The official name is XM WX Satellite Weather. Most pilots just call it XM Weather. The neat thing about it is that it offers virtually all available weather information and can be received and displayed on handheld units or on a variety of panel-mount units. There is an instrument panel docking station system available for the Garmin 396 and 496 handhelds that eliminates many of thewires that adorn the cockpit when these are used as a pure handheld, or mounted on the control column. Some of the panel-mount units don't yet have the capacity to display all the information that XM broadcasts.
Another neat thing is that a popular Control Vision PDA-based unit with a moving map and XM Weather display sells for $1,495. A Garmin 396 is advertised for $2,195 and a 496 for $2,795. Any serious pilot, whether renting, flying club airplanes or owning an airplane, can now have the same in-flight weather information that is available to those flying the most sophisticated turbine airplanes, and for not a lot of money. This includes GPS navigation, even WAAS in some cases. And it is an easy and sure bet that more pilots will fly into this thunderstorm season with Nexrad displayed in the cockpit than ever before. Those Garmin units literally flew off the shelves last Christmas.
Most of the buzz about XM Weather relates to the Nexrad weather radar mosaic and lightning display. That is as it should be. The other items, the metars, TAFs, airmets, sigmets, winds, freezing levels, synoptic pictures and more, are and have been available elsewhere. Many of these products have a relatively long life and can be obtained before takeoff. When flying, the metars, TAFs and convective sigmets are readily available by calling an FSS or, in the case of metars, by listening to ATIS, ASOS and AWOS broadcasts. But the radar picture you look at before takeoff is guaranteed to change and XM Weather brings those changes to the cockpit.
To this ancient pelican XM Weather is nothing short of miraculous. As I first became acquainted with thunderstorms and airplanes we had only a diagram of temperature and dewpoint with altitude (from weather balloons) to use to get an idea of the possibility of storms. How stable is the air? How much moisture is available? A storm is reported on a metar (formerly a sequence report) when thunder can be heard by the observer. (With the automatic stations it is done electronically.) Reported storms were, in the bad old days, the only indication that the stability, or lack thereof, and the humidity were just right for the formation of a storm. Now, the big picture of weather is available at a glance.
Before we talk about how this is used, a grain of salt. There is no magic enabler here, no yes/no windows. It is information that has to be interpreted and used properly in conjunction with everything else that is available. At least one pilot flying with Nexrad information has come to grief in proximity to a thunderstorm. As this becomes more widely used, only time will tell whether or not we'll have less thunderstorm-related accidents. If pilots use it to push too hard in weather, we could have more accidents.
Where to start learning about the possibility of storms before takeoff? The area forecast combines the more complex convective outlooks to simplify the expectation of thunderstorms. Isolated means not many storms are expected. Widely scattered means to pay attention. Scattered or areas means you might have to work at it. Numerous or widespread means the forecasters think it will be an active day.
The synopsis is important too. Where is the low pressure area and where are the fronts? These features do not even start to pinpoint the location of possible storm activity but they can be the triggers.
Finally, before flight look at the radar picture and the convective sigmets. This can show you what is happening now. It might also suggest a best initial routing if there are storms nearby. However, and especially if the flight is a few hundred miles or more, that flight planning look at convective weather is valid only for a few minutes or miles. It will change.
One other thing to look at is your watch. This is a total generalization but it has long been a rule of thumb that if thunderstorms have a weakest moment, it is around midmorning. Then as the day wears on, conditions are likely to become livelier.
The beauty of XM Weather is in being able to keep up with the weather radar picture as the flight progresses. Maybe the look before takeoff suggested that a zig or zag of, say, 50 miles might work. The operable word there is "might." Thunderstorms ebb and flow and move, and only a dynamic picture of activity can help you keep your airplane out of harm's way.
In relation to convective weather, XM displays Nexrad, lightning, echo tops and a satellite mosaic. In relation to individual storm cells it will also show tops of precipitation return, the strength of the return, and a motion arrow, the tip of which indicates where the storm is expected to be in 15 minutes. That is a forecast, though a 15-minute forecast is usually pretty accurate.
For light airplane pilots, the tops don't mean much other than the higher they are, the meaner the storm is likely to be. There's no way you are going to top a storm. For pilots, the lightning information has to be used with a grain of salt because it is only cloud to ground lightning. Cloud to cloud lightning often begins well in advance of cloud to ground and it is also something to stay away from.
So, while all the XM Weather information is good, it is the Nexrad mosaic that says the most about where the nose of the airplane should be pointed. The other information might be considered supplementary, to be used only in connection with Nexrad.
Let's look at some scenarios.
First, consider a trip about 500 miles long with an area of weather related to a stationary front along the way. This area is showing some red and yellow return but the gradient is shallow. Gradient is simply the distance over which the picture changes color. A steep gradient, where the green and yellow give way to red and darker red in a short distance is indicative of convective weather.