From the Lee Side
The El Reno, OK Tornadoes
Posted by: Lee Grenci, 4:25 PM GMT on June 10, 2013 +8
Despite a slow start to the 2013 tornado season, the number of headline tornadoes in May has kept me really busy thinking about instructive ways to present these events to my faithful Wunderground readers. The last one on my list is the EF-5 twister that ripped through central Oklahoma on May 31, 2013.

Our fearless leader, Jeff Masters, suggested I write a blog about the anticyclonically rotating tornado that formed in concert with the cyclonically rotating EF-5 monster (whose width, as many of you already know, was measured at a record 2.6 miles at a location roughly south of El Reno...here's the relevant Public Information Statement).

To get a sense for the structure of the EF-5 tornado and its parent supercell, check out the twin images of base reflectivity and storm-relative velocities (below) from the radar near Oklahoma City (KTLX) at 2323Z (6:23 P.M. CDT; larger image; annotated version). Focus your attention on the image of storm-relative velocities (on the right). The bright pink and orange colors represent strong outbound velocities, while the dark blue and magenta indicate strong inbound velocities. This velocity couplet marks the mesocyclonic circulation of the El Reno supercell. I can go one step further. This pattern of strong inbound and outbound storm-relative velocities qualifies as a tornado vortex signature.


The 2323Z (6:43 P.M. CDT) image of base reflectivity (left) and storm-relative velocities (right) on May 31, 2013. Larger image. Courtesy of NOAA.

For the record, the anticyclonically rotating tornado (a clockwise circulation in the Northern Hemisphere) spun up along the flanking line of the El Reno supercell (definition of flanking line from the AMS glossary; annotated photograph).

Before you jump to any conclusions, I point out that meteorologists don't classify these flanking-line twisters as mesocyclonic tornadoes. That's because these tornadoes tend to be anticyclonically rotating, non-mesoscyclonic tornadoes, which is a mouthful for saying that these flanking-line twisters are essentially landspouts (focus on the second entry from the AMS glossary). As it turns out, there's almost always anticyclonic vorticity (spin) on the anticyclonic-shear side of the outflow generated by a supercell's rear-flank downdraft (RFD). The word "shear," in this context, refers to horizontal wind shear generated by the juxtaposition of faster winds near the rightmost anticyclonically curved streamline and slower winds near the leftmost anticyclonically curved streamline. To understand how horizontal differences in wind speed can create spin about a vertical axis, check out my rather simple illustration from a PowerPoint presentation I used in class.

So that you can better visualize the anticyclonic vorticity, check out the idealized plan view of a tornadic supercell below (the "T" marks the site of the tornado; larger image, courtesy of, and copyrighted by, the Penn State online certificate program). For this discussion, the three anticyclonically curved arrows just behind the rear-flank gust front are streamlines that indicate the direction of the outflow. These three streamlines represent the source of anticyclonic vorticity (spin) on the anticyclonic shear side of the RFD outflow.


An idealized plan view of a tornadic supercell. Thin, curved arrows represent streamlines. Shades of gray indicate clouds and colors indicate levels of radar reflectivity. "T" marks the position of the mesocyclonic tornado, and "A" marks a favored position for any anticyclonically rotating landspout that might develop in response to the anticyclonic wind shear in the rear-flank outflow (the last three anticyclonically curved arrows are streamlines that depict the outflow winds that generate anticyclonic spin). Larger image. Courtesy of, and copyrighted by, the Penn State online certificate program.

Staying with the idealized schematic above, I note that there's also a center of cyclonic vorticity on the cyclonic shear side of the RFD outflow (closer to the tornado). Such a juxtaposition of cyclonic and anticyclonic translates to a vorticity couplet that typically straddles the main axis of the rear-flank downdraft. Of course, for this discussion, we're primarily interested in the anticyclonic portion of the couplet. Not surprisingly, anticyclonically rotating twisters can sometimes spin-up when anticyclonic vorticity on the anticyclonic shear side of the supercell's rear-flank outflow gets vertically stretched by a fortuitously positioned updraft within a cumulus tower building along the storm's flanking line (recall that vertical stretching increases vorticity (spin)...study the first part of this flash animation, courtesy of, and copyrighted by, the Penn State online certificate program). Again, note the "A" on the schematic (above)...it corresponds to the preferred position for an anticyclonically rotating twister. I caution that the presence of anticyclonic vorticity on the anticyclonic-shear side of the RFD outflow usually isn't enough to produce a tornado, so, in the context of supercells, non-mesocyclonic anticyclonically rotating twisters (in other words, landspouts) are relatively rare. For the record, over 99% of all tornadoes worldwide rotate cyclonically (keep in mind that any cyclonic circulation in the Southern Hemisphere is clockwise).


The zoomed-in version of the 2337Z (6:37 P.M. CDT) storm-relative velocities on May 31, 2013. Courtesy of NOAA.

Despite the relative rarity of anticyclonically rotating tornadoes, the strong rear-flank outflow associated with the parent supercell that spawned the EF-5 cyclonically rotating tornado near El Reno, OK, set the stage for the anticyclonically rotating twister roughly three to four miles to the south-southeast of El Reno. On radar imagery, the anticyclonic landspout first appeared as a weak circulation at 2328Z (6:28 P.M. CDT) and disappeared by 2342Z (a lifetime of 15 minutes; the nearby EF-5 lasted 40 minutes, according to the National Weather Service). I suspect that the anticyclonic twister was likely undercut (choked off) by the relatively cool, increasingly stable rear-flank outflow (the same outflow whose anticyclonic shear helped to spin up the twister). Take some time to examine the 2337Z (6:37 P.M. CDT) twin images of base reflectivity (left) and storm-relative velocities (right). Here's a zoomed-in version. I annotated the 2337Z close-up image of storm-relative velocities (above) so you can see the clockwise circulation of the anticyclonic landspout. To get your bearings, the blue pixel indicates motion toward the radar site at Oklahoma City (KTLX lies to the east-southeast...on your right), while the red pixels indicate motion away from the radar site.


An anticyclonically rotating landspout near El Reno, Oklahoma, on April 24, 2006. The twister formed on the anticyclonic shear side of the rear-flank outflow generated by a supercell that also spawned a mesocyclonic tornado near El Reno (spatial relationship between the two twisters...see inset on this photograph). Courtesy of Aaron Kennedy.

To me, this anticyclonic landspout was fascinating because practically the same scenario occurred on April 24, 2006, when a supercell spawned a mesocyclonic tornado as well as an anticyclonically rotating landspout on the anticyclonic shear side of the supercell's rear-flank outflow (a fortuitously positioned updraft in the storm's flanking line vertically stretched anticyclonic vorticity). Storm chaser Aaron Kennedy took a photograph of the anticyclonic twister (see above). Check out the inset on this Aaron Kennedy photograph to see the spatial relationship between the two tornadoes.

I just found it fascinating that Nature would repeat itself by producing two relatively rare anticyclonically rotating landspouts in pretty much the same region within a period of several years.

Lee

I meant to post this blog last week, but I was hit by a car while riding my bike on Thursday. I'm okay, but I didn't feel much like writing until this morning. Thanks for understanding. Lee
Categories:Severe Weather Mesoscale Forecasting
Updated: 12:04 PM GMT on June 12, 2013   Permalink | A A A
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The Long-Lived, Nearly Stationary Tornado in Ottawa County, KS
Posted by: Lee Grenci, 5:55 PM GMT on June 05, 2013 +7
These days, it seems like the slow movement of any weather system gets attributed to a blocking high-pressure system, whether one exists or not. The criteria for blocking highs are pretty clear-cut (see my previous blog on blocking highs). Yet, these criteria too often seem to get watered down and diluted to the point that blocking highs have become a default explanation for any slowdown in weather systems.


The long-lived tornado in Ottawa County, K...
Categories:Severe Weather Mesoscale Meteorology Opinion
Updated: 12:40 PM GMT on June 06, 2013   Permalink | A A A
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The Moore Tornado
Posted by: Lee Grenci, 3:23 PM GMT on May 28, 2013 +12
On Tuesday, May 21, in the aftermath of the EF-5 tornado that devastated Moore, Oklahoma, the previous day, I watched Dr. Greg Forbes of The Weather Channel give a live interview standing in front of some of the damage. At the time, the media referred to the twister as an EF-4 tornado, but Dr. Forbes pointed to a heavy propane tank that had been ripped from its moorings and then thrown a distance of a quarter mile, providing evidence, Dr. Forbes stated, that support...
Categories:Severe Weather Mesoscale Meteorology
Updated: 12:09 AM GMT on May 29, 2013   Permalink | A A A
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The Granbury Tornado
Posted by: Lee Grenci, 7:20 PM GMT on May 19, 2013 +8
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.
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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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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