January Flooding and a Look Ahead

January was a remarkable month in what has been a very unusual winter. This year we have seen a pattern consisting of extended dry spells punctuated by intense rainfall.

Case in point: December was on its way to being the driest December in our history (records go back to 1889), with barely a quarter inch of rain by Christmas Day. In the last week of the month, however, more than 4 inches fell, giving the month a respectable 4.77 inch total – still far short of the monthly normal of 6.94 inches, but not bad.

Then it got dry again; January was relatively quiet until the 16th, when a couple days of snow occurred. This was followed by heavy rain, which began on the morning of the 17th. By 8 am the next morning (when observations are taken at Hyslop Experiment Station, our official local station), the rainfall totaled 2.55 inches – a very wet day but not a record-breaker. Over the next 24 hours, however, the rain intensified, and by the morning of the 19th another 4.02 inches had fallen. This was just the third time we have received more than 4 inches in one day, and it was a new daily record for January 19th.

More significantly, the two-day total of 6.57 inches was the highest two-day total ever recorded here, surpassing.the record of 5.64 inches set in February, 1996.

And all that rain, not surprisingly, led to a flood – one of the more significant in recent decades. But unlike the 1996 and 1964 floods, this wasn’t a huge flood on the Willamette; instead, it was the tributaries that had the most significant flooding.

Locally, the Mary’s River near Philomath in Benton County crested at 21.3 feet, setting a new record. The previous record — at 21.1 feet — was set Dec. 30, 2005. Flood stage for Mary’s River is 20 feet, according to the National Weather Service. Another high spot in the region was on the Luckiamute River near Suver, which crested at 32.6 feet, 5 feet above flood stage.

Two things kept the Willamette from exceeding flood stage: (1) the two-day length of the heaviest rain; and (2) the relatively low snow pack. In general, the bigger the river, the longer the rain storm needed to cause an extreme flood. The February, 1996 flood was caused by a very wet 4-day event. The rainy period in a comparable flood in December, 1964 lasted nearly a week. Both caused extreme flood events on the Willamette and the Columbia.

Shorter events, however, tend to affect the tributaries more significantly. Thus, this year’s two-day storm, which produced such extreme values on Willamette tributaries, had a less significant effect on the bigger rivers.

Snow (or lack thereof) also had an effect on the flood. In 1996 the snow pack in the Willamette drainage was 110% of average when the rains began. Four days later most of the snow had melted and was filling the rivers. Ditto in 1964. This year, however, there was much less snow when the rain storm began – only 39% of average on January 17th. A big snowpack would have caused a much different result.

And now we’re back to an extended dry spell again. Spring approaches. The Climate Prediction Center (NOAA) is predicting a cool, wet spring. Again! The last two springs were much cooler and wetter than average.

But I disagree. I’m predicting a spring that’s closer to average. Followed by another world-class Oregon summer.

I am SO ready!

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2 Responses to January Flooding and a Look Ahead

  1. Snowed at my place this morning (450 ft, eastside W. Valley). I suppose that is average for late February, but… The extended La Nina plus PDO cold phase portends colder than average temps this Spring. Not sure about wetter or dryer, but I’m guessing wetter. And as much as I too would like a long warm summer, my magic 8-ball says not likely.

  2. Record March snow. Looks like I was right in my prognostication. Hope I don’t get rotator cuff injury from patting myself on the back.

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