Squall Line Ahead – Now the Real Fun Begins – Flight Weather

Squall Line Ahead – Now the Real Fun Begins – Flight Weather

Squall Line Ahead – Now the Real Fun Begins – Flight Weather

Coping with supercell thunderstorms

Read any book about flight weather and flying near thunderstorms and the standard advice will be to avoid the storm by 20 miles and don’t fly under the anvil. You’re also told to never try to out-climb building cumulonimbus cloud formations. Good advice, but is there anything specific that might help you avoid the grim reaper?

If the wheels on the atmospheric slot machine look like they will line up and cause you to hit the supercell thunderstorm jackpot, don’t launch. Stay home. Penetrating a line of thunderstorms is never a good idea, even though you MAY survive if you are lucky and don’t hit an active cell. Still, trying to fly through a squall line containing supercell thunderstorms, and/or their grumpy cousin, the tornado, is akin to suicide, as renowned test pilot Scott Crossfield unfortunately discovered.

Severe Thunderstorm Strategies

Before you actually depart, go online and satisfy yourself that a severe weather setup is not present or, if it is present, that it can be safely circumnavigated. If it’s a 50% or less coverage situation, you might consider launching after reviewing the TAFs and METARs for all potential diversion airports along your flight path, so you can formulate a plan “B”. But divert at the first indication – and I mean the very first indication – that plan “A” is coming unraveled. The thing you want to avoid is penetrating a line of severe thunderstorms in IMC. If you’re certain you can maintain VFR and circumnavigate the severe TRW cells with at least 40 miles of separation from each cell, your odds for survival are much, much greater than any IFR penetration attempt, no matter what equipment you are flying and what avionics are onboard.

Do not ever– as in never – rely only on a DUATS or Flight Service briefing. You must get a legal briefing, so talking to FSS or logging on to get a DUATS brief is therefore required, but when potential severe weather is afoot, involve yourself in the process so you understand the dynamics present in the atmosphere.

For this type of weather in particular, once you’re aloft you must do extensive enroute “nowcasting”, meaning you must continually look at what is SUPPOSED to happen compared to what IS happening and adjust your plans accordingly. In essence, continually check on what’s happening 100 miles ahead and 45 degrees to either side of your intended route and compare it to what the briefer said would happen. If the peanut butter and bread aren’t coming out even, divert and live to fight another day.

A quick aside: There’s a story that’s been making the rounds about an airline first officer who was looking for a “soft spot” in a squall line that contained supercells. He asked ATC if anyone had penetrated the line near a particular fix. ATC said, “We had a 757 go through there about 20 minutes ago.” The captain then dryly asked ATC if the five-seven ever came out the other side – and then he diverted. Diversion is the single best countermeasure you can take when things become dicey.

The Obligatory “There I was….” Story

There’s this thing called fate and it lies in wait for pilots every year. It could have gotten me some years back. My only defense was “nowcasting”.

I was slated to fly from Dallas to Louisville one mid-April day. A squall line had come through Dallas in the wee small hours, prior to my launch time of 8 AM, waking me up with its violence.

A 7 AM look at the charts – you could still walk into a Flight Service Station in those days – indicated that the squall line had discharged the lift and moisture in the atmosphere along my route and there was nothing in particular happening on radar with the cold front that lagged about 300 miles behind the squall line. So, wheels up time came and off we went.

As we passed Memphis off to the southeast, I could see the back side of the squall line, whereas it should have been long gone. That meant it had slowed down. Looking northwest, I could clearly see the anvil tops of another line of thunderstorms. The front had found moisture to chew on and I was now in a tunnel between the slowing squall line and the onrushing front. It was totally clear where I was, but something told me that a trap was closing on me.

I called Flight Watch and they cheerfully told me that, yes, what I thought was happening was indeed happening. The front was catching up with the squall line and it was questionable whether I could make it to Louisville before that happened.

Long story short, I diverted to Evansville, Indiana, put the airplane in a hangar and watched the show from the safety of an FBO lounge.

Tornadic_classic_supercell_radar  A couple of pictures are worth a few hundred words

In case you ever find yourself in a bad situation in spite of your due-diligence, it helps to be aware of how a supercell thunderstorm works internally and also how it appears on radar.

Some may not have onboard radar but if you know what to ask, you can still discover some very useful information from Flight Watch (122.0 MHz on your radio dial). When they tell you what they see on their radar, their description will make much more sense to you if you’ve absorbed the imagery that follows.

Study the supercell diagram and learn where to expect roll clouds, in-flows and out-flows, gust fronts and tornados. (You’re looking into the TRW from a vantage point southwest of the storm, so the tornado is directly in front of you.)

Next, look at the image from the Oklahoma City radar display. The radar display is oriented north up and the first thing you’ll notice is that the southwest quadrant of the storm has a hook echo component – just where the supercell diagram shows a tornado would most likely be.

The hook echo is an indicator of a probable tornado, and something you’ll want to ask Flight Watch about when you contact them. If they don’t see a hook echo on the storms ahead of you, ask if they see any storms with Bounded Weak Echo Regions. (The radar image has one just above and to the right of the hook echo, so you’ll know what you’re asking about.; it’s the precip free area). A BWER indicates the possibility of internal circulation – a precursor to a tornado. If there is no hook but there is a BWER, the storm is still almost certainly a supercell. It may not have spawned a tornado yet but it could easily do so at any moment.

Ask if their radar display has Doppler capability. If the briefer says it does, ask if there is any rotation in any of the cells ahead on your flight path. A rotation component is also called a Mesocyclone. It’s a signature of a supercell and an excellent reason to double the usual 20-mile exclusion zone for thunderstorms to at least 40 miles.

Supercell_side_viewAlso ask if they see any Line Echo Wave Patterns on the squall line and, if so, where are they along the line. A LEWP (also called a Bow Echo) indicates straight-line winds in excess of 70 mph. They can be associated with supercells but also occur within a plain-vanilla squall line. A LEWP will ruin your whole day, should you attempt a landing near one or decide to penetrate the line where one is located.

Finally, ask what the TRW coverage percentage is. If it’s less than 50%, you can consider continuing, but have a way out and use it immediately if things change or if you can’t maintain 40 miles separation. If there is more than 50% coverage, divert then and there.

Remember to reference your present location and destination to fixes, airports, navigation aids, etc., since Flight Watch has no idea where you are, where you’re going, or your route of flight, until you tell them.

Further Reading

There’s an excellent book on Severe Thunderstorms, and how to cope when they stalk the land, entitled “Severe Weather Flying” by Dennis Newton, ISBN-13: 978-1560270720. It’s available on Amazon.com.

Under the heading of shameless self-promotion, yours truly has written a book entitled

“WeatherWise – Forecasting and Nowcasting Techniques for General Aviation Pilots”, ISBN-13: 978-1475190199. It’s also available from Amazon.com and the above strategy was adapted from one of the chapters.

John Loughmiller is a freelance writer, commercial pilot and CFII/MEI-A. He retired from the business world a few years back and is now living the dream as a contract pilot flying various piston and turboprop twin.

About the Author

Leave a Reply