Weather-Guessing and Computers

Look at that picture above. Now that is something no pilot wants to see. The XM Weather (as depicted on my Garmin 496 GPS) is showing an area of heavy precipitation adjacent to and around the Brewton Airport (12J). Red is bad. I was only 32 miles out when I took that picture, but I had been watching that storm blossom from 50 miles away or more.

It's summertime, and here in the South that means lots of thunderstorms. Earlier in the day I had been heading for Birmingham, Alabama. Before taking off, our employee there told me I'd be dodging thunderstorms that were all around the area. In flight, the Boss looked at his iPhone and told me the same thing. But I am usually not too concerned. Thunderstorms usually either move or burn themselves out. Using the scroll/zoom feature of the 496, we looked at Birmingham's weather. Sure enough, storms ringed the city. I just shrugged and said that things would absolutely be different by the time we got there...IN AN HOUR, our enroute time. And sure enough, they were. We had no problems getting in or out.

But coming home, there weren't any storms around that I could see other that one over Home Base had been lurking and building for a long time. I wasn't really worried; there are plenty of other places I can go to wait it out. But I would be annoyed. The day had started early (as most of them do). Now, I wouldn't land until six p.m., meaning I wouldn't get home until after seven. Deviating or diverting would delay things even more. Hey, we pilots are human! I try to be philosophical about such crap, but after a long day I just want to put the helicopter away and go home and get something to eat.

The closer I got to home, the more I realized that the XM weather was just wrong. For some reason it thought there was "weather" in a place there was none.


In the picture above I am over the city of Brewton. The airport is dead ahead - that little sliver of green just below the horizon (kind of hard to see if you don't open the picture up in a new window). No rain around. Allas klar, herr kommissar!


Here we are closing in on the airport. Nothing but blue sky do I see. So much for XM Weather! HARUMPH! (By the way, that little gray thing with the pyramid on top is my ZAON XRX collision avoidance device - which every aircraft ought to have! It also plays through the incredible Garmin 496. And just above the ZAON is a sandbar on a winding river between me and the airport.)

Well, wait. Normally the XM Weather is very accurate. But once in a while it shows stuff that is simply not there. Fortunately the reverse is not true: If there is a storm cell, it WILL be displayed. I suspect that there had been a storm of some sort over Brewton, but that it had just dissipated in the interim and was still showing up in XM's computer for some reason. Typically there is a "lag" of five minutes or so between what the screen shows and what's really happening. But this was ridiculous; I'd been watching this storm for a half-hour.

Which brings to light one of the weaknesses of XM Weather Radar. It is a computer generated "product," compiled from various radar sources. It does not show precipitation but only moisture. And there's a difference! Big clouds can hold a lot of moisture. Generally, at some point that moisture gets too heavy and falls out as rain. Thus, if the radar measures a certain amount of moisture we assume that rain is falling underneath that area. But that's not always the case.

I've seen huge red blobs on my GPS directly in my flight path. But me, being down low, can see that there is no rain coming out the bottom. ATC controllers, who see the same red blobs on their screens, will start freaking out that I'm about to fly into such an area of bad weather. But truly understanding what's going on with the weather takes input from more than one source. Sometimes all you need is a good set of plain ol' Mark II eyeballs.

We pilots, if we're diligent about our jobs, learn a lot about weather since it so directly affects us. I'm grateful for every modern tool that we've developed to help us understand what's happening right now (like the weather radar displayed on my GPS screen) and make fairly intelligent guesses about what's going to happen in the future. But I'll tell you what: The more I learn about weather, the more I realize how little I know about it. And that's the funny thing about Mother Nature - she doesn't always play by hard and fast rules. Sometimes she fools the computers.