer is predicting 112 MAF, give or take 14 MAF, which is 109 percent of normal.
Dittmer is predicting a similar winter to last year’s with a weak La Niña. He is predicting near normal precipitation in the mountains, with November and March producing the most snow.
He predicts five snow events in Portland for the winter (last winter’s actual number of events), with the first snow event in early December. But his prediction also includes intense rain, floods, fog, gorge winds and freezing rain for the Portland area.
Precipitation in November will be 114 percent of normal, in December it will be 99 percent of normal, January will be 102 percent of normal, with 100 percent of normal in February and 117 percent of normal in March.
He predicts slightly lower than normal temperatures in the Columbia River Gorge during the winter of 2017-18 with lots of snow (about 135 percent of normal), with a seasonal total snow of 35 inches.
Presentations from the conference are at tinyurl.com/2017-Winter-Outlook.