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What I Learned In School Last Week

What I Learned In School Last Week

by Kent Ehrhardt

KMOV.com

Posted on March 18, 2011 at 9:11 PM

Updated Friday, Mar 18 at 10:49 PM

We are so fortunate to have a terrific National Weather Service (NWS) office serving our area. The staff there is dedicated and knowledgeable. I've always been impressed with the level of expertise and, in particular, the awareness of the importance of their relationship with their media partners. Every year the staff takes a proactive stance and organizes a half-day workshop focusing on the coming season's weather threats and shares the latest forecasting techniques.

Last week they hosted the second "Midwest Bow Echo Workshop". While you may not know what a bow echo is you have most likely dealt with them recently. Most of the severe outbreaks we have had this year, including the New Years Eve tornadoes, were a result of bow echo systems. Actually the scientific term is "Quasi-Linear Convective System" (QLCS). Let's just stick with "bow-echoes". The "echo" refers to the signature return or echo on radars. Although Doppler radars can help us see detailed dynamics within a bow echo, a conventional weather radar can also provide more than enough data to diagnose this rather common Midwest phenomena.

Operational, academic and broadcast meteorologists from across the Midwest gathered on the campus of St. Louis University for this valuable and extremely relevant 2-day workshop. One of the leading experts in the country on bow echoes is Ron Przybylinski, a staff member at the St. Louis NWS office. Ron is also known for his expertise in storm damage surveys. It's part of Ron's job to determine whether damage was caused by a tornado or straight-line winds and how strong the winds were. The result of Ron's surveys is often an official declaration of the strength of a tornado based on the damage using the enhanced Fujita scale (EF scale). Damage surveys can be particularly difficult in the wake of a QLCS.

So what is a QLCS? The image above is a Doppler radar image of a QLCS that resulted in a severe weather outbreak in April of 2006. Notice how the line of storms is almost liner (quasi-linear). You can also see how the line is bowing in the middle, as opposed to being a straight line. Where we see these bows is often where strong straight-line winds occur. Simple enough, right? Whenever we see a bowed line of storms we can expect straight-line wind damage. Not exactly, but that's a start. If the straight-line winds weren't enough of a threat tornadoes are often embedded with in the QLCS. These tornadoes can be particularly dangerous as they are often rain wrapped and difficult or even impossible to see. Which is why you should resist the temptation to stand outside in hopes of getting a glimpse of a tornado. By the time you see it it may be too late.

Damage from a QLCS is a challenge to survey. Tornado damage is characterized by the divergent pattern of the debris. Often a QLCS embedded tornadoe's damage can be masked by strong straight-line winds.

A new tool is on the horizon that will help forecasters glean more information from QLCS radar data. "Dual Polarity Doppler radars" are the next generation of technology. These new machines will provide forecasters with a higher data resolution for interpreting QLCS signatures as well as hail size and rainfall rates. The result will most certainly be longer lead times, and fewer deaths and injuries.

The meteorologists at the National Weather Service and here at KMOV as well as the entire News 4 team are committed to providing you the most timely and accurate severe weather warnings possible.

 

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