A Squall Line Hit Our Area On Friday ... What In The World Is A Squall Line?
If you were watching our severe weather forecasts or coverage on Friday, you probably heard us predicting a squall line and talking about a squall line in the forecast. A lot of people are wondering what the term "squall line" means? In this blog, I am going to discuss what a squall line is and how they begin \ mature \ & ultimately die off.
Defining A Squall Line
Before we talk about how squall lines work, we need to define what they are to begin with. A squall line is defined by the NWS as "A line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms." Basically a squall line is a long line of thunderstorms (appear to be a solid line of red on the radar) and they are notorious for producing wide swaths of strong winds. The strongest squall lines can produce widespread damaging winds with isolated tornadoes. Most of the times, squall lines are formed when you have linear forcing along long \ strong cold fronts. Here is a great example of a what a squall line looks like on radar...
Watching The Life Cycle Of A Squall Line?
May 26th, 2011, we saw a mature squall line move across our area. The line developed to our west over Illinois, matured in the western part of our area, then "gusted out" in the eastern part of our area. This gives me a great case to show you how these squall lines operate. Let's starting looking mid afternoon on May 26th, 2011 as the line was intensifying in Illinois. Notice the line is fairly straight and really has no breaks in whatsoever.
As the line approached our area, it really began to mature. What you notice is squall line appearance changes from a straight north south line, to the appearance of a bow. When a bow occurs, we know that stronger mid level flow is being translated down to the surface and the wind threat is increasing. At the "apex" of the bow, we will see the highest likelihood of damaging to destructive winds. "Bow echos" or bows within a squall line are concerning and they need to be watched the closest.
As this line approached Louisville on the evening of May 26, 2011, something started to happen. On the radar, we noticed a green line started to advance out ahead of the squall line (just like what happened on Friday night). This green line indicates that the storm is "gusting out". When a storm system gusts out, it means the cold air begins to advance faster than the t-storms themselves. Since warm air is the fuel for t-storms, this gusting out phase can signal the end of a squall line. Notice as this is occurring, the line does not look as intense as it appeared an hour earlier and it was losing the continues bright reds in the line.
When these storm systems gust out, we traditionally get a "shelf cloud" out ahead of the storm system. The NWS has a great example of a shelf cloud with key illustrations.
These shelf clouds occur quite frequently in our area and here is a great example of one sent by Craig Peak of Floyds Knobs, IN.
When you are under these shelf clouds, the sky will look very turbulent but this only indicates the stronger winds pushing ahead of the storm. Here is a great example of that turbulent sky that appears once the shelf cloud has moved through and it was taken my Jason Mills of Georgetown, IN.
As this gusting out phase occurs, the storm is giving the meteorologist signs that it is likely losing it's punch. As we move into the mid evening on May 26, 2011, we saw that weakening occur. Notice the green line (gust front) has now advanced way ahead of the storms and the line begins to look ragged. We no longer see a consistent line of red on the nose of the squall line and everything is blending into a yellow color. This is occurring because the gust front has advanced so far ahead of the line that it begins to ingest cold air. Since warm air is the fuel for storms, cold air should cause the storms to lose intensity. Think about it like this... when you put good gasoline in a corvette, the car is very fast but if you put water in your gas tank of the corvette then it simply won't run. The same thing happens when storms ingest cold air, so when these gusts fronts advance ahead of squall lines it normally means the end of the line.
The Squall Line In Kentuckiana
Squall lines are actually fairly common in Kentuckiana. We probably have 20 of these per year and they are the most common severe weather producers in our area. When we have squall lines, normally they occur as powerful fronts are heading into our area. Since the atmosphere has to share the energy over a ton of storms, we normally see damaging winds as the primary threat. It is not uncommon to get a few spin-ups with squall lines, but they traditionally are the shorter lived type of tornadoes.
I hope this helps you understand the squall line, the phases of a squall line, and how to know when the wind threat is high (bow echo) or diminishing (gust out phase).
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