Home' Afloat : AFLOAT March 2015 Contents Take monthly with water March 2015 47
by Malcolm Riley*
Thunderstorms affect the areas under
and adjacent to the storm track with
lightning, rain, hail, squally winds
and occasionally tornadoes. Most lighting
strikes are within about 16 kilometres of
the storm. However, in extreme cases
lightning has been recorded 80 kilometres
from a storm centre.
Cold air downdrafts storms reach the
ground and then spread out as squally and
gusty winds, often in different directions to
the prevailing surface wind. These winds
often extend from the thunderstorm by
about 10 kilometres – sometimes more.
I received a query the other day asking
“ what was the strange formation that
appeared on the Yarrawonga radar in
central Victoria? ”
The sender also passed on this
Tocumwal Airport 22:50
The sky is completely clear. No
thunderstorms/lightning could be seen in
the NE. Suddenly the wind raises to more
than 40km/h with peaks higher. After about
45– 60 min the event is over.
I then went in weather CSI mode to
see what was happening. Looking at the
archive loop of the Yarrawonga radar there
was indeed a strange bow-shaped echo.
com /radar-loop /IDR493-yarrawon
The image below from the Yarrawonga
radar was taken at the same time the report
from Tocumwal, as the band was passing
through that town.
There was no rain withthis phenomenon
and the echoes on the radar were caused by
raised dust. The echo was moving fairly fast
at about 75 km/h. Reversing the speed and
direction of the strange echo’s movement
led straight to a severe storm just north
of Canberra, 350 kilometres away, nearly
five hours beforehand.
For a definitive explanation of what
happened a full study would need to be
conducted. However, it is likely that a burst
of strong winds from a storm started off
a wave or gust front. The air came out of
the storm as strong downdrafts and just
This wave in some ways was similar to
the ‘morning glory’ clouds that can appear
in north Queensland. As the wave moved
away from the storm it traversed some
of the drier areas of New South Wales,
raising dust that was then carried along
by the wave. It was this dust that was seen
by the radar.
The wave kept moving as far as it did
due to light winds over the area, and
because it was moving downhill. The
area where the storms generated the
initial wave was around 600m above sea
level, compared to around 100m in the
Yarrawonga area. As it was in the evening,
the lower levels of the atmosphere were no
longer being mixed by the heat of the sun.
Bureau of Meteorology radars point
upwards from horizontal about one
degree. This angle, combined with the
curvature of the earth, means the further
out from its source the radar beam goes
the higher in the atmosphere it is sampling.
In the image below the raised dust
caused by the winds was relatively shallow
in height. When the dust first moved in
from the far right, it was under the beam
and not seen by the radar. As it moved
closer to the radar it was just seen as a
weak echo. By the time it was close to the
radar, maximum reflection from the dust
showed as a stronger image.
As the dust moved away from the
radar this process was reversed. The
wave became visible on the Yarrawonga
radar about 75km to the northwest. The
feature disappeared from the radar as it
moved about 75km to the southwest of
Yarrawonga, but was probably still going.
At this stage it had travelled over 400km.
While all this is very interesting, what
does this mean for the mariner, especially
There is usually less friction over the
water than the land (which has hills and
trees) to slow down the initial gust front
from a storm. The gust front can travel a
long way from the storm, bringing wind at
speeds and directions that are not forecast
and probably un-forecastable.
If you are seeing lightning on or
even over the horizon there is the
possibility that one of these events may
be experienced. The heating of the day
mixes the atmosphere in the lower levels
and this mixing would reduce the strength
and the distance travelled by these gust
fronts. They are more likely to travel further
at night. h
Severe storm just north of Canberra.
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