How it all came down

Deep snow and ice atypically accumulates on the banks of rivers in Eastern Oregon before the Pineapple Express sweeps the region. (Corps of Engineers photo)

Deep snow and ice atypically accumulates on the banks of rivers in Eastern Oregon before the Pineapple Express sweeps the region. (Corps of Engineers photo)

 

By Dave Elson and Andy Bryant, National Weather Service

Extreme weather conditions deliver the 1964 Christmas Flood

Rain steadily fell around the state. (Data source: National Weather Service)

Rain steadily fell around the state. (Data source: National Weather Service)

Flooding can be a common occurrence in the Pacific Northwest in the late fall and winter. Indeed, the wettest months of the year in a region known for its rainfall are generally November and December. Storms moving in off the Pacific are responsible for the frequently wet weather. Occasionally these storms will pull moisture-rich air up from the tropics, often from the vicinity of Hawaii. Known locally as a Pineapple Express and broadly as “atmospheric rivers,” the connection between storms and tropical moisture can result in impressive rains. The windward sides of the mountains of western Washington, Oregon and northern California see the heaviest rains in these storms, where 24-hour totals on occasion exceed 6 inches.December 1964 brought such a storm, but on a scale rarely matched in the region’s history. Torrential rains falling on an unusually high snowpack produced one of the region’s great floods, known today as The Christmas Flood.

November and the first half of December were a little wetter than normal across the Pacific Northwest, but not extremely so. It was an outbreak of arctic air in mid-December that set the stage for the floods that would follow. On Dec.15, arctic air pushed into northeast Oregon, dropping the temperature in Pendleton from 41 degrees Fahrenheit during the day, to just 8 F by midnight. The temperature would fall to 12 below zero the next day. Two inches of snow accompanied the cold front in Pendleton.

A 'silver thaw' encases everything it touches in ice. (National Weather Service photo)The cold air continued to push southwest and by Dec. 17 covered all but the far southern part of Oregon. The morning of Dec. 17 saw new record lows for the month. Minimum temperatures west of the Cascades were generally from 5 to 15 F, while east of the Cascades minimum temperatures dropped to between -5 and -38 F. This was the coldest December weather since 1919.

The end of cold weather outbreaks in the Pacific Northwest are often connected to warm Pacific storms that meet the cold air, resulting first in snow and freezing rain and eventually just rain. In 1964, storms off the Pacific began to push in Dec. 18, dumping heavy snow and gradually warming the air mass. All but the warmest valley locations saw snow, and within 36 hours near-record snow depths for this time of year had accumulated around the state, including much of the Willamette Valley. Up to one foot fell in the Coast Range, with three to four feet in the Cascades. In the lower western valleys, snow amounts ranged from just a trace in Medford to 11 inches in Portland.  Substantial snows also fell in central and northeast Oregon, while blizzard conditions trapped hundreds of motorists and stopped rail traffic in the Columbia River Gorge.

Out over the Pacific Ocean, the weather was no less interesting. Super-typhoon Opal formed over the west Pacific on Dec. 9 and grazed the Philippines as a Category 5 storm on Dec. 13. Opal dissipated by the Dec. 16, but remnants of the storm moved east and energized the atmosphere over the northeast Pacific Ocean. A strong jetstream then developed and stretched from north of Hawaii to the Pacific Northwest. This persistently strong atmospheric river brought moisture-rich and much warmer air, including the remnants of Typhoon Opal, into Oregon and northern California. This combination of conditions resulted in extremely heavy rain over an extended period.

An unusually heavy snowpack initially absorbed some of the rain. The melt water, however, ran directly into creeks and rivers, since the ground was already saturated and frozen in many areas. The freezing level rose from the valley floor on Dec. 18 to over 8,000 feet by Dec. 21. The high temperature at Salem was only 28 F on the Dec. 18, rising to 61 F on Dec. 22.

An arctic cold front brings a “silver thaw,” encasing a pumphouse on the Clackamas River. (Corps of Engineers photo)

An arctic cold front brings a “silver thaw,” encasing a pumphouse on the Clackamas River. (Corps of Engineers photo)

The snowpack in the Coast Range and Cascades collapsed over three days, creating massive amounts of rainfall and snowmelt running off together. At Government Camp on Mount Hood, located at 3,900 feet, the snow depth was 55 inches on the morning of the Dec. 20. The amount of liquid water contained in the snowpack was 5.4 inches. On the morning of Dec. 21, the snow depth had dropped to 45 inches, but water content increased to seven inches. By Dec. 23, snow depth was only six inches, with most of the seven inches of water content released from the snow. Meanwhile, nine inches of rain had fallen for a total of 16 inches of runoff.This storm toppled many 24-hour and monthly rainfall records across the state and many of those records still stand today.

For more, visit the Corps’ Christmas Flood of 1964 website.

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