Pre-frontal troughs and warm sector convection

[Rob H]

Can anyone help me understand pre-frontal troughs and especially how they help with warm sector convective development on chase days?

Most sources don't go into too much detail, and simply say that they are an area of lower pressure preceding a cold front and may be accompanied by a wind shift. I did find an AMS journal article that looked at them in depth, but the conclusion seemed to read like a Dr. Seuss book - sometimes they do/don't happen, sometimes they're colder/warmer, sometimes they're near to/far from the front.

On a day like 6/17/10 I remember thinking that the southern play looked decent, but I wondered what feature would kick off storms down there. After looking at the data, the storms initiated deep in the warm sector and perhaps 100 miles or so ahead of the cold front. There were no discernible outflow boundaries, and the convective pattern was a long, narrow band that mimicked the shape of the cold front fairly well.

Was a pre-frontal trough responsible as the triggering mechanism for this or other events, and if so, is there any method of forecasting when this will happen or to what extent? I get the feeling the answer could be "show up and hope for the best" but I like to remove those factors as much as possible when chasing obviously.

 

[Skip Talbot]

Was a pre-frontal trough responsible as the triggering mechanism for this or other events, and if so, is there any method of forecasting when this will happen or to what extent? I get the feeling the answer could be "show up and hope for the best" but I like to remove those factors as much as possible when chasing obviously.

Yes, the storms in southern Minnesota on June 17 definitely initiated on a prefrontal surface trough. This almost bit us too. We went all the way to Luverene, MN expecting storms to go up on the dryline. Our first clue was driving underneath a robust looking cu field in central Minnesota and then as we continued west it thinned out. We were able to double back in time though to catch the show.

This trough was a lot more subtle than the dryline/cold front and warm front that was lifting north through there. Watch the surface obs for pressure and wind changes. Mesoanaylsis might help with picking out elongated areas of low pressure. These troughs are a source of lift and so will be where storms initiate and where updrafts can be amplified. The fact that these storms went up so far into the warm sector, where the air was juicer and the directional shear better, probably partially explains the tornado outbreak.

 

[Rob H]

Thanks, Skip. So is it safe to say that with our current understanding there isn't much about pre-frontal troughs that can be forecast - they're pretty much in the realm of nowcasting?

I've read some interesting case studies about warm sector convection and they don't really mention this type of feature. Perhaps they are glossed over as there isn't much information available. The best information I've found so far is in this AMS journal.

 

[Fred Gossage]

Thanks, Skip. So is it safe to say that with our current understanding there isn't much about pre-frontal troughs that can be forecast - they're pretty much in the realm of nowcasting?

I've read some interesting case studies about warm sector convection and they don't really mention this type of feature. Perhaps they are glossed over as there isn't much information available. The best information I've found so far is in this AMS journal.

Like the name suggests, a pre-frontal trough is a trough of lower pressures ahead of a surface cold front. These are often accompanied by a low-level wind shift... and also low-level convergence. That is why they are often a focus for convective development. Pre-frontal troughs can often be an aid in the development of tornadic storms because they initiate the storms further away from the stronger frontal forcing... making the storm mode more likely to be discrete. As was the case on June 17th, they can also cause the focus for convective development to be deeper into the richer boundary layer moisture, allowing storms to develop in a lower LCL environment, which is more favorable for tornadoes. Also, because of the surface pressure falls along and ahead of a pre-frontal trough, the surface winds are more likely to back ahead of it, and this increases the low-level directional shear.

The pre-frontal trough is a boundary that can initiate convection. Generally speaking, the forcing mechanisms aren't as strong and linear as with a cold front or even dryline; so, this helps with discrete storm mode. However, this also means that it CAN be a little tougher to bust through the cap by low-level convergence mechanisms.

 

[Gene Moore]

Allow me to suggest a good case study that's had papers published and a bit of digging will yield much data: 8 June 1974. This day was not a pure synoptic case known for pre-frontal supercells as much else happened worthy of investigation, to include the dumping of chaff in the Harrah OK tornadic storm. That said, for us as chasers this high powered fast moving event would have/ could have burned us had we not noted the pre-frontal wind shift in the field (chasing with no data). That event kept us from traveling further west from our position at Clinton. When the convection went up just behind us we were ready to bolt east on I-40 to get in position for the first tornado.

Here is a nice surface map (with special thanks to Jon Finch) that clearly shows the windshift:
http://bangladeshtornadoes.org/UScases/060874/06087419zsf.gif

Additional information from Stormtrack:
http://www.stormtrack.org/forum/showthread.php?t=16834

As an aside: When considering convection displaced ahead the front another feature to consider is the depth of the moisture on a capped warm sector. For example, if the LLJ or 850 jet is displaced to the east of the frontal boundary the depth of moisture in this regime may be better. Thus this (N-S) linear band, especially the west edge, may be prone to reach the level of free convection sooner. Aided by strong cloud base winds this area will develop storms faster.

Gene Moore