All the technical folks will tell you that the levels of precipitation are depicted the same on airborne weather radar and Nexrad. In actual practice, that is simply not true, especially when the airborne radar uses a 10-inch antenna. I have flown through a lot of red on the Nexrad picture that with my ship's radar I would never have done so.
With XM there are other tools to use to verify it is okay to fly through an area of rain. If no storm cells are identified, that is a definite plus. If there is no cloud to ground lightning, that is also good. But the best thing is always the Nexrad picture where shallow gradients and a mostly green and yellow picture suggest that there is no wing-bending turbulence in there. I'd add that working out a route that traverses only green or no return gives the best ride.
We do have to realize that any time it is wet it can be bumpy. And there is one particular condition where it can be extremely turbulent in the lower levels without a clear warning from lightning or Nexrad or a ship's radar. When a front is occluding, usually a cold front overrunning a warm front because of a strong circulation around a low, the air is really bothered. The turbulence is wind shear turbulence and even though it lacks the extremely strong up- and downdrafts of a thunderstorm, it can really blur the view of the instrument panel.
After tangling with one occlusion, I have flown hundreds of miles out of the way to stay away from others. Get far enough away from the low and all you have to fly through is a plain old cold front. If ever you are flying from where there is a strong southeasterly flow into an area where there is a strong southwesterly flow, you might be about to learn the occlusion lesson. The closer the two points are, the stronger the lesson.
The strong southwesterly flow aloft ahead of and just behind a surface cold front is another time to pay attention. There is a lot of wind shear turbulence here and any showers that show on Nexrad are best avoided if possible because the turbulence will be enhanced in the showers. Green and yellow would mean a lot more here than in a stationary front.
Next, consider a condition where severe thunderstorms are forecast. Where there is usually a correlation between lightning and precipitation, in severe storms there might be lightning shown ahead of the storm's precipitation. This in itself is an indication of severity. Give it a wide berth.
There are guidelines on the distance to stay away from storms. Five miles from the precip is the word for garden-variety storms. For severe there is a bigger number but everything here is open to question. Severe turbulence can be found in any cloud associated with a thunderstorm so the key to a better ride when there are severe storms around is in staying out of all the clouds as well as away from the precip. And, regardless of how far away Nexrad shows the precipitation to be, watch out for those wispy looking clouds. They can grab you by the tail and give you a thorough shaking. I remember a controller once questioning the extent of my deviation around a severe storm but I was basing it on the view and the ride, not on the distance from precipitation.
Another thing that Nexrad gives is a radar picture of the terminal area toward which you are flying. It is here, looking at the relationship between thunderstorms and an airport, that you get the best demonstration of how conditions change rapidly and how we didn't really have a handle on this until we got Nexrad.
If there is any suggestion of storms for an arrival, I start looking at the arrival area right after takeoff to get a picture of how things are moving and developing. And there have been times when my en route planning changed several times over a few hundred miles.
The gradient of a storm is steeper on the advancing side of the storm and the direction from which it is getting its moisture feed. It is normal to say the east side around to the south is strongest in the Northern Hemisphere, but that is not always true. A westerly moving storm bested an airline 727 at New Orleans years ago, and I always wondered if the crew could have been thinking in terms of an easterly moving storm when scoping that one out. Whatever, they made the wrong decision. With Nexrad we can see the direction of movement and properly deal with a maverick storm.
When we are looking at a storm and an airport from a distance, the decision we are trying to make is whether it would be better to get around on the east side and go in ahead of the storm, or wait until the worst of the storm is past the airport and then come in from the back side. The beauty of Nexrad is that you can see all of the storm. With airborne weather radar, looking at one from the back side, you may not see the worst because of attenuation, which occurs when nearby rain uses up all the radar's ability to detect and display precip. Likewise, when looking at a storm visually, the back side will look a lot tamer than the advancing side. The complete picture helps in the decision-making process.
Nexrad isn't going to make an on-time arrival possible if the storm gets there at your ETA. What it does is keep you abreast of how things might work. I would hasten to add that when the weather is active, the best word on the surface weather at the airport comes from the ATIS, ASOS or AWOS. The metar from XM Weather just won't be as current as the dynamic broadcast.
I got weather in my cockpit five years ago when it first came out and I still have that now-obsolete Bendix/King uplink system. I'm also flying with XM through a Garmin 496. When in-cockpit weather started, the big buzz about the Nexrad picture was latency. That's bureaucrat for age. How old is the radar picture we look at and how much difference does age make? The picture on the panel does have some age because the radar information has to be gathered, processed and then broadcast to the airplane. The age shown on the screen is the time since it was received in the airplane. In the beginning we figured a total of 15 minutes as an average from the Nexrad radar sweep to our screen.