When you're deciding which side of the storm to deviate around, here's what you should take into consideration.

Two Flights, Two Choices

Two flights are leaving Chicago O'Hare (KORD) for Traverse City, Michigan (KTVC). A long line of storms moving at 40 knots WEST to EAST with tops between 30,000-40,000 feet. The storms block the most direct routing, so it's time for some creativity and decision making. The screenshots below are from two actual flight paths, with departures just a few minutes apart.

The shorter route will get you through a gap along the eastern side of the storms in a total of 255 miles. The longer route will allow you to clear the upwind side of the storms, for a total of 310 miles enroute. Both flights make it just fine and land safely. Which side of a thunderstorm will you deviate around and why?

Thunderstorm Movement

While no two thunderstorms are the same, the graphic below explains the basics of what thunderstorm movement looks like. There are times you'll find stationary storms, but with winds aloft, your decision making must adjust accordingly. If you have to fly around thunderstorms, you want to find clear air to visually separate yourself from the storm. Unless you have onboard radar, embedded thunderstorms are difficult, if not impossible to navigate around. And even with onboard radar, navigating through embedded storms can be challenging.

In this example, let's make staying in clear air the first priority.

Generally speaking, you'll find the best chance for clear, smooth air on the upwind side of a thunderstorm. On top of that, you're typically in more sunny conditions on the upwind side. From a passenger's perspective, sunny weather is almost always more enjoyable than cloudy weather, even if the same amount of turbulence is present in both conditions.

As winds aloft blow the thunderstorm downwind, the anvil begins to spread out. Precipitation usually falls on the downwind side, and odds are you won't find conditions nearly as clear. Severe or extreme turbulence, hail, lightning, and strong straight-line winds can exist outside of the visible thunderstorm. Most of the time, these elements occur downwind of the thunderstorm, in the direction of its movement.

The Anvil Tells You A Lot

The flat, spreading top of a cumulonimbus cloud is called the anvil because it looks like one. While anvils usually don't spread very far upwind, they often spread hundreds of miles downwind of powerful storms, with strong winds aloft. Many pilots have experienced dangerous, large clear-air hail by flying beneath overhanging anvil clouds. This hail is essentially sucked upwards through primary updrafts near the core of the storm, and then spit out with the direction of the wind.

One rule-of-thumb is to avoid the downwind anvil side of a thunderstorm by at least 1 mile for every 1 knot of wind at that flight level.

You're Looking At The Radar...

When you're on the ground planning your route, pay close attention to the storm's direction, and how fast it's moving. Satellite-based weather radar will have latency, so it's likely the storm will move significantly downwind by the time you approach it. If you're worried an upwind route may take too long, it's likely you can plan to cut the corner closer than you think due to the direction of the storm.

The worst outcome is to plan your flight around the downwind side of the storm, or through a small gap, which could close with little warning.

If you approach a line of storms and you're faced with a "should I turn right or left?" moment, turning toward the upwind route is typically the best decision.

If you have to pick your way through cells or a small gap, be cautious as cells can join together rapidly. On top of that, some of the worst turbulence can be found right between building cells.

What's Your Decision?

Have you ever been faced with a long line of storms and had to divert or make a substantial deviation? Tell us about your experiences in the comments below.

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