Winthrop hit 71 degrees, thanks to ‘Chinook winds’
It was a beautiful, balmy – and somewhat unsettling – day, with the temperature hitting a record-breaking 71 degrees in Winthrop on the first day of December. Then – within a couple of days – the Methow was blanketed with a few inches of snow and barely got above freezing.
That spring-like temperature is more than twice the normal high of 33 degrees recorded on Dec. 1 over the past 30 years, said Service Hydrologist Robin Fox with the National Weather Service (NWS) in Spokane. The low this Dec. 1 was 37 degrees. The normal low is a frosty 19.
Omak also set a record, at 74 degrees, but other areas, while warm, didn’t rival Winthrop. It was 68 in Moses Lake and 69 in Ephrata, Fox said.
“When you get 70 degrees in the Methow Valley, that isn’t too good for cross-country skiing,” noted another NWS meteorologist.
While December’s start shattered records, November was also warmer than usual, with temperatures in Winthrop 1 to 3 degrees above average for the month, Fox said.
By early morning on Dec. 2, things had normalized somewhat. People across the valley woke up to snow on Saturday (Dec. 3) and again on Monday. The temperature dropped into the low 20s and didn’t get above 29 degrees on Sunday.
A few factors contributed to the anomalous warm spell. The jet stream was exceptionally far north, well north of the Canadian border, and was accompanied by vigorous winds from the south, NWS Weather Forecaster Greg Koch said. While it’s not unheard of for the jet stream to veer north in the winter, last week was really unusual, he said.
The effects of the jet stream were magnified by strong, Chinook winds blowing across the Cascade ridges, which significantly warmed the downslope side, Koch said. Without those phenomena, the Methow is too far north – and days are too short – to warm the air with sunshine alone, he said.
Warm and wet
November brought more precipitation than usual to most of Washington, although that effect wore off in the eastern part of the state. The western slopes of the Cascades saw record-breaking precipitation – mostly rain, because it was so warm. But, east of the mountains, the wet weather was confined mostly to the Methow Valley and the east slopes of the Cascades. Eastern Okanogan County actually received less precipitation than usual, Fox said.
With cold weather in early November, the mountains got considerable snow. But, by the end of the month, the warmer weather dropped rain instead, melting much of the snowpack.
At the end of November, there were 37 inches of snow at Harts Pass, down from a high of 41 inches earlier in the month, according to the Natural Resources Conservation Service (NRCS), which maintains 70 mountain SNOTEL sites around the state. And it’s a soggy blanket of snow, with the density at 36%, compared to a normal of about 25%, said NRCS Water Supply Specialist Scott Pattee. The snow had compressed to 34 inches by Dec. 3 before adding 5 inches from the weekend snowfall.
Although total precipitation for the water year, which starts on Oct. 1, is above normal in many basins, snow-water equivalent is low almost everywhere. For precipitation alone, Harts Pass is at 140% of normal and Rainy Pass at 152% of normal, Pattee said.
But Harts Pass, with a snow-water equivalent of 110%, is one of just three SNOTEL sites – those at the highest elevation – above normal. In most basins of the state, including the Central and South Puget Sound and the Olympics, the snow-water equivalent was just a third of normal on Dec. 6. In the Lower Yakima basin, it’s only 4% of normal. Still, because it’s early in the season, there’s plenty of time for them to catch up, Pattee said.
The December runoff is less concerning than accelerated snowmelt in the spring, since there’s plenty of snow in the mountains to absorb any rain. Particularly once colder temperatures set in, the mountain snow should provide a good base throughout the winter, Pattee said.
While no single weather event can be attributed to climate change, climate change does cause more extreme weather events, Fox said.
The long-term forecast is for weather east of the Cascades to be normal – or warmer than normal – in the first half of December. Later in the month it could be cooler than normal, she said.
Rain and melting snow have caused rivers to surge, aquatic ecologist John Crandall said. “It’s not unprecedented to have a high flow, but these are very, very high flows for this time of year,” Crandall said.
Flows in the Methow River at Pateros reached 3,230 cubic feet per second (cfs) on Dec. 2, before cooler weather brought the flows down to 2,260 cfs as of Dec. 6.
The highest recorded flows for this time of year were in 1996, when the river reached 5,770 cfs on Nov. 30, before dropping to 3,300 by Dec. 2. The lowest recent flows were in 2003, at 267 cfs, according to the U.S. Geological Survey (USGS). The mean flows for the first week of December range from 528 to 583 cfs.
Spring and summer Chinook, coho, and bull trout have all deposited their eggs in riverbed gravel, where they’ll remain until they hatch in the spring. Because high flows and rapid velocity can pull the redds apart, they pose a potential threat to the eggs, Crandall said.
These fish evolved to deposit their eggs in the water at the right time of year, when rivers are typically calm and flows are low. If rivers run high in the winter more often, it could affect reproductive potential over the longer term, Crandall said.