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Storm Cell Identification and
Tracking (SCIT) Markers 
Satellite-based weather is one product that the
average corporate jet jocky or airline captain has little or no
experience using. However, approach most general aviation aircraft
owners and they will rave about how it has allowed them to fly with
much greater confidence. Crack open your AIM, Instrument
Flying Handbook or Pilot's Handbook of Aeronautical Knowledge and you
won't find any guidance on how to safely use satellite-based
weather. Nevertheless, this particular technology has some important
limitations - and you won't find these limitations in the documention
that comes with your satellite-based weather system.
There are
two providers of satellite-based weather at the moment, namely, WSI and
WxWorx. XM-based satellite weather is a product of WxWorx
headquartered in Huntsville Ala. Wxworx processes the data
from the NWS WSR-88D NEXRAD Doppler radars and broadcasts it to your
receiver. Before the broadcast, they clean the image up quite
a bit to remove non-precipitation returns and weave it into a mosaic
product using the highest reflectivity from all elevation angles.
This radar product is refreshed every five minutes.
Every minute and 15 seconds, they evaluate the NEXRAD
reflectivity and velocity data (Doppler) to determine if any cells
exhibit severe characteristics such as shear, heavy rain rate and hail.
These cells are identified with a Storm Cell
Identification and Tracking (SCIT) marker. How it is produced
is a well-kept secret, although there are dozens of similar algorithms
publically available.
The image shown above from the WxWorx
on Wings display is an example when SCIT markers can be incorrect or
misleading. The SCIT marker is the magenta square containing
a directional arrow. This marks the location of the cell
exhibiting severe characteristics and provides the direction of
movement of this cell. As you can see, two of the three SCIT
markers contain some agreement with respect to the direction of
movement, but the southern-most SCIT marker is pointing in nearly the
opposite direction.
This was a very isolated and slow
moving cell that had just developed. Often with developing
pulse thunderstorms or pulse thunderstorms with little or no movement,
the SCIT markers can show a very chaotic direction of movement.
Essentially what the WxWorx algorithm thinks as movement is
actually a new area of development while another area in the cell may
be dissipating. SCIT markers provide better guidance when the
cell is mature and has a well-defined speed and direction of movement.
There wasn't any movement for this
particular cell. It drifted just a bit north before
dissipating. The SCIT markers continued to update every
minute and 15 seconds giving a different answer with each update.
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