Anyone that has moved here from another part of the country east of the Cascades has probably noticed we can go long periods, even in the summer, with no thunder or lightning. That’s especially obvious west of the Cascades. Portland only averages a few thunderstorms each year; many of those are just one-hit wonders too. By that I mean just one rumble or a couple flashes.
The reason for the lack of action? There are two. The main ingredient is the very stable atmosphere over us during the warm season. The cold Pacific assures that most of the warmer part of the year the airmass overhead doesn’t become very unstable. To get an unstable atmosphere you need either very warm and humid air below or cold air above. In summer it’s relatively cool at the lowest elevations compared ot the very warm atmosphere above; we don’t get strong rising motions that way. Of course sometimes it does get very hot here, so why few storms then? The hot weather is almost always associated with a dry airmass and sinking motions associated with an upper level ridge of high pressure.
In the rare circumstance that we have: a warm atmosphere, no push of cool ocean air, and some moisture in the mid-upper levels of the atmosphere? Those are the few times we get some good thunderstorms west of the Cascades.
Looking ahead…wow…Dullsville the next week. I think October bores me weatherwise more than any other month of the year. We are done with extreme heat, but too early for a big freeze or snow. Low clouds and fog increase dramatically from September; it gets darker and gloomier in general. But still no big storms or fronts like November and December. No windstorms either (at least during my lifetime so far). Now when we DO get a ridge of high pressure it can be REALLY nice. The crisp Fall days are great with the colors on the trees; but this year that has been and is forecast to be non-existent for the first half of the month at least.
Tomorrow looks especially depressing with solid onshore flow bringing dark and gloomy skies. Maybe more so than today. Weak lift in the westerly flow should give drizzle at times near hills and mountains. Beyond tomorrow another weak system drags through Saturday night and Sunday morning, then maybe sometime slightly more impressive Monday and Tuesday. Possibly a brief break the middle of next week, then more weak fronts later next week.
Chief Meteorologist Mark Nelsen