Avidyne FlightMax EX600
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IS XM "BETTER" THAN WSI? Does "Better" even matter?

IMPORTANTLY: I absolutely love BOTH products. In the final analysis, there is very little difference in that BOTH services will save a lot of lives and BOTH have made our flying so much easier...but I have come to believe that WSI's airborne NEXRAD (and the related WSI weather products) is a slightly better airborne service (at least, on the EX600 I operate.)

At the Beech Museum in May 2011 at Tullahoma TN, I listened with great interest to a Dr. David Strahle talk in-depth about "The Miracle of NEXRAD". I was generally impressed at the depth of Dr. Strahle's knowledge. I also thought he overlooked a few points.

By now, I have 800+ XM hours and 400+ WSI hours. Before I switched to WSI, I asked a number of questions of WSI. (Remember, I used to repair radars).

Dr. Strahle spoke mostly about individual radar sites and explained raw NEXRAD data; he talked less about the important matter of the mosaics we aviators use; ... and very little on the really important subject of how these mosaics are composed by the government and then tweaked (in various creative ways) by our several weather service providers.

He pointed out, correctly and importantly, that the vertical dynamics of a major storm need to be displayed prominently. I'd agree completely, but then he never mentioned the "Cell-Zits" in which both XM & WSI already deliver to us all of the features—more features, really—than he asks for and proposes be called the “Strahle Index” of vertical energy. Lightning Strikes give us the important hint that we are dealing with a very dynamic system.  The CellZits, Cell Tops, Cell Direction, Cell Ground Speed, Hail Likelihood, and Lightning Strikes give us a very clear picture of where to go..or not!

Example. Here, we were “end running” a vicious line of weather; we are out over the Gulf, ~300+ people having just died from this line, the day before.

I thought that his omission of the manner in which WSI creates its mosaic was a bit misleading. So, a few days later, I talked to him at length; he agreed that a balanced presentation was important and he said he would change his future emphasis to say how very good BOTH products really are.

But here's why I like WSI NEXRAD a bit better than XM NEXRAD:

Base reflectivity provides a more accurate snapshot (think of an image taken in an instant) of the atmosphere; a snapshot gives us a better look at reflectivity gradients and shear which are the real concerns. The composite image is more like a time-exposure of a dynamic and rapidly moving subject; it’s blurry at the edges; by definition, it has to be blurry as it’s collected over a full scan sweep cycle of 10+ minutes. If Dr. Strahle’s patients were allowed to move around for ten minutes as he’s imaging their breasts, those images would be blurry, too. A medical technician immobilizes the patient for long scans and/or uses snapshots for the same reason that WSI does...to achieve precision imagery. But there is no way to immobilize a line of active weather.

Further, WSI does indeed depict radar returns well above the “Local Station’s Base-Sweep Height”. The important point is that WSI "mosaics" all of the overlapping NWS NEXRAD stations, such that for a given storm’s location, there are multiple base reflectivity values based on returns from different stations.
 
The center of the beam at ~ 30 NM is at ~2,000 ft AGL. Obviously, WSI depicts radar returns above that altitude. For example, a station at just 100 NM from the cell produces a base beam height of ~10,000-13,000 ft; it’s even higher when further away from the XMTR, of course.
 
The WSI mosaic software shows us the highest value of the base reflectivity from different stations; multiple stations are looking at the same storm at different heights. This is why they don't miss storms. I have yet to see a cell that WSI missed. In the very few cases where a CellZit appears alone with very little NEXRAD, that's a perfect example of the dymanic verticality measure that Dr. Strahe suggests be offered us. He's right about that, it seems to me, and that unstable verticality is already in-cockpit (from both XM & WSI) as CellZits and Lightning strikes.
 
WSI's meterologist told me last year--and my somewhat-experienced eyes believe--that the WSI methodology preserves the reflectivity gradient, which is important for interpreting the presence of shear and turbulence.  

That makes sense to me; I have long disliked the blurry XM presentations, and early on I said it was “too-alarming”, far too much “Wolf!”, but I’d readily agree that it’s easy enough to ignore the full screen of green...except that we can’t see the NavData underlying the green, and I didn’t like that, because we have to turn it OFF.

Other than these omissions, Dr. Strahle offers a very good summary of NEXRAD, and how to use it. His is a lecture absolutely worth your time, if and when you get the opportunity. A few weeks later, a friend and I made a same-time, same place, same EX600 MFD comparison of the XM WX and WSI NowRad images. I'd love to know what you think of our study.

And come back next year! This technology is changing fast, improving monthly. All providers are getting better at delivering FABULOUS weather information services.

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