I was cleaning out a file tonight here at work (slow Sunday), and found this old BPA rain map I’ve had for at least 20 years. I think it is from Charlie Feris, who worked at the BPA for a long time. He (and maybe someone else) had a collection of rainfall observers around our area. Note it’s a 15 year period from the late 1960s to early 1980s.
Rainfall distribution in our area is totally dependent on local topography. Much of the year air flow is from a westerly direction. Where air rises over hills or runs into mountains, rainfall increases as clouds dump their load of moisture. And the effect begins before the land begins rising too as clouds pile up ahead of the increasing elevation. When air descends off hills/mountains, rainfall accumulation is lower.
Those of you that have lived here a long time know much of this, but it might be new info to some of you. Take a look at the map and then I have a few thoughts below. Feel free to add your own comments of course.
- The driest parts of the metro area are in the lowest elevations in the Tualatin Valley and along the Columbia River from Vancouver Lake to NE Portland. That includes the Hillsboro, Portland, and Vancouver airports. These areas typically average less than 38″ of rain each year. Another dry spot, although not quite as dry, is the northern Willamette Valley south of Wilsonville.
- No surprise, but the wettest locations are the eastern suburbs which are nestled up against the Cascade foothills and highland areas in eastern Clackamas/Multnomah/Clark counties. Those locations receive 50″ or more. That includes much of Gresham, Boring, Damascus, Battle Ground, Camas, & Washougal. I see Oregon City and West Linn get enough lifting from terrain in those areas to push the average to 50″ as well.
- Some of our far eastern towns/cities get even more. Looks like Estacada, Sandy, and eastern Washougal area gets 60″ or more most years! That’s an extra 22″ over PDX.
- Chehalem Mountain (between Newberg & Hillsboro/Sherwood) is a sharp rise from showers/rain coming in from the southwest, so that area gets 54″ or so. So do the Tualatin Mtns (West Hills), although to a lesser extent. Still, it’s amazing that the West Hills up around Germantown Road averages 50″ yet just a few miles northeast along the Columbia River it’s more like 38″.
- There is a bit of an increase on the far west side of the metro area near the Coast Range, but even Forest Grove and Banks average under 50″. That’s because most of the time the flow is from the south/southwest or west, so rarely do we see much “upslope” component to enhance rain there. It does happen; I have seen situations with a southeast low-level flow where those areas get far more rain than the east side of the metro area.
I can tell you from living around the 62-66″ location…it sure does rain a lot more in the hills. 11 years ago I moved, but only about 2 miles as the crow flies farther east in the Corbett area. It’s noticeably wetter in the “new location”.
Chief Meteorologist Mark Nelsen