Here is a snip-it of an essay by John Sauder, a Canadian meteorologist, who describes a "cold core funnel":
"A cold core funnel is a vertically tilted rotating column of air under a rapidly growing convective cloud, but the atmospheric conditions are different than those conditions that produce typical funnel clouds or tornadoes"
Imagine a typical Oregon spring rainy day. You know, those days where the weather doesn't seem to be able to make up its mind? Rain, sun, clouds, hail, sun, rain. The environment described by Suder above is very similar to those kind of Pacific Northwest days. Tornadoes in the Midewest and Southeast do not form this way. Tornadoes in "Tornado Alley" are much easier to see coming, not to mention much larger, and thus it is easier to forecast and issue warnings. No tornado or severe thunderstorm watches or warnings were issued with yesterday's McMinnville cell.
Radar is an amazing weather product that helps meteorologist identify characteristics of a tornado. This radar image has almost NO characteristics of a typical tornado. Take a look at the radar comparison between McMinnville's tornado and the tornado that ripped through Moore, Oklahoma last month:
it, while the McMinnville did not. Why was no warning issued for yesterday's storm? Sauder offers up an explanation:
"...these cold core events happen on a small scale (mesoscale) and are typically rather short lived."
This would answer why it is so difficult to issue warnings on Oregon twisters. They just pop-up! Just like those showers and sun-breaks that are so common during our spring weather. Another tool radar offers up is called "velocity". That helps identify wind directions in a storm. Here is a typical tornado signature using the velocity tool:
"The velocity signatures are very small and usually blend in with background noise...makes cold core funnel or cold core tornado detection using radar almost impossible. This difficulty in forecasting such events results in short , if any, warning times for the public"
On top of the weak signatures, the elevation at which these signatures occur are rather close to the surface. The radar beam that sweeps over McMinnville hits at somewhere near 4,000 feet. That is too high to detect significant velocity circulations at the surface.
All these factors challenge any meteorologist in the Northwest when it comes to tornadoes. Seeing the radar triggers described above in real-time make issuing watches and warnings much easier in other parts of the country than in our neck of the woods. So while these events are very rare and not significantly strong, the tornadoes we see still present a very real danger when the spawn.