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Satellite-delivered weather -
manual gross filter strikes again! 
Do you notice anything unusual about the
XM-delivered satellite weather radar image on the right? (Click here to view a complete
image). This image was taken from the WxWorx on Wings display
and shows a grid of five
recent lightning symbols and a single Storm Cell Identification
& Tracking (SCIT) marker just to the west of Charlotte, North
Carolina. If this were truly a thunderstorm, where are the radar
returns? Is it possible to have lightning without any precipitation?
The radar feature is definitely selected and the
radar portion of the broadcast was indeed received. This can
be verified by looking at a wider view of the Southeast on the image
below (click here to view a larger
image). This view clearly shows that the radar broadcast was
received since  there are valid
returns shown in eastern Alabama. In addition to the missing
returns west of Charlotte, notice that there is also a similar issue in
northern Georgia. So what could cause such an anomaly?
First, let's take a look at the source
of this data, namely, the NWS WSR-88D NEXRAD Doppler radar out of
Greer, South Carolina. If the data is missing from the source, then
that may explain why the broadcast was incomplete. The radar image
shown below is valid at the same time and clearly shows a cell to the
west of Charlotte in the same location as the lightning symbols and
SCIT marker. The complete image also clearly shows
precipitation over northern Georgia this is also missing from the image
above. Consequently, the cell west of Charlotte is a real
thunderstorm in progress. But where's the composite radar
data associated with this cell?
This issue
has nothing to do with the XM weather receiver or its display.
The broadcast was received successfully. The
display was updated with the latest and greatest composite radar
mosaic. But somehow, the radar data containing real
precipitation over a fairly large region was obviously not included in
this radar mosaic.
This anomaly is caused by a delay in removing a
manual gross filter. These gross filters or masks are applied
by meteorologists at WxWorx in Huntsville, Alabama to regions in the
coverage area that have a very low probability of precipitation,
normally in areas with a totally clear sky. A filter such as
this helps to completely remove annoying ground clutter and other
returns caused by anomalous propagation. In other words, it
produces a high quality image while masking out unwanted returns.
However, if that mask is not removed once convection starts
to develop, it will not only  remove the clutter, but it does a great
job removing any real precipitation in the areas where the mask was
applied.
In this case, the mask was finally
removed before the next broadcast at 1815Z
(1415 EDT). As you can see from the image below, the
composite radar data magically appears out of nowhere (click here to view a complete
image) to show a very healthy thunderstorm in progress over 10 miles
wide.
How long was this data missing?
Looking back at the NWS NEXRAD data, this cell first appeared
at 1743Z or nearly 30 minutes prior to the broadcast shown to the left.
Could echo tops have also provided some
clues that a thunderstorm was in the making? Unfortunately
not. Echo tops are also generated by the NWS WSR-88D
radars. As a result, they were conveniently filtered as well.
While these tools provide us with much needed guidance in the
cockpit, they also cannot be used solely as primary guidance.
If your subscription includes lightning data, make sure it is
always displayed. Also try to remain in visual meteorological
conditions when traversing through a convective environment.
This allows you to integrate what you see outside of the
cockpit with the technology inside the cockpit.
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