Weather Blog

'Scud clouds' -- Don't be fooled by Mother Nature's ultimate prank

'Scud clouds' -- Don't be fooled by Mother Nature's ultimate prank
Nope, nothing to see here folks. Just harmless scud cloud approaching Mukilteo on March 30, 2014.

Coming off of April Fool's Day and coming into the month that traditionally has the most "severe" weather (at least by paltry Pacific Northwest standards) I wanted to highlight a trick of atmospheric dynamics that fools countless people every year: "Scud" clouds and their tornado-like funnel impersonations.

Tornadoes are quite rare around here -- Washington just averages about 1-2 a year somewhere in the state, the last notable one being the tornado that damaged a Frederickson plant last September. The vast majority of them are quite weak thanks to our climatology that greatly inhibits tornado development.

But we do have frequent days -- especially in the spring -- when the air mass is unstable and we get those billowing clouds that bring heavy showers and a few bolts of lightning. These types of cells do tend to have a ragged cloud base with "strips" of clouds that can sure look like a funnel at first blush.

But 99 times out of 100, and perhaps even more frequently, these are harmless "scud" clouds. NOAA defines them as: "Low ragged and wind-torn appearing cloud fragments, usually not attached to the thunderstorm base, often seen in association with, and behind gust fronts. Scud clouds DO NOT produce severe weather. Scud clouds are often mistaken for wall clouds and tornadoes, especially when attached to the thunderstorm base."

There have been a few examples of these clouds recently that look ominous but are harmless. How to know if a cloud is just scud or a budding funnel? Three words: Rotation, rotation, rotation. A truly developing funnel cloud will have tight rotation forming and also be attached to the base of the cloud. Scud clouds tend to float aimlessly just below the clouds or not really attached.

For example, here is a cloud spotted over Queen Anne late last month. Looks a little but like a funnel?



But now here is a one-minute video and that's when you realize there is no rotation and it's just harmless scud:



Whereas this video, also from last month, *does* show some rotation (middle/center of the video - note the water spray beneath it) and was likely the initial formation of a waterspout:



I've put in the photo gallery some other ominous looking photos that were in fact harmless. So don't be fooled - be sure to look for the rotation. If not, it's just a funnel imposter.