Horseshoes not lucky in supercell thunderstorms – visual clue of potential tornado

This blog post is a version of an article that appeared in the April, 2015 issue of “Allen County HamNews” and is used here with permission of the author (me).

During a severe weather seminar in DuPage County, Ill. March 14, storm chaser and amateur meteorologist Skip Talbot pointed out that a horseshoe shape in a supercell thunderstorm’s updraft base is often visible before the storm forms a tornado (see figures one and two). Talbot said that this feature is a more reliable indicator of a potential tornado than is the formation of a wall cloud, which does not always happen before a tornado forms.

Horseshoe-shaped updraft base drawn on example radar image of a supercell thunderstorm
Figure 1. Horseshoe-shaped updraft base drawn on example radar image of a supercell thunderstorm. The “T” in a circle represents the location of a tornado. Image from presentation by Skip Talbot.

 

Photo of actual supercell thunderstorm with horseshoe-shaped updraft base indicated by orange line
Figure 2. Photo of actual supercell thunderstorm with horseshoe-shaped updraft base indicated by orange line. Image from presentation by Skip Talbot.

Talbot’s presentation was one of several during the annual, day-long Severe Weather Seminar hosted in a western suburb of Chicago by the DuPage County Office of Homeland Security and Emergency Management.

Talbot’s presentation was titled, “Anticipating Tornadoes in Visual Clues,” and focused primarily on tornado-producing supercell thunderstorms. Fortunately, we don’t often see supercells in northern Indiana but all spotters need to know what to look for when those huge storms do arrive.

How big is a supercell?

Speaking of huge, Talbot cleverly superimposed a radar base-reflectivity image of a Plains supercell over a map of the Chicago area. This provided a clear idea of the scale of such storms (see Figure 3).

Simulation of a supercell thunderstorm radar image over map of Chicagoland.
Figure 3. Simulation of a supercell thunderstorm radar image over map of Chicagoland. From presentation by Skip Talbot.

Tornado red flags

Talbot also provided a valuable list of “tornado red flags:”

  • Rain moving rapidly from left to right (as viewed from beyond the right side a supercell)
  • A sudden surge of wind flowing into the supercell
  • A sudden barrage of positive cloud-to-ground lightning.

While none of the above meet the SKYWARN storm spotter reporting criteria, they can help spotters remain alert to the strong possibility that a storm is about to produce something reportable!

Storm spotting squall lines (QLCS’)

Meteorologist Ben Deubelbeiss of the Chicago National Weather Service (NWS) office did a presentation that focused on “squall line” thunderstorms, to which meteorologists refer as quasi-linear convective systems (QLCS’). These types of storms are quite common here in northern Indiana, especially during the summer.

A QLCS is a line of thunderstorms that can extend hundreds of miles and persist for hours. It typically produces straight-line wind damage. Shelf clouds form along the leading edges of many QLCS’. Deubelbeiss pointed out that the bottoms of shelf clouds often contain a great deal of turbulence that results in many false reports of funnel clouds (See Figure 4). As trained storm spotters, we must guard against getting fooled by that turbulence.

Turbulence that often forms on the bottom of shelf clouds, resulting in false reports to the NWS
Figure 4. No funnel cloud or wall cloud, just turbulence that often forms on the bottom of shelf clouds, resulting in false reports to the NWS. Photo from presentation by Ben Deubelbeiss, NWS Chicago.

Deubelbeiss also offered some important comparisons between QLCS’ and supercell thunderstorms. One relates to where the action is. In a supercell, our attention is at the right, rear portion of the storm, where tornado formation is most likely. But with a QLCS, “worst is first” as Deubelbeiss put it. The heaviest wind, occasional weak, short-lived “spin up” tornadoes and damage occur along the leading edge of a QLCS, followed by heavy rain.

Deubelbeiss also made an important point about spotter location. We’ve long been told that the best place to view supercell thunderstorms is where the storm’s heavy rain and hail are to our right and updraft is to our left (typically southeast of the storm). But where should we be when a QLCS approaches? Deubelbeiss strongly advises taking shelter in a building before the leading edge arrives, putting as many walls as possible between us and the outdoors. He reminded spotters that QLCS’ can have storm motion of more than 60 mph. He suggested waiting until the wind dies down to look outside for wind damage to report.

Technology for storm spotters

College of DuPage meteorology professor Dr. Victor Gensini provided an informative presentation, “Technology and Software for Spotter and Emergency Personnel.”

Among his recommendations was the GRLevel3 radar software available for PCs for a one-time fee of $79.95. The latest version is available for download at www.grlevelx.com/grlevel3_2/. GRLevel3 displays live and archive NEXRAD Level III data. It displays high resolution base products, dual polarization products, and derived products along with local storm reports, severe weather warnings, the positions of spotters who are registered with SpotterNetwork.org and other data.

For mobile devices, such as smartphones and tablets, Gensini recommended Radarscope (for iOS and Android) and PYKL3 (for Android only). You can find information about Radarscope at www.radarscope.tv and PYKL3 at www.pykl3radar.com. Like GRLevel3, both products integrate with SpotterNetwork.org.

Another valuable resource for spotters – especially the serious “weather geeks” among us – is a Web page maintained by the College of DuPage meteorology department (weather.cod.edu). The “Weather Analysis Tools” menu of that page offers a wide range of nationwide data, including surface maps, upper air maps, upper air soundings, satellite and radar data, numerical model data, etc.

Late season start

Finally, a note about complacency. The severe weather season nationwide got off to a late start this year, without a tornado reported anywhere in the country until March 25. Meteorologist Greg Carbin of the NWS Storm Prediction Center advised the spotters at the DuPage County seminar that climatologically speaking, severe weather someplace in the continental United States is likely to increase through the month of April. Remember also that in the 37-county area served by the northern Indiana NWS office, tornadoes are nearly twice as likely in April as in March, 1.8 times as likely in May as in April, and 1.7 times as likely in June (our peak tornado month) as in May.

Similarly, in the northern Indiana NWS area, severe thunderstorm winds (58 mph or stronger) are nearly 2.5 times as likely in April as in March, three times as likely in May as in April and nearly twice as likely in June (also our peak severe thunderstorm month) as in May.

So, don’t let the late start to the severe weather season fool you. As we get closer June, it becomes much more likely that our area will see severe weather and SKYWARN spotter activation. Be ready!

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