November 10, 2009
November is a unique month in that the weather changes very rapidly throughout the course of it. The beginning of November is not notoriously stormy; yes, it is usually moderately wet, but there are stormier periods of the year. By the end of November, though, we have reached the stormiest part of the wet season. The last week of November is, on average, the stormiest week of the whole year for the Pacific Northwest. After that week, things usually settle down a tiny bit but remain fairly constant in terms of storminess up until February or so.
The upcoming week looks fairly wet, but not a soaker. We saw a weak system come through tonight, and we will see some continued showers tomorrow, although many people could see no rain. Thursday we will see a continued threat of showers, but most of them will end in the afternoon.
Things start to get a little interesting Thursday night. A COLD system – the coldest we have seen since last spring – will drop down over the area from the north, exiting on the day Friday. This system will bring dramatically lower snow levels to the area, with snow a certainty above 1,500 feet. The thing I am really watching closely is the potential for some lowland snow (yeah that’s right I said lowland snow) Friday afternoon and evening. After the front passes, a Puget Sound Convergence Zone will form. As of now, it is expected to first form by the northern Snohomish county border line before sliding south to Seattle where it will fizzle and die out. The atmosphere will be very unstable with this zone, and thunderstorms are a possibility. With thunderstorms and intense precipitation, snow levels can often be temporarily lowered to the surface. All in all, I am NOT expecting any major accumulations in Seattle proper. If we do get any accumulations, it will be in the foothills. It is very early still and very VERY hard to pinpoint convergence zone events, particularily ones where snow is involved, but I’ll keep you posted on details if things change.
The big story I’m looking at right now is the potential for some flooding on the area rivers next week. Models have been fairly consistent with the idea of bringing heavy precipitation to our area early next week. This corresponds to an ongoing MJO (Madden-Julian Oscillation) event. An MJO is basically a pattern of equatorial rainfall traveling around the planet. When it gets to a certain stage, the Pacific Northwest often gets “Pineapple Expresses” and flooding. That certain stage would occur next week, which lines up perfectly with what the models are predicting precipitation wise. The details are still fuzzy, but at this point, heavy rains look probable for Vancouver Island, the Olympics, and the North Cascades. The lowlands also look like they will receive a lot of precipitation as well. As the details become clearer, I will relay them to you. If you know of any people who live on a floodplain, now is a good time to let them know that a Pineapple Express event is possible.
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