Dirty Ridge

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A term often used in meteorological circles is "dirty ridge." No, this is not some kind of meteorological pornography. Rather its when there is a "ridge" or area of high pressure that is not strong enough to keep us dry and cloud free. Weather disturbances with sufficient amplitude can inject clouds and rain into the northern portions of the ridge...that is the dirty part. Take a look at the upper level pattern for Thursday at 4 PM (see graphic). This represents the heights of the 500-mb pressure surface about sea level and is roughly at 18000 ft (sea level pressure is typically around 1012 mb). The ridge is obvious.
I have included the 24h precipitation for the next two days...you see some precipitation over the northern portion of the domain...particularly over the mountains. Lots of rain shadowing. This is typical for dirty ridges since the flow tends to have a strong westerly component (from the west), which produces good rain shadowing (and strong orographic enhancement).
By the way, when do you think we typically get the lowest temperatures of the day? 9 PM, midnight, 3 AM, 6 AM, or 8 AM...or perhaps some other time? Will give the answer in the next blog.
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