Outflow Dominant and other terms

[Elinor McLennon]

When dealing with a tornado/supercell, what does it actually mean when they say the storm is going outflow dominant?

What is Cape?

When talking about cap, I have a vague understanding of that. It's kinda like a lid on a pan and when things heat up, it pops? What determines how strong or weak the cap is?

 

[Chris Allington]

When dealing with a tornado/supercell, what does it actually mean when they say the storm is going outflow dominant?

What is Cape?

When talking about cap, I have a vague understanding of that. It's kinda like a lid on a pan and when things heat up, it pops? What determines how strong or weak the cap is?

1) Outflow dominant means the storm has started to 'gust out' and there isnt much of a tornado threat any longer typically as the storm no longer has a well defined inflow region. Often times storms will become outflow dominant and form into a big squall line.

2) CAPE is Convective Available Potential Energy and is just a measure of instability ( in joules). Obviously the higher CAPE values indicate greater potential instability.

3) Your 'lid on a pan' acronym is pretty close. The cap is usually a layer of warm air aloft. To be unstable, warm moist air is rising into cooler air so when the rising air hits the cap, it is unable to rise farther as it is no longer bouyant. Atleast not until some kind of mechanism can break the cap or remove that warm air aloft (i.e. cold air advection in the mid-levels of the atmosphere) and allow air to continue to rise.

 

[John Wetter]

What is Cape?

From Wikipedia: http://en.wikipedia.org/wiki/Convective_available_potential_energy

 

[Elinor McLennon]

What visual clues do you have for a storm becoming outflow dominant? Do you start to lose the wall cloud?

 

[Skip Talbot]

Hi Elinor, here's a few pictures I got that show a supercell cycling:

The RFD is going to cut a notch in the back of the wall cloud, and then this notch is going to expand until the base fans out into a more shelf cloud like feature. At this point a cyclical supercell will usually shed this fanned out base and reorganize with a new base, tightening up with a new wall cloud to the north of the base. An outflow dominant storm will continue to fan out, however. You'll see lots of scud as the surface becomes saturated with rain cooled air, and rain under the base. I'd say the third photo in the sequence is when the storm is really starting to look outflowish. As the base continues to fan out, sometimes you get a dramatic feature known as the Whale's Mouth:

 

[Elinor McLennon]

Thanks Skip. Good pics. That makes sense. I hear that expression alot but wasn't quite sure what it meant. You're right, the third pic does not look as ominous as the first. I was on a storm in March(i think) and that's exactly what it did. I heard other chasers say it went outflow dominant but all I really knew was that it had died and there was no longer a funnel
Thanks again.

 

[Richard Dickson]

Skip,
Thanks for taking the time to put together that series of photos. They were excellent. It is quite one thing to read a description of a particular weather feature and try to visualize what was transpiring, but in this case, it truly is the old "picture is worth a thousand words". I'm sure there are now several folks that have a better idea of what to look for the next time they hear the term.

Richard (TANK) Dickson

 

[Jason Blum]

Fantastic! I had a vague understanding of this, but the photos really helped me to understand what I was looking for much better.
Many thanks, Skip.