Relationship between a storm and a boundry?

[Austin Jones]

Let me start by saying in the last year I have learned so much through Storm Track just by reading but never really participating, and really appreciate the great people and knowledge on the site. This will be my first question thread, and I hope to start participating in discussion much more in the future. Here it goes:

It is my understanding that boundaries/fronts act as a source of lift for storm initiation. I would like to know more about the relationship between the boundary and the storm, after the storm has initiated and/or matured. This question has arisen after reading and hearing others mention that a storm "attached to, or rode a boundary". I have also heard discussion of storms being either in front of or behind a boundary and that this effected the life of the storm. Can anyone tell me about the processes that are occurring between the boundary and the storm in these and/or other instances?

 

[Dan Robinson]

Not only can boundaries initiate storms, they contain vorticity that, when ingested by an updraft, can greatly enhance tornado potential. I've been convinced by fellow chasers and experience that outflow boundaries make some of the best chase targets. They are often out in the open warm sector - updrafts that ingest surface-based CAPE air around an outflow boundary don't have the same concerns of cold, dry air entrainment or cold undercutting like you see on the cool side of cold/warm fronts. Boundary orientation is also important - a storm that tracks along a boundary will ingest its vorticity for longer than one that crosses it at a right angle. Updrafts that cross boundaries at shallow angles can 'latch' onto them and cause the storm to turn and move along the boundary, increasing storm-relative helicity.

 

[Jeff Duda]

This question has arisen after reading and hearing others mention that a storm "attached to, or rode a boundary".

Generally, the phrases "attached to a boundary" or "rode a boundary" mean that a storm was essentially forced to track along a boundary because any attempt by the storm to deviate or move off the boundary would cause the storm to die. Something in the environment wouldn't be supportive of keeping the storm going off the boundary. Many times that factor is high CIN. It also helps if mid-upper level winds have a low angle to the boundary, thus helping it stay along the boundary instead of pushing it off the boundary, as would be the case when the winds are closer to being perpendicular to the boundary. It's just a matter of keeping the updraft sustained and where the favorable environments for doing so are.

If a storm is ahead of or behind a boundary, that just means it is not riding the boundary and is fully immersed in one of the air masses the boundary separates. Typically, "behind the front" means a storm is in either cooler/drier air behind a cold front or north of a warm front, or hotter/drier air behind a dryline. That is typically a less favorable environment for severe weather compared to being on the other side of the boundary ("ahead of"). However, sometimes the air mass ahead of the same boundary is inhospitable to convection (e.g., too much CIN), and so sometimes storms may preferentially fire behind the boundary. Typically, "ahead of the front" means a storm is in warm/moist air east or southeast of a cold front or south of a warm front or east of a dryline where CAPE is typically the highest. The bigtime, monster storms that rage for hours and produce multiple huge tornadoes are storms that fire either on or ahead of a boundary and then move out into the warmer/more moist airmass. Shear, especially in the low levels, tends to be higher out ahead of a front (in what we call "the warm sector"). This helps give storms the rotation they need to go insane.

 

[Austin Jones]

Thanks to both Dan and Jeff, you guys have both enhanced my understanding.

Typically, "behind the front" means a storm is in either cooler/drier air behind a cold front or north of a warm front, or hotter/drier air behind a dryline.

One more question in reference to the quote above, and I feel this is probably just a typo or I could be totally wrong, but it is my understanding that warm fronts typically move to the north, and I would have called a storm north of a warm front "ahead of the front".

 

[Jeff Duda]

Downstream of a front, I guess, may be a better way to define "ahead of a front".

 

[Austin Jones]

Interesting read I just found that applies well to my question: http://www.spc.noaa.gov/publications/garner/ejssm.pdf