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ASOS Lightning Reports
Does
an Automated Surface Observation System (ASOS) have a built in
lightning detection system? How does it know to add a TS, VCTS or tell
you about lightning in the distance?
Some ASOS sites do have a single-site
lightning sensor. If there isn't a lightning sensor at the site, it is
still possible for the ASOS to report lightning. For FAA-sponsored ASOS
sites without a lightning sensor, lightning data is made available to
the ASOS through the Automated Lightning Detection and Ranging System (ALDARS) which is a ground-based
lightning detection system. ALDARS is not co-resident with the sensor
and sends the data to the ASOS. Here's how it works.
An ASOS will format a METAR or SPECI
for lightning in one of three ways: TS, VCTS (thunderstorms in the
vicinity) or lightning in the distance.
1) If the cloud-to-ground lightning strike is within five miles of the
ASOS, the ASOS will make a special (SPECI) observation and carry "TS"
(for thunderstorm) in the body of the special observation in the
present weather field. "TS" will continue to be carried in the present
weather field in subsequent METAR observations until no cloud-to-ground
lightning strikes are observed for a 15 minute period. At that time,
the ASOS will make a SPECI observation and end the thunder (removes the
TS).
2) If the cloud-to-ground lightning strike is between five miles and
ten miles of the ASOS, the ASOS will make a SPECI observation, and
carry "VCTS" (for thunderstorm in the vicinity) in the body of the
observation in the present weather field. "VCTS" will continue to be
carried in the present weather field in subsequent METAR observations
until no cloud-to-ground lightning strikes are observed for a 15 minute
period. At that time, the ASOS will take a SPECI and end the "VCTS."
3) Separate from above, or independently, if the cloud-to-ground
lightning strike is between 10 miles and 30 miles of the ASOS, the ASOS
will carry a "LTG DSNT xx" remark indicating distant lightning, with
"xx" being the direction of the lightning in octants. This will be
appended as appropriate on all SPECI and/or METAR observations.
Of course, at locations with a human observer, the
ASOS
observation can be overridden including adding remarks such as FRQ LTGICCG
OHD TS OHD MOV NE.
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