How Shelf Clouds Form

Thank you to everyone who shared pictures of the approaching storms Tuesday afternoon.  While most of the severe weather stayed south of the immediate Stateline, there were some pretty impressive looking clouds that rolled through.

This is a picture from Paul Hackerson from Stillman Valley earlier Tuesday.  It's what we call a 'shelf cloud' and can sometimes be very ominous looking.  There's a lot of stuff going on in the atmosphere to produce this type of looking cloud, and here's how it happens.

Shelf clouds are the leading edge of a gust front or an outflow boundary.  When rain cooled air hits the surface of the earth it spreads outward.  When the air spreads out it forces the warm, moist air ahead of the thunderstorms northward.  When that air condenses, it forms a shelf cloud.  As the shelf cloud passes, you can feel a shift in wind direction and usually an increase in wind speed followed by heavy rainfall.  Sometimes the wind gusts with shelf clouds can be very strong, upwards of 60 mph.  Tuesday we had wind gusts near 40 mph.