December started mild at first but very cold arctic air pushed into the Pacific Northwest on the 3rd and 4th, bringing the coldest air since 1972. On the 6th, snow fell across the region
with 7 to 12 inches reported across much of the south Willamette Valley. On the 7th, skies cleared and the temperatures steadily dropped from the 20s during the day into the single digits in the evening. Cold air over the surrounding higher terrain drained through the river valleys and settled in the south Willamette Valley. To make it worse, snow covered ground and clear skies combined to make temperatures drop further. By late evening on the 7th, the temperature was down to 7 degrees below zero. Hours later on the 8th, it would be 10 degrees below zero
. This would make for the 2nd coldest night since 1890. Coincidentally the record coldest for Eugene was 12 below zero and was set on the same day back in 1972. December 2013 tied 1985 for the coldest average daily low temperatures. Overall, the average daily mean temperature of 33.5 degrees made December the 3rd coldest on record.
Warmer weather returned by the 12th with seasonable weather. But it remained on the dry side as persistent high pressure remained over the western united states. This high pressure brought light winds and set up strong inversions, which prevented good mixing of the air in the valley. This led to persistent fog and low clouds and stagnant air. NEW RECORDS:
December 4th set a new low with 18 F, beating the 20 F set in 1972. December 5th set a new record low of 14 F, beating the 20 F set in 1992. December 5th also set a record low high temperature of 31 F, beating the 33 F set in 1904. December 6th set a new daily snowfall record of 7 inches, beating the 4 inches set in 1956. December 7th set a new record low of 7 below zero, beating the 6 below set in 1972. The month was the 3rd coldest December for average daily highs and tied 1985 for coldest average daily lows.