Avoiding Thunderstorms is the Best Policy

One beautiful, hot summer day, I provided a standard briefing to a VFR-only student pilot. The radar was indicating a broken line of thunderstorms about 30 miles away and moving toward the station. Convective SIGMETs (WST) and other advisories were associated with the line.

The departure forecast called for scattered clouds with an occasional ceiling of four thousand overcast, visibility three to five miles with thunderstorms and moderate rain showers. The pilot's route of flight penetrated the line. His destination on he other side of the line was reporting scattered clouds and haze. I issued the VFR flight not recommended statement and the pilot asked "why."

I have briefed pilots when the mere mention of a thunderstorm in the forecast had them planning an alternate route or waiting for the thunderstorms to dissipate or move through the area–no questions asked.

A pilot's respect of thunderstorms more than likely begins with the flight instructor or ground school training. This respect intensifies if the pilot has been caught in or near a thunderstorm, or listened to conversations of pilots who had come face-to-face with one.

Flight service specialists know that thunderstorms mean danger. Thunderstorms imply hazardous conditions, whether it is a microburst, low level wind shear, turbulence, hail, up- or downdrafts, or the worst of all scenarios, a funnel cloud. We know the extra danger thunderstorms can produce.

The National Weather Service reports that an average thunderstorm cell is 15 miles in diameter and lasts an average of 30 minutes. Thunderstorms need certain elements to mature including moisture, instability and a lifting mechanism. A mature thunderstorm has considerable depth, often reaching 40,000 to 60,000 feet. Strong updrafts and downdrafts co-exist. This is the most dangerous stage when large hail, damaging winds and flash flooding may occur.

Forecasters issue advisories stating conditions are right for thunderstorm formation, but the confirmation–the where and how big, mean and ugly–comes from your pilot reports once you are safely clear of the area. Reporting squall lines, icing, turbulence of moderate or greater intensity, rain, lightning, hail, roll clouds or frontal activity will confirm the weather radar activity and alert your fellow pilots to avoid the area.

The National Severe Storms Center reports that at any given moment there are roughly 2,000 thunderstorms in progress around the world. It is estimated that there are 100,000 thunderstorms each year with about 10% of them reaching severe levels. During the summer months, the forecast will include at least a chance of thunderstorms almost every day of the week. According to professional weather personnel, operation within 20 miles of thunderstorms should be approached with extreme caution, as the severity of turbulence can be markedly greater than the precipitation intensity might indicate.

Take a moment to review the do's and don'ts for thunderstorm flying in the Airmen's Information Manual. The most advantageous practice is stated in the first paragraph–avoiding thunderstorms is the best policy.

This article was written by Nancy Causey, LMFS Communications Manager