Don't say no-go, say maybe
Learn to identify and understand the differences between steady state and air mass thunderstorms. FAA Advisory Circular 00-6, Aviation Weather, says that steady state storms are associated with weather systems such as fronts, converging winds, and troughs of low pressure; air mass storms result from surface heating.
Steady state storms can last for hours and be extremely wicked. They are given a wide berth by all pilots, usually 20 or more miles. Air mass storms, however, are the type illustrated in most textbooks in order to show the three stages of development--cumulus, mature, and dissipating. Aviation Weather states that the life cycle of an air mass storm is usually between 20 and 90 minutes.
Learn the terms used for thunderstorm coverage, and be familiar with the terrain features that exist in the area you're evaluating for thunderstorm activity. Isolated (ISOL) means single cells with no percentage given for the area. Widely scattered (WDLY SCT) indicates less than 25 percent coverage. Scattered (SCT) or areas (AREAS) indicate 25 to 54 percent coverage. Numerous (NMRS) or widespread (WDSPRD) indicate 55 percent coverage or more.
Your first flight planning responsibility is identifying possible hazards--thunderstorm activity in this case. Go to the FAA's PilotWeb site. Click on FAA Aviation Digital Data Service (ADDS) followed by Standard Briefing, listed under Related Information. Look at two sections on that page: Under En Route Forecast, select Low Level SIGWX Progs. Under Additional Information, select the Day 1 and Day 2 convective outlook charts.
Do the prognostic charts show that your flight will be in an area of high pressure that's void of weather systems? If so, that's a good sign. However, if the convective outlook charts forecast a slight, moderate, or high chance of thunderstorms, that's bad, because all three of those classifications mean severe thunderstorms are forecast. If that's not the case, plan your flight, but more information is needed to make a go/no-go decision.
As departure time approaches, return to ADDS and Standard Briefing on the Internet. Look at the surface analysis and weather depiction charts to see if the area in question is still under the influence of a high pressure system that's void of weather systems. Look at the area forecast, listed under en route forecasts; and convective sigmets, sigmets, and airmets, listed under adverse conditions. That's where you'll find the terms ISOL, WDLY SCT, SCT or AREAS, and NMRS or WDSPRD, if applicable.
If SCT or AREAS or NMRS or WDSPRD are forecast or reported, cancel the flight; there's too much risk. If ISOL or WDLY SCT is forecast, you still have a chance. Go to the radar summary chart, listed under weather charts, to check storm locations in relation to the terrain features that you identified. If all is well, it's time for a second opinion. Contact FAA flight service for a standard briefing in order to validate your decision.
Now ask yourself if the risk is acceptable. It is, providing the storms are over areas that you can easily circumnavigate--terrain that's elevated or more susceptible to surface heating. However, you must assume the worst. Are there VOR stations along your route that you can monitor for hazardous in-flight weather advisories: airmets, sigmets, and convective sigmets? If the storm coverage increases unexpectedly while en route, will you have enough fuel to fly to a suitable diversion airport? Are there suitable alternate airports along the route? If a storm develops over your destination airport just when you approach the area, will you have enough fuel to fly to a diversion airport or hold until the storm dissipates?
If a towering cumulus cloud is drifting toward your destination airport and it starts to emit rain as you make your approach, exercise extreme caution. That storm has reached the mature stage, and the resulting downdraft will generate a gust front that will cause turbulence, change wind direction, and increase wind velocity at the airport. You must use good judgment. Abort your approach and remain well clear of the area until the storm has dissipated or moved away.
In review, you identified a hazard and determined that you could operate without undue risk based on current information, you monitored the hazard while en route, and you guaranteed yourself a safe deviation if the risk became unacceptable. That's the sign of a truly safe pilot.
Ralph Butcher, a retired United Airlines captain, is the chief flight instructor at a California flight school. He has been flying since 1959 and has 25,000 hours in fixed- and rotary-wing aircraft. Visit his Web site.
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