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What is an upper level low pressure system?


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I have plans to do Rainier over the weekend. The forecast indicates the approach of an upper level low pressure system. Does this mean very high, but stable clouds above the summit, or lower layer on the mountain with high winds on the summit?

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Pretty much what Phil said. Upper-level low pressure systems are not typically as strong as full-blown storms and they're more common in spring and summer. What you typically get out of them is more showery weather. The potential for thundershowers is higher. There's an old weatherman's adage: "A-cas in the morning, hiker take warning." A-cas stands for Altocumulus castellanous. This type of cloud comes in large groups that look like small popcorn clouds scattered about the sky at mid-altitudes. These are typical of upper level lows. They tend to presage thunderstorm growth in the afternoon, thus the adage. If a-cas doesn't drift by until the afternoon then storms probably won't happen (or I should say it has a lesser probability; i.e., never say never when it comes to weather). In this case, the setting sun will not have enough time to spur T-storm growth of any worrisome magnitude.


I have used my morning sighting of these a-cas to great effect. I'll warn my friends that it "could be stormy this afternoon" upon seeing these. I won't always tell them how I know.


In regard to Mt. Rainier, since this is a peak that reaches into mid-altitudes, an upper level low will have the same affect up there as a normal storm. However, the chance for sunbreaks is better.

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