Meteorologists at last weekend’s annual winter
weather forecast conference 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 Nina, 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 Nina 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 Nina 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
Evan Bentley of the National Weather Service
in Portland, is predicting a weak La Nina with cooler than normal sea surface
temperatures this winter (2017-18), after last year’s weak to neutral La Nina.
No weather models, he said, are predicting any kind of El Nino 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 Nina, he said. The sea surface temperature at this
time is identical to the 2016 temperature, signaling a weak La Nina. 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 Nina 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 Nina comes greater than normal snowpack in the Cascade
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 Nina 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 Nina 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 Nina, 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.
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
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
Overall, the winter in Portland was 1.6
degrees cooler than normal with 152 percent of normal precipitation, Dittmer
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 Nina. 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 https://oregonams.wordpress.com/2017/10/28/presentations-25th-annual-winter-weather-conference/
--CBB, October 20, 2017, “NOAA Still
Predicting La Nina Could Shape Coming Winter; Unrelated To Big Rain Hitting
This Weekend,” http://www.cbbulletin.com/439753.aspx