From Supercell to Bowing Squall Line

By: 24hourprof , 3:27 PM GMT on August 12, 2013

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As a follow-up to my last blog regarding the hail-producing HP supercell in northwest Nebraska on August 3, 2013 (I recommend reading The Case of the Unwitnessed Hailstorm before you proceed), the storm that I presented on this 12Z mosaic of composite reflectivity (here's a closer look with counties) transitioned in a little over an hour to a bowing line segment of thunderstorms. For confirmation, check out, below, the 1330Z mosaic of composite reflectivity on August 3 (larger image).


The 1330Z mosaic of composite reflectivity on August 3, 2013. Larger image. Courtesy of Penn State.

Before you jump to any conclusions, supercells transitioning to bowing line segments is a fairly common evolution. And this kind of transition in storm mode (type) typically occurs when the outflow of cool air associated with the rear-flank downdraft (RFD) is both relatively strong and deep. In other words, such transitions occur when there's a strong RFD surge.

By way of review, there are two downdraft regions associated with the structure of supercells (see idealized schematic below). The rear-flank downdraft forms at the rear of the storm (obviously, not rocket science) in the vicinity of the rotating updraft (the mesocyclone) in response to drier air entraining (mixing) into the backside of the updraft. This entrainment of drier air promotes evaporative cooling and, in turn, increases negative buoyancy, which favors downward accelerations. Moreover, when hailstones and other icy hydrometeors (snow and graupel) get tossed into the rear-downdraft region, melting and sublimation of ice hydrometeors also help to cool the air and thereby increase negative buoyancy (melting of ice requires energy, which the surrounding air supplies).


An idealized schematic of a supercell thunderstorm. Courtesy of, and copyright by, the online Penn State Certificate Program.

Although I really can't say for sure what exactly caused the RFD surge on the backside of the HP supercell in northwestern Nebraska on August 3, I'm betting that the partial (or complete) melting of some hailstones probably played a role in enhancing the depth and strength of the rear-flank cold pool. The bottom line here is that the leading edge of the surging deep and strong RFD outflow initiated new thunderstorms, thus paving the way for the HP supercell to transition into a bowing line segment of thunderstorms early on August 3.

Below is the 1216Z image of storm-relative velocities (larger image; twin image of base reflectivity) I showed in my previous blog. This time I circled the footprint of the rear-flank downdraft...greens indicate negative (inbound) velocities. While speeds near 30 knots might not seem impressive, remember that the HP supercell was approximately 64 nautical miles from the radar site, placing the footprint of the RFD at roughly 6000 feet. Most cold pools have a depth of one kilometer, although cooler air flowing outward from thunderstorms have been as shallow as a few hundred meters and as thick as four kilometers (the depth of a cold pool largely depends on the longevity of the parent convection). Nonetheless, in the case of August 3, the footprint of the RFD at roughly 6000 feet (about two kilometers) was sufficiently large to convince me that rear-flank winds at lower attitudes were much stronger (keep in mind that the radar beam tilts upward and that the radar routinely misses strong low-level winds when the storm is relatively far from the radar site).


The 1216Z image of storm-relative velocities from the radar at North Platte, Nebraska (KLNX), on August 3, 2013. The white circle indicates the footprint of the rear-flank downdraft of the HP supercell that allegedly produced hail as large as four inches in diameter (or greater). Larger image. Courtesy of NOAA.

So, based on this image of storm-relative velocities, I believe there was an RFD surge associated with the HP supercell on August 3, 2013, and that the surging rear-flank gust front initiated new thunderstorms that paved the way for the supercell to transition into a bowing line segment of thunderstorms in a little over an hour.

Here endeth the lesson.

Lee

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23. 24hourprof
10:40 AM GMT on August 17, 2013
Quoting 22. calkevin77:
I absolutely love the science and explanations in your blog entries. Much appreciated. This one was particularly informative. Thank you.


You're welcome!!!!
Member Since: October 24, 2012 Posts: 91 Comments: 803
22. calkevin77
12:36 AM GMT on August 17, 2013
I absolutely love the science and explanations in your blog entries. Much appreciated. This one was particularly informative. Thank you.
Member Since: June 9, 2006 Posts: 0 Comments: 848
21. 24hourprof
10:16 PM GMT on August 15, 2013
Quoting 20. georgevandenberghe:

Quoting Lee Grenci from an earlier post response

"Well, I got "undressed" right on the spot and learned, the hard way, about the limited role tropical cyclones play in maintaining the global heat and moisture "balance."


Unless you are leaving out some details, the professor could have
offered a more constructive correction rather than a dressing down.
Still given the choice between destructive criticism or none at all I'll take the former since I'm at least made aware of an error.




Exactly George! I never get upset when I'm wrong and someone points it out because it's an opportunity to learn.

But I agree with you...the professor could have done it in a more polite way. All is forgotten and forgiven.
Member Since: October 24, 2012 Posts: 91 Comments: 803
20. georgevandenberghe
6:35 PM GMT on August 15, 2013

Quoting Lee Grenci from an earlier post response

"Well, I got "undressed" right on the spot and learned, the hard way, about the limited role tropical cyclones play in maintaining the global heat and moisture "balance."


Unless you are leaving out some details, the professor could have
offered a more constructive correction rather than a dressing down.
Still given the choice between destructive criticism or none at all I'll take the former since I'm at least made aware of an error.


Member Since: February 1, 2012 Posts: 18 Comments: 1760
19. 24hourprof
1:14 PM GMT on August 15, 2013
Quoting 18. georgevandenberghe:
Quoting Lee Grenci

"Your question reminds me of the mistaken notion that hurricanes play a major role in the transport of heat energy and moisture to northern latitudes and thus play an integral role in smoothing out temperature differences between high and low latitudes (i.e., the global energy budget). I've heard this on TV, and I'm not making this up."


I wasn't aware this was mistaken but looking at the number of tropical cyclones, the number of tropical thunderstorms (which is hugely larger), the number of midlatitude cyclones and waves in the westerlies and the ocean currents, the conclusion is sound. The point to take from this, besides what's in the sentence it to look critically even at what is said to be true and has been passed down through generations without critical examination.

By the way the craft of horticulture is full of these too and if I were either an academic or well connected with the horticultural popular press I would perform the experiments and publish refutations to some of them.


Thanks George.

I learned this the hard way a long time ago. I naively stated, in front of a professor, that we shouldn't forget the contribution of tropical cyclones to helping to smooth out the temperature and moisture differences between low and high latitudes. Well, I got "undressed" right on the spot and learned, the hard way, about the limited role tropical cyclones play in maintaining the global heat and moisture "balance."

It is a hard lesson I never forgot.

I went through the calculations about 20 years ago, and I've forgotten the precise results, but I remember the mathematics verifying what the professor told me.
Member Since: October 24, 2012 Posts: 91 Comments: 803
18. georgevandenberghe
1:03 PM GMT on August 15, 2013
Quoting Lee Grenci

"Your question reminds me of the mistaken notion that hurricanes play a major role in the transport of heat energy and moisture to northern latitudes and thus play an integral role in smoothing out temperature differences between high and low latitudes (i.e., the global energy budget). I've heard this on TV, and I'm not making this up."


I wasn't aware this was mistaken but looking at the number of tropical cyclones, the number of tropical thunderstorms (which is hugely larger), the number of midlatitude cyclones and waves in the westerlies and the ocean currents, the conclusion is sound. The point to take from this, besides what's in the sentence it to look critically even at what is said to be true and has been passed down through generations without critical examination.

By the way the craft of horticulture is full of these too and if I were either an academic or well connected with the horticultural popular press I would perform the experiments and publish refutations to some of them.
Member Since: February 1, 2012 Posts: 18 Comments: 1760
17. 24hourprof
11:00 PM GMT on August 14, 2013
Quoting 16. Some1Has2BtheRookie:


Not only did you answer my question, Lee, but you helped me to understand why the influence of a hurricane on a blocking high would be miniscule, at best.

Thanks!

Mike


You're quite welcome, Mike.
Member Since: October 24, 2012 Posts: 91 Comments: 803
16. Some1Has2BtheRookie
3:09 PM GMT on August 14, 2013
Quoting 14. 24hourprof:


An interesting question. I would say that, in general, the answer is no.

I hope I answered your question.


Not only did you answer my question, Lee, but you helped me to understand why the influence of a hurricane on a blocking high would be miniscule, at best.

Thanks!

Mike
Member Since: August 24, 2010 Posts: 0 Comments: 4737
15. 24hourprof
2:17 PM GMT on August 14, 2013
Quoting 12. vis0:
You Mr. Grenci, teach (VERY WELL*) as to how real weather performs.
One subject i've read on your blog is as to how simply cooler rising air doesn't always led to storms or the building of clouds.

*RECOMMEND that new or young READERS not be afraid to get information via the hyper linked (blue) words. Some see the "big" words and give up, don't give up, once you understand the complex words Mr. Grenci makes learning a breeze. (pun intended)

The following are just my words DO NOT represent Wxu.

If one decides to try to read the PDF (here)
please remember i post in clues & unconnected paragraphs. One of
the clues is on the raising of cooler air and influencing weather.


Many thanks.

You gave me a great idea for a blog!!!!!!! Thanks.

I often hear on television that "Rising air causes lower pressure."

That's hogwash...rising air tends to act as a check and balance against lowering pressure. Stay tuned!
Member Since: October 24, 2012 Posts: 91 Comments: 803
14. 24hourprof
2:11 PM GMT on August 14, 2013
Quoting 11. Some1Has2BtheRookie:
I do have a question, Lee. This is off topic, but I am curious about it. A few years back there was a person on Dr. Master's blog that has what I would consider to be an interesting hypothesis. The thought is that a hurricane can become strong enough that its outflow would help to strengthen a blocking high pressure system and thus steer the hurricane a little further west before it would begin its movement towards the north. Would you consider this be be possible? ... Don't blame me. You said that I could ask questions. :)


An interesting question. I would say that, in general, the answer is no. A hurricane is, in most cases, a meso-alpha system (a spatial scale of 200-1000 kilometers).

A blocking high, on the other hand, is a "higher-end" synoptic- to planetary scale (2000 kilometers or more). I have great difficulty making the leap that a meso-alpha system could significantly alter a system whose spatial scale is several thousand kilometers.

Is the interaction zero? No, I don't think so. But when you're talking about blocking highs, you're talking about planetary waves, particularly wave-numbers 1, 2 and 3, which are primarily determined by the distribution of continents and oceans and large mountain ranges. I just don't believe that most hurricanes can compete with such large-scale features.

I suspect that this blogger didn't know the criteria for a REAL blocking high (in these days of hype on television, "blocking highs" seem to be a dime a dozen).

Your question reminds me of the mistaken notion that hurricanes play a major role in the transport of heat energy and moisture to northern latitudes and thus play an integral role in smoothing out temperature differences between high and low latitudes (i.e., the global energy budget). I've heard this on TV, and I'm not making this up.

There's no doubt that hurricanes recycle some heat energy and moisture from the tropics to higher latitudes, but their overall contribution pales in comparison to other transport mechanisms such as mid-latitude low-pressure systems and ocean currents. In my view, hurricanes are "small potatoes" in the "business" of heat transport.

So, while hurricanes are awesome storms, they are not large enough or of sufficient number to have large impact on global energy budgets or on planetary-scale features such as blocking highs.

I hope I answered your question.
Member Since: October 24, 2012 Posts: 91 Comments: 803
13. 24hourprof
1:38 PM GMT on August 14, 2013
Quoting 10. taistelutipu:
I only recently discovered your blog so I'm at beginning of the learning process and I cannot contribute much to the conversation yet. I appreciate your clear writing style so that even a newbie like me can follow.

In the previous blog on the unwitnessed hail I just happened to have a related question and I was very grateful for your and george's input. This time, I didn't have anything to ask or to add. If I have any question regarding future posts, I'll ask.

Being a lecturer at university I fall into a related category of busy people: I have to prepare my lectures for the next term which is why I have less time to hang out on wunderground.


Understood and much appreciated!

In many ways, I miss preparing my lectures...I don't miss grading, however.
Member Since: October 24, 2012 Posts: 91 Comments: 803
12. vis0
6:49 AM GMT on August 14, 2013
You Mr. Grenci, teach (VERY WELL*) as to how real weather performs.
One subject i've read on your blog is as to how simply cooler rising air doesn't always led to storms or the building of clouds.

*RECOMMEND that new or young READERS not be afraid to get information via the hyper linked (blue) words. Some see the "big" words and give up, don't give up, once you understand the complex words Mr. Grenci makes learning a breeze. (pun intended)

The following are just my words DO NOT represent Wxu.

If one decides to try to read the PDF (here)
please remember i post in clues & unconnected paragraphs. One of
the clues is on the raising of cooler air and influencing weather.
Member Since: December 15, 2006 Posts: 247 Comments: 422
11. Some1Has2BtheRookie
2:36 AM GMT on August 14, 2013
I do have a question, Lee. This is off topic, but I am curious about it. A few years back there was a person on Dr. Master's blog that has what I would consider to be an interesting hypothesis. The thought is that a hurricane can become strong enough that its outflow would help to strengthen a blocking high pressure system and thus steer the hurricane a little further west before it would begin its movement towards the north. Would you consider this be be possible? ... Don't blame me. You said that I could ask questions. :)
Member Since: August 24, 2010 Posts: 0 Comments: 4737
10. taistelutipu
10:31 PM GMT on August 13, 2013
I only recently discovered your blog so I'm at beginning of the learning process and I cannot contribute much to the conversation yet. I appreciate your clear writing style so that even a newbie like me can follow.

In the previous blog on the unwitnessed hail I just happened to have a related question and I was very grateful for your and george's input. This time, I didn't have anything to ask or to add. If I have any question regarding future posts, I'll ask.

Being a lecturer at university I fall into a related category of busy people: I have to prepare my lectures for the next term which is why I have less time to hang out on wunderground.
Member Since: August 20, 2007 Posts: 12 Comments: 639
9. 24hourprof
10:23 PM GMT on August 13, 2013
Quoting 7. Astrometeor:


Lol, you sound like another blogger I know who was griping about a lack of comments.

I fall in Rookie's #1 explanation, school started up a week ago for me, and I have homework combined with college apps combined with Boy Scout work on the road to Eagle. Real busy.

I enjoyed the blog, I never understood why super-cells bow out. I appreciate you writing this Lee.

Thanks,
-Nathan


Wow! You are indeed busy!

Thanks for taking the time to write. I enjoy hearing from you.
Member Since: October 24, 2012 Posts: 91 Comments: 803
8. 24hourprof
10:22 PM GMT on August 13, 2013
Quoting 4. georgevandenberghe:
I have interests in transition of bowed lines (very common) to derechos (rare). or MCCs I missed the 2012 June 29 derecho, leaving DCA Friday early AM June 29 for the Bahamas. I was worried about MCCs in my absence both June 29 and June 30 coming over the top of the ridge but had no idea we'd get a derecho. MCCs always trigger a caution in my mind because my home is vulnerable to flash flooding.

I was caught in the open in July 2010 by a very strong bowing line which produced a sustained 60-70 knot wind for over a minute at my location. I was in the parking lot of a big box store and decided I'd best stay OUTSIDE of the building because I was afraid of roof collapse but at that
point choosing between inside, outside near the building or inside my car, I really wanted an option D. I chose outside the building and took my chances with lightning. I was worried about either a tornado or 100 knot winds(didn't verify) which could move the car and seeing what these winds
were doing made me think not entirely rationally. Option D in this case
was a concrete embankment where I would have been safer but I couldn't safely get to it with the flying debris on the path.







Now that's a great story, George! Thanks.
Member Since: October 24, 2012 Posts: 91 Comments: 803
7. Astrometeor
10:21 PM GMT on August 13, 2013
Quoting 1. 24hourprof:
I thought storm mode was an interesting topic, so I'm surprised nobody has commented. This is the first blog I've written at WU without any comments from readers.

For future reference, should I stay away from topics like this?

Many thanks in advance for your guidance and interest.

Lee


Lol, you sound like another blogger I know who was griping about a lack of comments.

I fall in Rookie's #1 explanation, school started up a week ago for me, and I have homework combined with college apps combined with Boy Scout work on the road to Eagle. Real busy.

I enjoyed the blog, I never understood why super-cells bow out. I appreciate you writing this Lee.

Thanks,
-Nathan
Member Since: July 2, 2012 Posts: 100 Comments: 10277
6. 24hourprof
10:20 PM GMT on August 13, 2013
Quoting 5. Some1Has2BtheRookie:


The topic is fine, Lee. I would imagine that there are several reasons as why the questions and comments have slowed here.

1. School age children preparing for school again.

2. Recent high school grads preparing for their first year of college or their first realistic chance of a better job.

3. Some have less time to post because they are the parents of the first two possibilities.

4. The CV season is about to start up and many bloggers here will cling to Dr. Master's blog during this time of the year.

And then there are people like me. I love to read your blog topics, but my lack of knowledge allows for me to just read and lurk instead of making posts. ... Should we both live long enough, I may actually become knowledgeable enough to join in these conversations. ... What do you think, Lee? I may be ready by the year 2025??? fingers crossed


I think you're there already. Your scientific curiosity is keen, and that makes you a great partner to carry on conversation. Feel free to ask questions no matter how basic you deem them. I am ready and willing to teach.

And I agree with all your reasons...maybe I should go tropical now and then. :-)
Member Since: October 24, 2012 Posts: 91 Comments: 803
5. Some1Has2BtheRookie
6:53 PM GMT on August 13, 2013
Quoting 1. 24hourprof:
I thought storm mode was an interesting topic, so I'm surprised nobody has commented. This is the first blog I've written at WU without any comments from readers.

For future reference, should I stay away from topics like this?

Many thanks in advance for your guidance and interest.

Lee


The topic is fine, Lee. I would imagine that there are several reasons as why the questions and comments have slowed here.

1. School age children preparing for school again.

2. Recent high school grads preparing for their first year of college or their first realistic chance of a better job.

3. Some have less time to post because they are the parents of the first two possibilities.

4. The CV season is about to start up and many bloggers here will cling to Dr. Master's blog during this time of the year.

And then there are people like me. I love to read your blog topics, but my lack of knowledge allows for me to just read and lurk instead of making posts. ... Should we both live long enough, I may actually become knowledgeable enough to join in these conversations. ... What do you think, Lee? I may be ready by the year 2025??? fingers crossed
Member Since: August 24, 2010 Posts: 0 Comments: 4737
4. georgevandenberghe
6:11 PM GMT on August 13, 2013
I have interests in transition of bowed lines (very common) to derechos (rare). or MCCs I missed the 2012 June 29 derecho, leaving DCA Friday early AM June 29 for the Bahamas. I was worried about MCCs in my absence both June 29 and June 30 coming over the top of the ridge but had no idea we'd get a derecho. MCCs always trigger a caution in my mind because my home is vulnerable to flash flooding.

I was caught in the open in July 2010 by a very strong bowing line which produced a sustained 60-70 knot wind for over a minute at my location. I was in the parking lot of a big box store and decided I'd best stay OUTSIDE of the building because I was afraid of roof collapse but at that
point choosing between inside, outside near the building or inside my car, I really wanted an option D. I chose outside the building and took my chances with lightning. I was worried about either a tornado or 100 knot winds(didn't verify) which could move the car and seeing what these winds
were doing made me think not entirely rationally. Option D in this case
was a concrete embankment where I would have been safer but I couldn't safely get to it with the flying debris on the path.





Member Since: February 1, 2012 Posts: 18 Comments: 1760
3. 24hourprof
6:02 PM GMT on August 13, 2013
Quoting 2. georgevandenberghe:


No this topic is interesting but I don't have anything to add to it.


Thanks George!

I just thought that folks might appreciate knowing that supercells can develop into another type of storm.
Member Since: October 24, 2012 Posts: 91 Comments: 803
2. georgevandenberghe
1:19 PM GMT on August 13, 2013
Quoting 1. 24hourprof:
I thought storm mode was an interesting topic, so I'm surprised nobody has commented. This is the first blog I've written at WU without any comments from readers.

For future reference, should I stay away from topics like this?

Many thanks in advance for your guidance and interest.

Lee


No this topic is interesting but I don't have anything to add to it.
Member Since: February 1, 2012 Posts: 18 Comments: 1760
1. 24hourprof
1:08 PM GMT on August 13, 2013
I thought storm mode was an interesting topic, so I'm surprised nobody has commented. This is the first blog I've written at WU without any comments from readers.

For future reference, should I stay away from topics like this?

Many thanks in advance for your guidance and interest.

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803

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Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.

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