PORTLAND, Ore. ( KOIN ) — A powerful thunderstorm produced a shelf cloud over downtown Portland Wednesday afternoon. This came as heavy rain, lightning and a rainbow were spotted in a thirty-minute time frame just after 4:00 p.m.
The National Weather Service defines a “ shelf cloud ” as, “a low-level horizontal wedge-shaped accessory cloud, usually attached to the thunderstorm base, that forms along the gust front. The leading edge of the shelf cloud is often smooth and at times layered or terraced while the underside is concave upward and appears turbulent, boiling, or wind-torn. Tornadoes rarely occur with shelf clouds. Remember that shelf clouds are usually found on the leading edge of an approaching thunderstorm.”
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A gust front is almost like a mini cold front that moves in front of a heavy round of rain. Those heavy and big raindrops pulls cold air aloft down towards the surface. As that cold air descends, it pushes the warmer air near the surface up and forward. That air movement helps build a shelf-like cloud in front of the heavy rain.