by Mike Bliss
Summer often brings some of the best flying weather, but also its share of challenges: turbulence from uneven surface heating, reduced visibility due to haze, and diminished aircraft performance due to the hotter air. The greatest threat, however, comes from thunderstorms.
To fully understand this threat, you need more than a standard weather briefing. You must grasp how thunderstorms form and the broader weather patterns behind them, especially given how quickly conditions can change
You can start this process by understanding the ingredients needed to form a thunderstorm. The three main ingredients are: moisture, instability, and a lifting mechanism. The moisture comes from oceans, lakes, and moist ground, in the form of clouds, which can be carried by wind many miles from their source.
Instability is often caused by warm air near the surface being overlain by cooler air aloft, allowing the warm air to rise freely.
The lifting mechanism can be triggered by convective heating, cold or warm fronts, mountains, or a few other less common causes.
To simplify your understanding of thunderstorms, you should consider the two basic types: air mass thunderstorms and frontal thunderstorms. The difference in how they are formed is the lifting mechanism. Air mass thunderstorms are caused by convective heating of the ground on hot summer days, so they are often an afternoon and evening phenomenon. They can be thought of as “pop-up” storms because they can quickly form and may or may not be long-lasting. These storms are often widely spaced, making navigation around them relatively easy, as long as visibility allows them to be seen. Though these storms are not always severe, if they continue to develop, they can reach very high altitudes, making them a serious threat to flight because of the strong up and downdrafts, which will frequently produce hail. Hail may be thrown out several miles ahead of the storm, so it is necessary to give these cells their distance.
Frontal thunderstorms are caused by either a warm front overriding a cold air mass or a cold front pushing under a warm air mass. They are called frontal thunderstorms because the thunderstorms develop along the edge of the advancing front. These storms are often more dangerous because they can form a wall that cannot be penetrated without sufficient gaps and good weather radar. These frontal storms are usually long-lasting and can travel for many miles as the front moves across the ground.
The key to safely managing a flight in conditions conducive to thunderstorm development is to obtain a thorough weather briefing and to understand the factors in play. A good place to start is with the Surface-Analysis Chart, especially if there is frontal activity near your route of flight. Get a good idea of where the front is, how it is expected to move, and at what speed. Next, move to the Radar Summary Chart to confirm where thunderstorms are actively occurring and take note of their intensity level.
If your flight is of some distance, examining the Convective Outlook will help solidify your understanding of the general thunderstorm threat level and timing over larger areas. The Significant Weather Prognostic Charts will add what to expect in terms of turbulence and the vertical extent of thunderstorm development. SIGMETs, Convective SIGMETs, METARs, and TAFs will provide what is needed to fill in the details. How much of this information is needed will be determined by how close you will be to the activity and how severe it is forecast to be.
Try to develop a picture of what is happening by comparing reports to forecasts to have a sense of how accurate the forecasts may be. Also, pay attention to trends. Is the weather trending as forecast, is it improving, or is it deteriorating? These trends can be helpful in determining how trustworthy the forecasts are.
Because of the fast-paced changes that can occur in the vicinity of thunderstorm activity, plan to get updates from Flight Service while en route. Don’t look for them to tell you what to do. You are the pilot in command and must make those decisions on your own.
One thing you should always be aware of is in what direction from your planned route of flight better weather lies. If conditions begin to deteriorate, have in mind which direction you will go for better conditions. Avoid the temptation to keep going when you are not comfortable with the conditions you are encountering. It is always better to be on the ground, waiting for the weather to pass, than to be caught in a thunderstorm wishing you were on the ground.
The forecast of possible thunderstorms should not automatically cause you to cancel your flight. Instead, gather all the information you need to make a safe go/ no-go decision, which should be based on your experience, the airplane’s capability, and your overall comfort level.