Thursday, July 7, 2011

Outflow boundary ... a storm's footprint

A dying thunderstorm leaves a piece of itself behind in the atmosphere.

Like one of those horror movies, where a supposedly dead monster comes back to life, sprouting from a small piece of itself, thunderstorms can regenerate along their outflow boundary.

In fact, the outflow boundary sometimes is an effective tool for predicting where storms might form the next day.

The outflow boundary is an area of storm-cooled air that moves into much warmer surrounding air.

That boundary also is called a gust front, because a brief period of gusty winds is observed as the area passes by.

The concept isn't difficult to understand. If you jump into a swimming pool, a wave is sent out in all directions. When a blast of cold air comes roaring to the ground in a thunderstorm downdraft, it reaches the earth and is sent out in all directions.

Meteorologists can locates these outflow boundaries by looking for density changes in the atmosphere. And they've learned that the boundaries can persist for 24 or more hours -- long after the storms that created them dissipated.

They've also learned that new thunderstorms often form on the outflow boundaries.

On Wednesday, we had a couple areas of thunderstorms in the Carolinas. One cluster was southeast of Charlotte, around Camden, S.C. Another moved across Cabarrus and Stanly counties. Today, we can look for storms to form somewhere on the edges (or maybe 25 miles out) from Wednesday's storms.

Incidentally, an outflow boundary (or gust front) was responsible for setting off that habob (sand storm) earlier this week in Phoenix. Strong wind gusts from dying thunderstorms that formed in the tropical air off the Gulf of California roared across the flat desert land, kicking up a monster sand storm.


Anonymous said...

I think it's called a 'haboob'?