Severe Weather over the Southern Plains Today

By: 24hourprof , 2:23 PM GMT on April 17, 2013

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The 13Z Convective Outlook issued by the Storm Prediction Center this morning included a moderate risk of severe thunderstorms over parts of the Southern Plains for this afternoon and this evening (graphic). It's a difficult forecast, primarily because of the uncertainty associated with the disposition of a shortwave in the subtropical jet stream over the Southwest. Nonetheless, discrete to semi-discrete supercells (storms with rotating updrafts) will likely develop, producing damaging winds, large hail, and some strong tornadoes.

The low-level jet stream, which will reach speeds in excess of 50 knots at 850 mb (roughly 1500 meters), will likely play a pivotal role in the outbreak of supercells in the area at moderate risk for severe weather later today. Check out the 12-hour NAM forecast of 850-mb isotachs (color-coded in knots) and 850-mb streamlines below (larger image).


The 12-hour NAM forecast (valid at 00Z this evening) of 850-mb isotachs (color-coded in knots) and 850-mb streamlines. Larger image. Courtesy of Penn State.

Before I elaborate, you might be interested to know that I use the term, "low-level jet stream," to indicate that the speedy, southerly winds are a consequence of the large gradient in heights at 850 mb. In my view, the driving force for a low-level jet stream is different than the physics associated with a nocturnal low-level jet...decoupling (fodder for another blog). The bottom line here is that, in my world, there's a fundamental difference between the underlying meteorology associated with a low-level jet stream and a low-level jet.

At any rate, the low-level jet stream will heighten the risk of supercells later today by increasing the vertical wind shear in the lower troposphere. Of course, the large vertical wind shear between the ground and an altitude of six kilometers is more important because it provides an environment that prevents updrafts and downdrafts inside individual thunderstorms from interacting, paving the way for longer-lived, more organized storms. But the large vertical wind shear in the lower troposphere, say from the ground to an altitude of two kilometers, paves the way for convective updrafts to rotate (see the annotated photograph of a supercell below).


Vertical wind shear in the lower troposphere produces spin around a horizontal axis. When this spin encounters a convective updraft, it tilts the spin into the vertical, causing the convective updraft to rotate and setting the stage for a supercell. Courtesy of Jessica Higgs.

Referring to the photograph above, vertical wind shear in the lower troposphere produces spin around a horizontal axis. When this spin around a horizontal axis encounters a convective updraft, it's tilted into spin around a vertical axis, causing the convective updraft to rotate and paving the way for a supercell to develop.

Okay, that's my stripped-down explanation. If the complete truth be told, the more technical version involves storm-relative winds and storm-relative inflow. Interested? Check out my more scientifically sophisticated explanation (it's rather brief, so don't get nervous).

When I was teaching mesoscale meteorology, I always likened the storm-relative inflow associated with supercells to a spinning noodle of spaghetti. The rate at which the convective updraft of a developing supercell "ingests" or "slurps up" the spinning spaghetti noodle can be likened to storm-relative helicity, a topic I will reserve for a future blog. The bottom line is that storm-relative helicity will be high over the moderate risk area (15-hour SREF forecast of probabilities of storm-relative helicity in the lowest kilometer greater than (or equal to) 150 square meters per square seconds, valid at 00Z this evening), so some strong tornadoes are likely.

Stay safe!

Lee

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32. WunderAlertBot (Admin)
1:52 PM GMT on April 19, 2013
24hourprof has created a new entry.
31. 24hourprof
8:39 PM GMT on April 17, 2013
Quoting Barefootontherocks:
Thank you.

And thanks for the "stay safe" today, your explanations and the links you provide.

Have a great day.


You're quite welcome.
Member Since: October 24, 2012 Posts: 91 Comments: 803
30. 24hourprof
8:38 PM GMT on April 17, 2013
Quoting BaltimoreBrian:
This is excellent! I can't believe I didn't know about this blogger before.


Many thanks. I'm kind of a quiet guy.
Member Since: October 24, 2012 Posts: 91 Comments: 803
29. Barefootontherocks
5:52 PM GMT on April 17, 2013
Quoting 24hourprof:


I like your use of the word, "bouncing."
Thank you.

And thanks for the "stay safe" today, your explanations and the links you provide.

Have a great day.
Member Since: April 29, 2006 Posts: 156 Comments: 18959
28. BaltimoreBrian
5:26 PM GMT on April 17, 2013
This is excellent! I can't believe I didn't know about this blogger before.
Member Since: August 9, 2011 Posts: 26 Comments: 8714
27. 24hourprof
3:42 PM GMT on April 17, 2013
Quoting Barefootontherocks:

Image courtesy of Oklahoma mesonet

Image updates every few minutes to help keep an eye on the bouncing warm sector - and the surface winds.


I like your use of the word, "bouncing."
Member Since: October 24, 2012 Posts: 91 Comments: 803
26. 24hourprof
3:41 PM GMT on April 17, 2013
Quoting Greg01:
Great post - keep them coming.


I'll do my best! Thanks so much for commenting.
Member Since: October 24, 2012 Posts: 91 Comments: 803
25. 24hourprof
3:40 PM GMT on April 17, 2013
Quoting Thrawst:


Thank you Lee! I was leaning the same way. The GFS however is very bullish with the warm front, putting it all the way to the NW oklahoma SW Kansas border area! I think it simply depends on that shortwave trough, and also the position of that warm front. Either way, a focused tornado outbreak is still possible!


Agreed. Very nice analysis.
Member Since: October 24, 2012 Posts: 91 Comments: 803
24. Barefootontherocks
3:32 PM GMT on April 17, 2013

Image courtesy of Oklahoma mesonet

Image updates every few minutes to help keep an eye on the bouncing warm sector - and the surface winds.
Member Since: April 29, 2006 Posts: 156 Comments: 18959
23. Greg01
3:20 PM GMT on April 17, 2013
Great post - keep them coming.
Member Since: July 13, 2012 Posts: 0 Comments: 153
22. Thrawst
3:15 PM GMT on April 17, 2013
Quoting 24hourprof:


Now that's a great question. I think, right now, based on how models are handling the short-wave trough in the subtropical jet stream, I would say "no."

According to SPC, "a HIGH risk area suggests a major severe weather outbreak is expected, with a high concentration of severe weather reports and an enhanced likelihood of extreme severe (i.e., violent tornadoes or very damaging convective wind events occurring across a large area). In a high risk, the potential exists for 20 or more tornadoes, some possibly EF2 or stronger, or an extreme derecho potentially causing widespread wind damage and higher end wind gusts (80+ mph) that may result in structural damage."

Having said all of this, I have to say that, as the day wears on and it comes closer to the time storms are initiated, I wouldn't be completely surprised by a "high risk." But the uncertainty of this forecast makes me lean more toward "no."

Best I can do. But it was a great question. Thanks!


Thank you Lee! I was leaning the same way. The GFS however is very bullish with the warm front, putting it all the way to the NW oklahoma SW Kansas border area! I think it simply depends on that shortwave trough, and also the position of that warm front. Either way, a focused tornado outbreak is still possible!
Member Since: July 18, 2010 Posts: 50 Comments: 1908
20. 24hourprof
3:11 PM GMT on April 17, 2013
Quoting Thrawst:
Do you think a High Risk is possible or is there simply not enough very favorable parameters in place? Would love your take on this.


Now that's a great question. I think, right now, based on how models are handling the short-wave trough in the subtropical jet stream, I would say "no."

According to SPC, "a HIGH risk area suggests a major severe weather outbreak is expected, with a high concentration of severe weather reports and an enhanced likelihood of extreme severe (i.e., violent tornadoes or very damaging convective wind events occurring across a large area). In a high risk, the potential exists for 20 or more tornadoes, some possibly EF2 or stronger, or an extreme derecho potentially causing widespread wind damage and higher end wind gusts (80 mph) that may result in structural damage."

Having said all of this, I have to say that, as the day wears on and it comes closer to the time storms are initiated (later model runs, more observational data), I wouldn't be completely surprised by a "high risk." But the uncertainty of this forecast makes me lean more toward "no."

Best I can do. But it was a great question. Thanks!
Member Since: October 24, 2012 Posts: 91 Comments: 803
19. 24hourprof
3:04 PM GMT on April 17, 2013
Quoting ncstorm:
Thanks Lee


You're quite welcome!
Member Since: October 24, 2012 Posts: 91 Comments: 803
18. 24hourprof
3:03 PM GMT on April 17, 2013
Quoting 24hourprof:


Thanks!

Right now, most of the region is capped...the shades of blue on this current Rapid Refresh mesoanalysis indicate CIN (Convective Inhibition), so right now, where TT values are high over this region, the atmosphere is actually stable.

But CAPE (based on mean potential temperatures and mixing ratios in the lowest 100 mb (roughly 1000 to 900 mb) is high (red contours), meaning that the potential for strong updrafts later today is high.

This is why I prefer CAPE, which is more of an integration than Total Totals, which is more of a bulk representation of the lower half of troposphere.

Appreciate your comments. Thanks!
Member Since: October 24, 2012 Posts: 91 Comments: 803
17. 24hourprof
3:02 PM GMT on April 17, 2013
Quoting KEEPEROFTHEGATE:


This is a contour plot of total totals index with a contour interval of 2. The TT field shows instability in the atmosphere based on the lapse rate from 850 to 500 mb plus dewpoint at 850 mb. Where TTs are greater than 45, thunderstorms are possible. The higher the number, the more unstable the atmosphere is and as a result, the bold the thunderstorms could become. Values of 52 or higher indicate areas where severe thunderstorms are possible. Values <40 indicate areas of stable weather where skies are generally clear.



Thanks!

Right now, most of the region is capped...the shades of blue on this current Rapid Refresh mesoanalysis indicate CIN (Convective Inhibition), so right now, where TT values are high over this region, the atmosphere is actually stable.

But CAPE (based on mean potential temperatures and mixing ratios in the lowest 100 mb (roughly 1000 to 900 mb) is high (red contours), meaning that the potential for strong updrafts later today is high.

This is why I prefer CAPE, which is more of an integration than Total Totals, which is more of a bulk representation of the lower half of troposphere.

Appreciate your comments. Thanks!
Member Since: October 24, 2012 Posts: 91 Comments: 803
16. Thrawst
2:58 PM GMT on April 17, 2013
Do you think a High Risk is possible or is there simply not enough very favorable parameters in place? Would love your take on this.
Member Since: July 18, 2010 Posts: 50 Comments: 1908
15. ncstorm
2:58 PM GMT on April 17, 2013
Thanks Lee
Member Since: August 19, 2006 Posts: 13 Comments: 16041
14. KEEPEROFTHEGATE (Mod)
2:51 PM GMT on April 17, 2013


This is a contour plot of total totals index with a contour interval of 2. The TT field shows instability in the atmosphere based on the lapse rate from 850 to 500 mb plus dewpoint at 850 mb. Where TTs are greater than 45, thunderstorms are possible. The higher the number, the more unstable the atmosphere is and as a result, the bold the thunderstorms could become. Values of 52 or higher indicate areas where severe thunderstorms are possible. Values <40 indicate areas of stable weather where skies are generally clear.

Member Since: July 15, 2006 Posts: 175 Comments: 54858
13. 24hourprof
2:49 PM GMT on April 17, 2013
Quoting VirginIslandsVisitor:
Thanks so much for your blogs, Lee.

Your explanations are done in such a way that even I, being a total non-weather person, can actually, "almost" understand you! I actually feel that I'm getting mini weather courses on your site!

Lindy


Keep it up, Lindy!!! It's all about the learning. Please feel free to ask questions!
Member Since: October 24, 2012 Posts: 91 Comments: 803
12. 24hourprof
2:49 PM GMT on April 17, 2013
Quoting wxchaser97:
Great blog Lee!


Many thanks for encouraging me. It's always worth my time and effort when folks like you appreciate it.
Member Since: October 24, 2012 Posts: 91 Comments: 803
11. trHUrrIXC5MMX
2:48 PM GMT on April 17, 2013
Quoting 24hourprof:


I do indeed. Check out the current dew-point analysis from the University of Illinois...large gradient over west Texas, etc.



yep..very tight gradient in that area...
Member Since: April 23, 2011 Posts: 104 Comments: 14873
10. 24hourprof
2:47 PM GMT on April 17, 2013
Quoting Slamguitar:
Thanks for the details on this severe wx! Helps me understand it better.


It's all about learning! Thanks a million!
Member Since: October 24, 2012 Posts: 91 Comments: 803
9. 24hourprof
2:47 PM GMT on April 17, 2013
Quoting FtMyersgal:
Thank you Lee!


You're most welcome. Thanks for the encouragement!
Member Since: October 24, 2012 Posts: 91 Comments: 803
8. 24hourprof
2:46 PM GMT on April 17, 2013
Quoting trHUrrIXC5MMX:
Great Blog Lee... appreciate it...
btw.. remember that dry line?


I do indeed. Check out the current dew-point analysis from the University of Illinois...large gradient over west Texas, etc.

Member Since: October 24, 2012 Posts: 91 Comments: 803
7. 24hourprof
2:42 PM GMT on April 17, 2013
Quoting Thrawst:
Great blog Lee! Thank you


You're welcome. And thanks so much for your encouragement!
Member Since: October 24, 2012 Posts: 91 Comments: 803
6. VirginIslandsVisitor
2:40 PM GMT on April 17, 2013
Thanks so much for your blogs, Lee.

Your explanations are done in such a way that even I, being a total non-weather person, can actually, "almost" understand you! I actually feel that I'm getting mini weather courses on your site!

Lindy
Member Since: July 30, 2011 Posts: 3 Comments: 679
5. wxchaser97
2:40 PM GMT on April 17, 2013
Great blog Lee!
Member Since: March 16, 2012 Posts: 127 Comments: 7959
4. Slamguitar
2:40 PM GMT on April 17, 2013
Thanks for the details on this severe wx! Helps me understand it better.
Member Since: July 2, 2011 Posts: 0 Comments: 1183
3. FtMyersgal
2:38 PM GMT on April 17, 2013
Thank you Lee!
Member Since: September 16, 2010 Posts: 0 Comments: 1219
2. trHUrrIXC5MMX
2:38 PM GMT on April 17, 2013
Great Blog Lee... appreciate it...
btw.. remember that dry line?
Member Since: April 23, 2011 Posts: 104 Comments: 14873
1. Thrawst
2:34 PM GMT on April 17, 2013
Great blog Lee! Thank you
Member Since: July 18, 2010 Posts: 50 Comments: 1908

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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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