The Record Breaking Blue Mountain Flooding of February 6-8, 2020

There has been a lot of discussion about heavy rain and flooding around the Northwest, but one region really stands out, with river levels and flooding unmatched in many decades, if ever:  the region on and near the northwest side of the Blue Mountains of northeast Oregon.

Walla Walla River from Hwy 12 Provided By Kevin Pogue of Whitman College

Walla Walla River Provided by Kevin Pogue

To orient you, a topographic map of the region is shown below.  The Blue Mountain's slopes on the northeast side are oriented southwest to northeast--that is going to be very important in understanding what happened a few days ago. 

Heavy rains on these northeast slopes flowed into rivers heading to the northwest, bringing most to flood stage and some to record flood levels not observed in a century.  I-84 and other roads were closed in places.

I-84 West of Baker Oregon

The rainfall totals for this precipitation event were extraordinary: some places received nearly 10 inches, and 3-6 inches were commonplace (see totals below for Feb 5-8).

The 7-day total precipitation from the National Weather Service show the situation with a wider view (see below).  You can also the even larger amounts on the western side of the Cascades, but the larger rivers on the western side of the Cascades can handle huge amounts of rainfall better. Still there was plenty of flooding in western Washington.

The intense precipitation on the northwest side of the Blue Mountains caused the rivers to surge, some to record-breaking levels.  Not for that day.  FOR ANY DAY ON RECORD.

For example, the  discharge of the Umatilla River (Near Gibbon, Oregon)...see map below... blasted the previous all time record (black line) and far exceeded daily records (red triangle).

So why was this event so extreme for northeast Blue Mountains?   Good question.

The answer was that we had an unusual multi-day atmospheric river that came in from the west-northwest, with wind optimized to push directly up the northwest side of the Blue Mountains (see water vapor channel satellite picture last on February 6th).  The result was intense precipitation and flooding.