"Jonathan Goodish" wrote:

In particular, since the NEXRAD provided is base reflectivity, ...

Not exactly true. Here's what WxWorx has to say about it (with minor
paraphrasing):
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WxWorx develops their own version of a composite radar picture using the 124
nm base reflectivity from each radar scan (revolution). They arbitrarily
undertake compositing at a five minute interval, compositing the latest scan
and going back in time so that all previous elevations are included (if
there are 14 "cuts" conducted by the radar and it is currently on "cut 4",
we include the latest cut, 4, then all others). The composited image from
each radar is then mosaic-ed for the whole U.S. image."

Each elevation scan or revolution produces a product called base
reflectivity. On a 5 minute schedule, WxWorx uses the most recent scan and
the previous 13 revolutions to build the composite reflectivity image. The
timing is arbitrary which means that the most recent scan may be at some
middle elevation. So when the timer goes off, they just dive in and grab the
last scan that completed and the previous 13. Essentially they have
"considered" all elevations by using this method.

The color of the pixel that gets displayed on your screen depends on many
factors. They look at each elevation over the last 13 revolutions (or 5
minutes) and choose the highest reflectivity value. This could have been
from the latest scan or from the oldest scan. You just don't know. They do
this for the entire 124 nm radius. There are other factors that I have not
mentioned that may change what you see (such as clutter filtering.

This is different than the composite reflectivity seen from the NWS SSR-88D
display. WxWorx does not try to align itself with the Scan 1 to Scan N
(called a volume coverage pattern) composite reflectivity image. Obviously
this makes good sense since they are trying to be schedule driven, not
product driven. In most cases, they will produce a product that is taken
from two volume coverage patterns. But who cares? Each elevation is
considered and they can provide the most current product.
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They refer to each scan as "base reflectivity", no matter what elevation the
radar is tilted to. The point is that the colors you see in your airplane
represent the highest level of all the different elevations scanned for that
area.

-Mike