Pilotage

See the weather, avoid the weather

July 1, 2007

Mark R. Twombly is the AOPA Airport Support Network volunteer at Page Field in Florida.

Here in Southwest Florida we have two seasons: winter and thunderstorm. The former is defined mostly by the absence of the latter. Winter flying is easy flying. An occasional cold front blows through to make the wind sock stand at attention saluting northwest for a couple of days, but for the most part, crisp, clear air, friendly breezes from the east-northeast, and baby-blue skies dominate. Ideal conditions for pilots.

Thunderstorm season is equally predictable, but for different reasons. The sun comes up in the morning and begins the prep work for the afternoon show. As the surface heats up, steamy thermals begin to rise. Puffy cumulus clouds are born and begin to grow in height and breadth. The hot, thin air is capable of absorbing a lot of moisture, and with Florida's two flanking oceans and an interior soggy with lakes and wetlands, there is no lack of source material.

In thunderstorm season I wake up to an easterly offshore breeze, feel it gradually slacken throughout the late morning, and then at about 2 p.m. see it switch over to a westerly onshore breeze. That's those thermals at work, sucking in sweaty air from over the gulf to fuel the stovepipe clouds. By midafternoon the more prominent among the building cumulus begin to assert their strength. Inevitably, a few will reach their maximum potential and become classic mature thunderstorms with intense lightning and drenching downpours.

The strategy for flying during thunderstorm season is to launch early, or go late in the day or in the evening after the storms have subsided. (See " Wx Watch: Tools of the Trade," page 125.) A midafternoon departure virtually guarantees a bumpy, uncomfortable ride at low altitude. Florida thunderstorms typically are not as tall and intense as Midwestern monsters, but they are far more numerous.

When I lived and flew in western Missouri I usually could count on being able to climb to smooth air where I could get a good look at those big, widely scattered storms and plot a way around them. Over the interior of Florida the summer sky often is too thick with bumpy clouds to allow for continued VFR above about 3,000 to 4,000 feet agl. I'm forced to stay at lower altitudes, scanning the gray horizon for shafts of rainfall marking the worst of the weather and menacing dark smudges that identify where the next downpour likely will occur.

That's why next-generation weather radar — Nexrad — in the cockpit is considered by many Florida pilots to be the most practical innovation in aviation since the airport restaurant. With thunderstorms such an ever-present feature of the flying life in Florida, Nexrad gives those of us who fly light airplanes without airborne weather radar, which means most of us, an unprecedented and highly useful tool for evaluating weather conditions during the flight.

L-3 Communications Stormscopes and Insight Strike Finders give us information about the general location and intensity of lightning strikes, and therefore about where we can expect to find wing-bending convective activity. Nexrad shows us where the National Weather Service's 159 radar sites throughout the United States are detecting precipitation. I want, and have, both of those tools when flying in the clouds, but in thunderstorm season in Florida the better bet usually is to stay VFR below the bases and request traffic advisories. In that situation the Nexrad display is my preferred tool. Looking at that graphical view of the location and intensity of precipitation on a day when the storms are really popping is like blowing the dust off a map showing escape routes out of a blind canyon.

Nexrad is so obvious an advantage and so intuitive that it's easy to ignore its limitations and overstate its usefulness to the point of relying on the information to go when you should say no, or to penetrate the weather. Even with the latest Garmin GDL 69 system in our airplane, the Nexrad weather displayed on the GNS 530 is at least eight to 10 minutes old. Doesn't sound like much, and in the big picture it really isn't, but developing storms can explode in a matter of minutes. The microburst that brought down Delta Air Lines Flight 191 at Dallas-Fort Worth in 1985 came from a cell that mushroomed from a harmless thermal to a mature, deadly storm in fewer than 20 minutes.

Also, it's easy to be lulled into penetrating an area displayed as yellow or red because the view out the window often does not live up to what the Nexrad picture is painting. That has to do with the age of the information (the rain could be dissipating but still displaying as intense), the fact that the display shows the maximum intensity of precipitation within about a one-square-mile area (the worst of it may be in a very small circle at the center of the display block, but the entire block will still show red), and the inherent limitations of using a very few colors to display an infinite range of precipitation intensity.

It's also important to keep in mind that in some parts of the country the radar sites are widely scattered, meaning there is little overlap and sometimes even gaps in Nexrad coverage. Here in Florida we fly out of coverage when eastbound to the Bahamas.

Any really useful tool is prone to abuse. I've poked myself good using a screwdriver to pry off a beverage cap. The bottom line when it comes to Nexrad is to vote conservative, and use it as the providers recommend — to see the weather and avoid the weather.