Misleading outflow

[Jordan Hartley]

I read Terry Tyler's report from yesterday's event in the south east and something struck me about his report. He observed a tornadic supercell that he thought was becoming outflow dominate yet it produced a tornado a short time later. In my short time of chasing I have witnessed on 2 seperate occasions the exact same thing, one on 5/5/07 SW of Great Bend, Ks and the other 5/26/08 near Pratt, KS. Anyone who was points E and SE of the storm S of Great Bend that day all thought the storm was going outflow dominate right before it produced. Im talking about looking like its done with outflow and what appeared to be a shelf and then producing 15-20 min later. Now one could say I was very near to the core or the rfd which would explain why I had winds coming from the storm. But in both situations I was lucky enough to be SE/E of the storm with an excellent view of what I thought was the base of the updraft at all times on both of those days. Plus each storm had shelf clouds which looked to make up the main bulk of the updraft portion of the storm which appeared at the time to be out in front of the precip giving the impression it was going outflow dominate. I know that with some HP sups the updraft portion of the storm can look like shelf clouds but what would explain the outflow coming from the storm beings that I was points S and E of the updraft? I and multiple other chasers in both situations were convinced the storms were outflow dominate, so why did they produce shortly there after? Can anyone shed some light on this subject and help me? Im confused...

 

[Skip Talbot]

I was also on that same 5/5/07 storm and was convinced that storm was outflow dominant. Chad and I were in the process of bailing when we saw the cone form. Anyway, I'm pretty sure what we witnessed was the occlusion of one mesocyclone and the formation of another, ie a cyclical supercell. That base had completely gusted out, but the tornado formed north of it. So yes a portion of the storm was outflow dominant, but the new meso was indeed inflow dominant and ramping up for a tornado.

 

[Greg McLaughlin]

I believe you are referring to the tornado near Radium, KS on 5-5-07. I was just to the north of the tornado and it appeared to develop on the northern edge of the RFD gust front where the meso had all but occluded. The tornado caught me and my chase partners by surprise, but we happened to be in a great location and probably had the best vantage point (high contrast view) of that tornado.

 

[Paul Knightley]

I guess this just shows that Mother Nature does not conform to textbook models!

 

[Jordan Hartley]

Can anyone give me a head start on where to look for some material on occluding meso's? Im going to do an internet search hopefully this weekend on the subject but will have to wait until after my wife gives birth to our 2nd child. Were leaving for the hospital in about 30 min the doctors are going to induce her today. Im so happy
Ive always had an interest in what an occlusion of a meso is and what it looks like. Now I just need to study up on it.

 

[Clarence Bennett]

Here ya go. This should get you started...

http://www.stormtrack.org/library/1981/warren.htm

 

[Mike Peregrine]

Seems like storms tend to phase quite a bit ... storms that appear to be transitioning to outflow often reorganize and balance themselves out again. How many times have chasers left a storm because it appeared it was transitioning to outflow dominance, only to have a tornado a few minutes later? ... Happens frequently. It's a good reminder when it does happen to stick with a storm in which you have observed rotation. Unless it is obviously moving into an unfavorable environment, anything is possible. Hiawatha, Kansas in 2005 is my favorite example (of course) ... but there are many others ... Throckmorton, Texas also comes to mind. I am convinced that it is a good idea to avoid second guessing yourself just because you see a base that has become really wet. Give the storm a chance to breathe ... that's basically what it's doing in my mind.

 

[Andrew Stoller]

I think many times that if you end up too far to the south southeast of a more classic to high precipitation supercell mesoclyclone as it wraps up and occludes with a wet RFD, you end up looking at the RFD gust front/shelf cloud and lots of precipitation, leading you to conclude that a storm is becoming outflow dominant while it is occluding. If you're brave enough and don't mind getting your car potentially obliterated with large hail on an HP storm, you need to position yourself right in the inflow notch just northeast of where the RFD meets the meso where a tornado would form. That way you have a better chance of an unobstructed view of the occlusion without looking at precipitation.

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/torn/dgr/evsch.rxml

This website was best thing I ever found for learning about supercell thunderstorms, how they form, and how they evolve. The page above depicts the occlusion process.

 

[Andrew Stoller]

Ive always had an interest in what an occlusion of a meso is and what it looks like. Now I just need to study up on it.

A mesocyclone in a way is like a synoptic scale surface low pressure system. The updraft/forward flank downdraft intersection area acts as a kind of small warm/stationary front when the storm is inflow dominant (visually many times this leads to a beaver tail). The leading edge of the RFD kind of acts like a mini cool front (although many times the RFD is warm). So as the storm rotates and the RFD accelerates and pushes forward past the mesocyclone, it will intersect the edge of the FFD and cut off warm inflow to the original updraft, just as a cold front in a strong surface low pressure system will wrap around and overtake the warm front near the surface low. That is what occlusion is. Visually on a storm, it is the clear slot that cuts into the updraft base and forms the RFD gust front.

 

[Paul Knightley]

I notice a reference to the Pratt storm on 26/5/08 - the two tornadoes developed on the leading edge of the mesocyclone on this HP beast, but relative to the meso/RFD, they where were you'd expect them - i.e. northern side of the mesocyclone/RFD cut.

Video here - watch for strong language!
http://www.youtube.com/watch?v=fbHEAfbXeyM