The Portland Rain Conspiracy

For years I've been boasting to out-of-towners, "Portland seems damp, but we only get 37 inches of annual precipitation on average, which is less than most East Coast and Southern cities." For example, Philadelphia gets 42 inches, and most of Florida gets more than 50 inches. In Portland, the rain (and rare snow) is simply spread out over more days. This has been my story. But after a recent conversation and some digital digging, I am concerned that our rain data is suspect - or, at the very least, not representative of the Portland region, or even the city proper.

A typical March rainbow in Sellwood. Photo: author

The conversation was with my mother, who lives up the hill from me in Portland's Woodstock neighborhood. As we drove to dinner last night, the rain started getting heavier. I mentioned how we're actually a bit low on our February rain total (I am a weather geek, among other geek types). Mom said, "Well that's surprising, because it seems like I've been emptying the rain gauge every few days." I'm not sure how voluminous her rain gauge is, but most store a few inches of falling water.

Yes, sometimes as humans, we think or say we do things more often than we actually do, but I believe my mother is on to something. There have been many days over my past six years in Portland when it has rained BUCKETS at our house in Sellwood, but then I check the National Weather Service (NWS) site the next day and find a rain total of, say, 0.04 inches. Is Portland's official, go-to rain gauge faulty? Is its location on NE 122 Avenue near the airport in some sort of rain shadow? Do Columbia Gorge winds blow the water out of the gauge? Or are my mother and I exaggerating about our city's famous rain?

I had no proof either way. Until now.

HYDRA gauge locations. USGS/Portland BES

A brief web search led me to the City of Portland HYDRA Rainfall Network, a collection of 36 rain gauges throughout the City of Portland and slightly beyond. It's a partnership between the Portland Bureau of Environmental Services and the US Geological Survey. Precipitation data is updated and totaled hourly, including averages by district. The gauges are located primarily at institutions such as schools and fire stations.

To truly and academically test my thesis - that the NWS Portland rain gauge measures less precipitation than other gauges citywide - I would need to analyze many years of data at all 36 HYDRA gauges and at the NWS gauge. But for today's simpler, less scientific approach, I looked at what was readily available on both websites: the running precipitation totals for yesterday (2/24), February, and the water year (which started October 1). Here are those three precip totals for a few locations:

NWS Portland official gauge: 0.15, 1.98, 20.02
Closest other gauge to NWS: 0.22, 2.55, 23.52
NE PDX average of all gauges: 0.20, 2.49, 23.95
Portland average of all gauges: 0.24, 2.59, 24.11
Downtown Portland gauge: 0.27, 2.59, 24.57
SE PDX average of all gauges: 0.27, 2.90, 25.83
Sellwood gauge (my 'hood): 0.34, 3.29, 30.45
Arleta gauge (Mom's 'hood): 0.25, 2.66, 23.94

As you can see, the NWS gauge is the driest of the bunch, and not by a small margin. The larger district of Northeast Portland also appears to be drier than Downtown, Southeast, or Portland as a whole. The gauge in my neighborhood has the highest totals of any of the 37 gauges. WOOT! The driest individual gauges, not shown above, are mostly in North Portland. The single driest, receiving a relatively scant 16.59" this water year, is on Sauvie Island. The West Hills appear to create a pronounced rain shadow.

But what's the deal with the gauge near, but not run by, the NWS? It's just a half mile east at NE 146th and Airport Way, but has collected three and a half more inches of precipitation since October 1. While I would tend to trust the trained scientists at the NWS over an unofficial gauge, this discrepancy lends more credence to my conspiracy theory.

So, while I need more data to back me up, I am going to go out on a soggy limb and say that Portland, on geographic average, gets several more inches of annual precipitation than what is measured at the NWS station near the airport. Most of us probably get more than 40 inches. Sure enough, digging deeper into the NWS site, Downtown averages 42.1 inches, five more inches than at the airport. That's like an additional month's worth of rain!

As for my house and my parents' house? I'll venture a guess and say that we get about....more than we would like.