Shelf clouds are probably one of the most foreboding types of cloud you’ll ever see roll across the sky. They are huge and span the entire skyline. They are the harbingers of the real storm to come, hidden behind the ominous wall.
Thunderstorms produce cold gusts of air — downdrafts — which is how shelf clouds eventually form. The cold air races out ahead of the line of thunderstorms and forces the warm, humid air ahead of it to rise rapidly and condense into a horizontal cloud. Multiple layers can form that resemble shelves.
This time lapse from Reed Meyer in Tysons Corner illustrates what the shelf cloud looks like as it approaches.
Shelf clouds portend two main threats — strong wind gusts and torrential rain. They pose a very low tornado threat, but on Monday night, rotation was spotted on the north side of the squall line. Sometimes the north edge of a squall line can start to spin, swirling in on itself as we saw last night around Leesburg, Ashburn and Reston. A funnel cloud was reported to the National Weather Service, but no tornado has been confirmed.
The worst weather associated with shelf clouds happens immediately when they arrive: the strongest wind, heaviest rain and largest hail. Once the shelf cloud passed last night, conditions tamed, and the sky turned pink and orange. Several rainbows were spotted.
Thanks to everyone who shared their shelf cloud photos with us last night!