From the Lee Side
The Granbury Tornado
Posted by: Lee Grenci, 7:20 PM GMT on May 19, 2013 +7
If you listen or read carefully to any of the media's typical account of the development of tornadic supercells (thunderstorms with rotating updrafts), you'll get the overall impression that strong winds aloft generate the strong vertical wind shear that favor supercells. Such impressions are not always true. Indeed, the supercell that spawned the EF-4 tornado that ripped through Granbury, Texas, on Wednesday evening, May 15, 2013, was a compelling example.

To see what I'm driving at, check out, below, the 00Z skew-T at Fort Worth, Texas (KFWD), on May 16, 2013 (the early evening of May 15). In case you're wondering, Fort Worth is the closest upper-air station to Granbury (the temperature and dew-point soundings appear in red and green, respectively). There's no doubt that lapse rates in the lower to middle troposphere were relatively large (the decrease in temperature with height was relatively large, rendering the lower half of the troposphere potentially and dramatically unstable). Moreover, there was ample moisture in the lower troposphere (the surface dew point, for example, was close to 70 degrees Fahrenheit). Having pointed out these favorable conditions, take a look at the wind speeds between 500 mb and 400 mb...only 25 knots or so, which is very weak by most supercell standards.


The 00Z temperature (red) and dew-point (green) soundings at Fort Worth, Texas, on May 16, 2013 (early evening of May 15). Note the weak winds between 400 mb and 500 mb. Courtesy of Penn State.

Before I explain how relatively weak, mid-tropospheric winds can still support supercells, allow me to provide some background. Model simulations conducted by Weisman and Klemp in the 1980s helped to identify the layer between the ground and an altitude near 500 mb as pivotal for predicting the mode (type) of a thunderstorm (their seminal paper). Specifically, their simulations indicated that thunderstorms tended to be short-lived whenever model environments lacked deep vertical wind shear. Later empirical research confirmed that vertical shear needed to be relatively strong in the layer of air extending from the ground to roughly five to six kilometers in order for supercells to form. As a general threshold, the Storm Prediction Center in Norman, Oklahoma, indicates that a bulk vertical wind shear of 35-40 knots between the ground and six kilometers characterizes a typical of supercells. By "bulk," I mean the overall vertical wind shear between the top and bottom of this layer of air. In other words, bulk shear does not account for "internal" changes in wind speed and / or wind direction that occur at intermediate altitudes between the ground and six kilometers.


The 23Z Rapid Refresh model analysis of bulk wind shear between the ground and an altitude of six kilometers on May 15, 2013. Larger image. For the record, the supercell that spawned the Granbury tornado was initiated before 23Z (5 P.M. CDT)...here's the 23Z visible satellite image (courtesy of SPC) and the corresponding loops (GOES-15 and GOES-13) from CIMSS.

So wind speeds in the neighborhood of 500 mb above Fort Worth, Texas (and, by proxy, over nearby Granbury, Texas), on the evening of May 15 would seem to fall short of the media's seemingly one-size-fits-all recipe of strong, upper-level winds suitable for supercells. But the 23Z Rapid Refresh model analysis (above; larger image) for the vertical wind shear between the ground and an altitude of six kilometers (roughly 500 mb) belied the seemingly tame wind speeds in the middle troposphere over Fort Worth (and Granbury). What's up with that?

Although winds near 500 mb might sometimes seem rather tame by most severe-weather standards, I never dismiss, out of hand, the possibility that supercells might still develop in such an environment (assuming, of course, that the environment supports the development of thunderstorms). That's because directional wind shear (winds changing direction with height) can help to produce enough bulk wind shear to tip the scales toward supercells becoming the dominant storm mode, even when there are relatively weak winds in the middle troposphere. Revisit the 00Z skew-T at Forth Worth, Texas (above), and focus on the lower troposphere to see what I mean by directional wind shear. Such non-classic environments (relatively weak, mid-tropospheric winds) that still favor supercells are more likely to occur when winds dramatically veer with height in the lower half of the troposphere. For the record, winds veer with height when they "turn" clockwise with increasing altitude (again, revisit the 00Z skew-T at Fort Worth, TX, on May 15, and note the pattern of veering winds in the lower troposphere).

In environments characterized by relatively weak, mid-tropospheric winds, how do veering winds in the lower troposphere contribute to an overall bulk wind shear strong enough to support supercells? Good question! Let's actually calculate the bulk wind shear between the ground and six kilometers over Fort Worth, Texas, at 00Z on May 16 by using the interactive flash animation which I introduced in an earlier blog (courtesy of, and copyright by, the Penn State certificate program). Here are the actual radiosonde data from the University of Wyoming. I'll have to interpolate the data associated with 490 mb and 483 mb to get the wind at six kilometers. I believe that a reasonable wind speed and wind direction are 35 knots and 260 degrees (west-southwesterly), respectively. At the surface, I'll use 14 knots and 145 degrees (southeasterly) for wind speed and direction. Putting my trusty flash animation into action, I get a bulk vertical wind shear vector whose magnitude is 43 knots and whose direction is 278 degrees (west-northwesterly; see flash frame below).


The magnitude of the bulk wind shear in the layer of air between the ground and six kilometers over Fort Worth, TX, at 00Z on May 16 was 43 knots...larger than the magnitude of the wind vector at six kilometers. By proxy, the bulk wind shear over Granbury, TX, was likely very similar. Courtesy of Penn State's online certificate program.

The main point here is that the magnitude of the bulk wind shear in the layer of air between the ground and an altitude of six kilometers over Forth Worth, TX, at 00Z on May 16 was larger than the magnitudes of the wind vectors at six kilometers and the surface, and the veering wind pattern in the lower half of the troposphere contributed to the surprisingly large magnitude of the bulk shear.

The bottom line here is that, even though mid-tropospheric winds looked a bit too weak to support rotating updrafts in severe thunderstorms, the veering wind pattern in the lower half of the troposphere helped to produce a bulk shear large enough to support supercells, one of which spawned the killer tornado that struck Granbury, Texas.

Lee
Categories:Severe Weather
Updated: 10:19 PM GMT on May 20, 2013   Permalink | A A A
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Frost Gets a Bad Rap
Posted by: Lee Grenci, 1:40 PM GMT on May 12, 2013 +5
During the growing season, the National Weather Service routinely issues frost advisories when "widespread frost formation is expected over an extensive area. Surface temperatures are usually in the mid 30s Fahrenheit." For this blog, I define frost as the formation of ice crystals on the ground and other surfaces (see photograph below).

The reference above to temperatures in the mid-30s might give you pause. Keep in mind that official air temperatures ar...
Categories:Thermodynamics Synoptic Meteorology
Updated: 7:00 PM GMT on May 19, 2013   Permalink | A A A
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Sloppy Language Can Breed Sloppy Science
Posted by: Lee Grenci, 5:35 PM GMT on May 07, 2013 +17
Perhaps I'm getting more eccentric in my old age, but I still cringe at some of the sloppy language I hear on television. In this world of shorthand texting, sound bites, and headlines that border on hyperbole, I sometimes feel like a dinosaur when I complain about sloppy language. So be it. I have bigger fish to fry today.

For whatever reason, the phrase "close proximity" has become popular lately, and it's driving me nuts. According to Merriam-Webster, ...
Categories:Synoptic Meteorology Mini-Blog
Updated: 2:20 PM GMT on May 11, 2013   Permalink | A A A
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Dramatic Dryness in Reno, Nevada
Posted by: Lee Grenci, 2:33 PM GMT on May 04, 2013 +5
The fires, winds, and low relative humidity in California made national news this week. I've been following the weather pattern in the West, and I sat up and took notice while reading the Area Forecast Discussion (AFD) issued by the National Weather Service at Reno, Nevada, on Wednesday afternoon (May 1, 2013):

VERY DRY AIR HAS SETTLED OVER THE REGION TODAY. IN FACT, THIS MORNING`S SOUNDING FROM RENO/KREV INDICATED THAT WE MAY HAVE HAD ONE OF THE DRIEST A...
Categories:Fire Weather Mini-Blog
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Tornadoes and Anafronts
Posted by: Lee Grenci, 7:19 PM GMT on April 30, 2013 +2
In the comments below my last blog (snow occurring at temperatures in the 40s), Thrawst asked me an interesting question about anafrontal cold fronts and tornadoes. I thought the question was worthy of a blog, so here goes.

By definition, a synoptic-scale cold front qualifies as an anafront when there's upward motion, clouds, and precipitation relatively far back into the cold air mass behind the front. For example, the cold front stretching from the Grea...
Categories:Mesoscale Forecasting Synoptic Meteorology Severe Weather
Updated: 1:24 PM GMT on May 03, 2013   Permalink | A A A
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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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