Cloud movement in different directions?

[Anthony Petito]

Just a quick question.

Some thunderstorms rolled through early this morning here in Nebraska and as they were clearning up I noticed something odd (to me atleast). The storm was moving east, and they seemed as if they were pretty high, but a lower level of clouds, thinner ones, were moving in the opposite direction. They were moving West. Just curious as to what this is called, how it happens, etc... Could it be wind direction at different altitutes?

 

[Mike Peregrine]

You're watching the result of wind shear ... the changing of wind direction and speed with height ... sometimes it's really cool to watch. Chasers love to see that on days like today -

 

[Anthony Petito]

That's kinda what I figured. It was pretty cool to watch!

 

[Jeff Snyder]

The westward motion of the low clouds behind the storm could likely be enhanced by the fact that they're probably associated with storm outflow, with tends to spread outward (sometimes remarkedly uniformly) from the storm. For example, if the storm was stationary, in a not-strong shear environment, chances are the outlfow would expand circularly outward from the storm. If it was only slowly moving east, the expansion of the outflow could very well be faster than the storm motion, and thus there could well be westward component to the outflow behind the storm...

 

[Carrie Halliday]

You're watching the result of wind shear ... the changing of wind direction and speed with height ... sometimes it's really cool to watch. Chasers love to see that on days like today -

I get alot of those and will sit forever and watch. Fun to see the black clouds rolling in and scud shooting off in other directions. And of course, the chaser aspect kicks in and makes it even more appealing!

 

[Andy Wehrle]

I have noticed clouds at different altitudes moving in different directions on several occasions this past year:

May 23, 2004: 3 brief tornadoes in the area, got hit with 3/4" hail from the edge of a supercell core.

May 30, 2004: Under high risk, was practically shitting myself, but nothing much happened.

October 23. 2004: Last Saturday a tornado warning was put on a cell approaching Green Bay, but nothing happened. Oh for another few j/kg of CAPE!

 

[Jeff Snyder]

LOL I'm assuming he was talking about the oddity of westward moving low clouds, and not just "changing wind directions with height" -- a.k.a. directional wind shear... As we all know, shear (and particularly directional shear) helps create an environment favorable for storm rotation and tornadoes (given sufficient instability, of course)...

 

[Bob Schafer]

Anthony, just as an FYI, you can monitor wind shear (both directional and speed) by looking at a VAD. Here is OAX's:

http://weather.cod.edu/cgi-bin/wwwradargif...OAX&product=VAD

At the moment, the data is only available up to 11K feet, but usually this data is pretty cool. I loved watching the VAD's during the hurricanes.

This is of marginal value for mesoscale data associated with a particular supercell, but is very valuable for forecasting in advance of severe weather events.

Bob

BTW, can someone please tell me what the colors are representing? Thanks!

 

[Robert Dewey]

The colors are the RMS... Basically, you want this number as close to zero as possible... The higher the number, the less accurate the data tends to be.

A sine wave is fit to the data plotted on the VAD product. The VAD Algorithm then calculates the Root Mean Square (RMS) error. The RMS technique averages the difference between the data points and the fitted sine wave. The RMS error expresses the reliability of the estimated wind speed and direction. A high RMS error signifies a large velocity difference between the data points and the sine curve, indicating that these wind estimates may be unreliable. If the RMS error exceeds the adaptable parameter (9.7 kts or 5.0 m/s) the wind direction and speed is not plotted on the VAD Wind Profile product.