A Neat Old Metro Rainfall Map

September 6, 2015

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.


  1. 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.
  2. 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.
  3. 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.
  4. 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″.
  5. 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

Welcome To Halloween! Likely a Record Cold September Day

September 6, 2015


Today was sure a dark, drizzly, & dank day (like the alliteration?). The afternoon high (as of 5pm), was only 59 degrees in Portland.  That’s what we typically see in late October!  Now at 5pm it’s 59 degrees at PDX and there’s a decent chance we jump to 60 next hour.  Update:  6pm it is 60 degrees at PDX.  If we don’t rise above 60, it’ll be a record cool high temperature for the day.


Thanks to Brian Schmit for pointing that out and the highlighted chart.  He also pointed out that IF we don’t hit 60, it’ll be the earliest such occurrence in PDX history in late August/early September!  That’s a huge change from the month after month of warm/hot weather we’ve been seeing.

My 14 year old son decided to come to work with me because I told him the cool skate park near the station on the westside would be dry.  He was skeptical all the way until we hit Sylvan.  Just like going over the Cascades and leaving the marine layer, suddenly the Tualatin Valley was under beautiful blue skies and roads were dry…I think I heard angels singing…funny how the sun can lift your mood (whether it’s warm or cold) after sitting through a day of drizzle/rain and clouds.

Notice how much warmer it was today south of the ‘ol Willamette Baseline(Stark Street on eastside and Baseline Rd westside).  Low 70s in the valley…much more comfortable.


Chief Meteorologist Mark Nelsen