October 21-22nd 2017 Flooding Rains

An early season atmospheric river brought substantial rainfall totals to northwest Oregon and southwest Washington over the weekend of October 21-22nd 2017. It resulted in a top 10 flood for the Wilson River at Tillamook with the river cresting just above 17 ft or ~24,500 cfs.  This was the earliest in the rainy season during the past ~50 years that such a large flood had occurred at this site.  Similarly, the Grays River near Rosburg also peaked just over 15.5 ft or at ~17,000 cfs, which was also the earliest in the rainy season that such a large flood had occurred in the past ~12 years.

8 AM - 2 PM Saturday October 21st 2017 Composite Radar

A warm front lifted northeastward across the area Saturday morning, but rainfall began to pick up across the area late Saturday morning, particularly across the Coast Range where west-southwest 850mb winds strengthened into the 40-60 kt range and Integrated Vapor Transport (IVT) values pushed well over 750 kg/ms. Rainfall amounts were generally in the 0.20-0.40" range across the Willamette Valley during this time period.  Across the coast range rainfall amounts ranged between 0.50-1.80" with the highest amounts falling across Lincoln and Tillamook Counties including 1.80" at Cedar RAWS.  The lowest amounts were across the far north Oregon coast range and the Willapa Hills where ~0.50" of rain fell.

2 PM - 8 PM Saturday October 21st 2017 Composite Radar

Strong west-southwesterly 850mb winds continued to enhance orographic lift and rainfall across the Cascades and Coast Range.  The main cold front can be seen starting to move southeastward towards the south Washington coast at the end of the radar loop.  Increasing lift ahead of this feature resulted in rainfall rates increasing across far northwest Oregon and southwest Washington with IVT values exceeding 1000 kg/ms.  This produced rainfall rates of 0.25-0.50" per hour for the entire period across the north Oregon Coast Range and Willapa Hills with Cedar, South Fork, Huckleberry & Abernathy RAWS all picking up over 2" of rain in the six hours.  Farther south, rainfall really backed off across Lane, Lincoln, Benton and Linn Counties during this six hour period.  Only ~0.10" fell around Eugene while PDX saw ~0.75" of rain in this six hour period.  

8 PM - 2 AM Sunday October 22nd 2017 Composite Radar

The lift ahead of the front, ample moisture and strong west-southwesterly 850mb winds continued to produce high rainfall rates across northwest Oregon and southwest Washington with IVT values continuing to exceed 1000 kg/ms.  The entire frontal system and atmospheric river dropped southeastward across the area between 8 pm Saturday October 21st and 2 am Sunday.  Rainfall rates peak between 1/3-2/3" per hour in this time period across the Coast Range with Abernathy RAWS in the Willapa Hills recording 0.74" between 9-10pm and Cedar RAWS measuring 0.67" between 1210-110AM Sunday.  Six hourly rainfall totals of 2.00-3.80" were recorded across the Coast Range during this time with Abernathy RAWS peaking at 3.82" in six hours ending at 11pm Saturday.  

Here is a look at the progression of IVT values and return intervals for those IVT values throughout the course of this event...

A number of rivers exhibited rapid rises due the rain that fell in the preceding 6-12 hours with the Grays, Nehalem, Wilson, Trask and Siletz Rivers all reaching flood stage.  You can find the corresponding hydrographs here...

2 AM - 8 AM Sunday October 22nd 2017 Composite Radar

The front really began to weaken during this time. Nonetheless, 1.5-2.5" fell across the Lane and Lincoln County Coast Range during this time.  

This produced a second spike in river levels across the Coast Range and Cascades.  

Coastal Locations

Inland Locations

The Time Delay Between Peak River Flows versus 6 Hour Rainfall

Email Colby at w-pqr.webmaster@noaa.gov for comments or questions.