The XM input on the G-1000 specifically states that the displayed NEXRAD
weather cannot and should not be used to aviod rapidly moving storm cells.

NEXRAD also suffers from attenuation larger cells blocking other cells "on
the other side" of the cell from the radar transmitter.

BT

"Peter R." wrote in message
...
I was listening to LiveATC.net's feed of Syracuse, NY, yesterday (Sunday)
during the approach of a large thunderstorm line that was moving towards
the
airport from the south. At one point a VFR a Cessna 172, approaching the
airport from the west, checked in stating his intention to land at SYR.

As the C172 progressed, the controller called the pilot to report that the
aircraft was eight miles from and heading directly towards a line of
extreme
weather. The pilot responded in a somewhat unappreciative tone that he was
"painting" it. The controller responded with a rather surprised,
"Roger."

Moments later the controller again called to warn the pilot of extreme
weather at his twelve o'clock. The pilot then replied in what sounded like
an
annoyed tone that he was "picking our way through the weather using our
XM."
Again, the controller replied with a surprised "Roger."

With a quiet moment on the frequency, the controller called the pilot a
third
time to ask him to explain what XM meant. The pilot described the concept
of
XM NEXRAD on his Garmin 396, but then added that the refresh rate allowed
him
to see heavy weather on the 398 from 2 to 8 minutes old.

When I heard that the refresh rate was up to 8 minutes old, I cringed to
think that this pilot (this one didn't sound like an experienced freight
dog
to me, but maybe I was wrong) was picking his way through a field of
thunderstorms on a day where returns went from light to extreme in less
than
ten minutes.

Granted that the pilot was VFR so presumably he was seeing and avoiding
using
the outside picture primarily. But this day it was a typical northeast US
hazy and humid day with visibility around the 6 to 8 mile range, so having
the Garmin 396 for strategic avoidance was a good thing. It still made me
sweat from the comfort of my office chair to imagine that this pilot was
flying through the weather line using up to 8 minute old data (not even
considering the NEXRAD delay before the picture is uplinked to the XM
satellites) and declining the more real-time weather guidance offered by
ATC
(SYR approach has excellent weather radar).

Eventually a thunderstorm cell erupted right over the airport, resulting
in
numerous wind shear alerts and at least one microburst alert, so the pilot
called approach to state his intention of diverting southeast towards an
airport 50 nm south of SYR to wait out the weather.

--
Peter