Darrin Rasberry
I've heard this term thrown around a bit in response to what happened last Sunday (April 26th). I haven't heard it before then. What is linear forcing, and what's the best way to check for it?
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Andrew Revering
EF3
You know thats a great question. Being able to identify linear vs 'point' forcing would be great to pay attention to.
I don't have a great answer, but a good example of linear forcing would be a fast sweeping cold front. The focus is in a line along the boundary in significant fashion.
point forcing would be something more like an 'airmass' thunderstorm or storms that initiate near a low pressure center... or perhaps just along a slow moving boundary... or a boundary that is well capped to keep things isolated.
I'd love to hear other responses about this and how exactly you would forecast a linear situation vs a point situation. I have a feeling there is no single quick and easy tool to make the difference.
I don't have a great answer, but a good example of linear forcing would be a fast sweeping cold front. The focus is in a line along the boundary in significant fashion.
point forcing would be something more like an 'airmass' thunderstorm or storms that initiate near a low pressure center... or perhaps just along a slow moving boundary... or a boundary that is well capped to keep things isolated.
I'd love to hear other responses about this and how exactly you would forecast a linear situation vs a point situation. I have a feeling there is no single quick and easy tool to make the difference.
Adam Lucio
EF5
Shear profiles that are more unidirectional can lead to linear forcing, especially in weakly capped enviroments. Im not expert on the subject but when trying to figure out if Im going to get isolated sups or a linear crapfest those are 2 things I look for.
Darrin Rasberry
I had the impression just from the term that "linear forcing" meant "development of linear storms," lol. But with what Adam said about unidirectional shear, wouldn't linear storms occur unless they move quickly into an environment of less unidirectional shear?
So what causes this for a dryline? Do vortmaxes have something to do with it? (What are vortmaxes, anyway?)
Andrew Revering
EF3
I think you're talking about the same thing be it a dryline or a cold front. however, a cold front usually moves faster, which can help to line out the storms more. I like what Adam said about shear.
Keep in mind a storm doesn't have to initiate in a unidirectional flow though. So they can be isolated from the start... and that tends to be the case... but as the storm evolution continues they tend to get multicellular and linear... probably in part due to them throwing off outflow boundaries and helping to generate more storms along its flank....thus making them multicellular storms and linear.
However, given enough shear (and maybe with some help of capping) they can remain isolated for a long time. Sometimes for their entire life cycle.
Take a look at what's going on in New Mexico today. This is a good example of 'point forcing' where its primarily orographic lift rather than a boundary causing the convection. Note that they are isolated...and probably beautiful cells.... not lines.
We'll see if that changes with time.
Vort maxes are areas of extreme vorticity in the mid levels. Strong vorticity advection with good warm air advection causes large scale synoptic lift. I'm not sure that this plays a 'direct' role in what we're talking about, but maybe.
Keep in mind a storm doesn't have to initiate in a unidirectional flow though. So they can be isolated from the start... and that tends to be the case... but as the storm evolution continues they tend to get multicellular and linear... probably in part due to them throwing off outflow boundaries and helping to generate more storms along its flank....thus making them multicellular storms and linear.
However, given enough shear (and maybe with some help of capping) they can remain isolated for a long time. Sometimes for their entire life cycle.
Take a look at what's going on in New Mexico today. This is a good example of 'point forcing' where its primarily orographic lift rather than a boundary causing the convection. Note that they are isolated...and probably beautiful cells.... not lines.
We'll see if that changes with time.
Vort maxes are areas of extreme vorticity in the mid levels. Strong vorticity advection with good warm air advection causes large scale synoptic lift. I'm not sure that this plays a 'direct' role in what we're talking about, but maybe.
Darrin Rasberry
Gorgeous cells, yes.
What is "orographic lift"? Does that have to do with (say, in this case) the mountains and/or high regions in NM? And perhaps the Caprock in Texas, hence "Caprock Magic"?
What is "orographic lift"? Does that have to do with (say, in this case) the mountains and/or high regions in NM? And perhaps the Caprock in Texas, hence "Caprock Magic"?
Andrew Revering
EF3
Exactly... it's just the terrain causing the air to go up... IE.. surface winds hit higher elevations or mountains and is forced upward. Get a hint of moisture in there with some mid level wind support and you've got storms.
