Ouch. That one hurt. Apologies to all for my forecast yesterday for snow today that never happened. And thanks for the interesting voicemails and emails. One voicemailer wondered if I had stock in Les Schwab Tires. One emailer suggested I wear a bunny suit on air next time I'm wrong. Interesting idea there. For all of you wondering what was I thinking, here’s a little insight. I've seen surprise Portland snowstorms with set-ups similar to this. There were a couple key factors on which I based my forecast. One was a process called evaporative cooling. When snow or rain falls into dry air below, some of it evaporates. Evaporation requires heat. Just think about a pot of water on a stove. If the stove is on, the water boils and evaporates away. In the case of evaporative cooling in the atmosphere, the stove, that is the heat source required to perform the evaporation, is the air into which the rain or snow is evaporating. This can cool that air (by taking heat out of it) very effectively and lower the freezing level, allowing snow to reach the ground. In one case like this back in the mid-90s, three inches of snow accumulated in Portland when the surface temperature was 40º. But in that case I suspect the air just above the surface was drier than it was today, which would've allowed for more evaporation. Strike One. The other key factor in my decision making process was the rate of precipitation. If it’s snowing hard aloft, there is more precipitation to evaporate on the way down, hence more cooling. It looked like we'd get some pretty serious precipitation rates based on the satellite imagery and the origin of the airmass coming in with this storm. The precipitation rates were very heavy, but they hit southern Oregon, not northern Oregon. Strike Two.

Everything else looked like a classic Portland snow set-up, with the very significant exception of the air mass east of the Cascades. I mentioned on the air that if that air had been of Arctic origin and was colder, this would've been a no-brainer snow forecast. As it was it was a borderline case, but I felt the two factors described above would overcome the marginally cold air coming through Gorge from the Columbia Basin. Strike Three.

So that's it. I learned a long time ago you're going to take some lumps weather forecasting. Rare events are difficult to forecast. We’ve seen a lot of Portland snowfall forecasts that have been wrong and a lot of Portland snowfalls not forecast at all. Every forecaster I know does his or her best job in every case. I've been on the other side of this coin, being the only one to hit a snow forecast. This feels worse than that felt good. Not that that matters or is any consolation to anyone who made plans based on my forecast.

A few emailers mentioned hype. If forecasting 2-6 inches of snow that didn't happen is hype for the simple reason that I was wrong, then I guess I'm guilty of hyping it. But I made my decision and went with it, based on what I thought was going to happen. There was no pressure from any source to hype the forecast for marketing reasons as one emailer suggested. Never in my 20 years of forecasting on television has that happened to me. If it ever does I'll have a much bigger decision on my hands than whether or not to forecast snow.

Thanks for watching KGW.

Sincerely,

Matt Zaffino
KGW Chief Meteorologist