14
BAMOS
Jun 2019
Figure 8: Wind speed and direction at Fawkner Beacon for (a) fluctuations in wind speed (example from 8 April 2017) and
(b) sudden drop in wind speed (example from 3 February 2005).
Wind speed decreases
For sailors, a sudden decrease in wind speed can also be
disconcerting. In the study data there were 56 events when
the speed decreased by 16 knots or more within half an hour.
As was the case with the events showing an increases in wind
speed, the events were concentrated between October and
April.
Twenty five of the events were associated with spikes or
squalls lasting 20 minutes or less and they included the nine
cases mentioned earlier where abrupt increases and decreases
occurred within adjacent or overlapping 30 minute periods.
Apart from these cases, the abrupt decreases in wind speed
fall into two main categories. Some are short term fluctuations
within a more extended period of stronger winds (such as the
case 15 October 2011 in Figure 6(b)). In some cases sudden
increases and decreases both occurred within the same day
associated with short-lived fluctuations in the wind speed.
The second category is a sudden drop‐off in wind with the
passage of a trough or front (see the case of 22 January 2006,
Figure 6a). As noted earlier, these sudden decreases can be
temporary with the stronger southerly winds appearing some
time later. The two largest decreases in wind speed illustrate
these two types as shown in Figure 8.
may also be short lulls in wind speed within an extended period
of strong winds.
There are several less familiar situations that also deserve
caution. In the warmer months, there may be a two‐stage
change in wind—the strong northerlies ahead of a front or
trough may give way to moderate, or even light, southerly
winds. Then the main burst of often squally south to south-
westerly winds may arrive two or more hours later. The "lulls"
can be pronounced and extended as can be seen in the "Black
Saturday" case. Boaties need to exercise caution. They may have
expected a change and be fooled by the initial change and
subsequent lull. Boaties should pay attention to the forecast
and not necessarily think that the main change has arrived
when the wind first changes to the south.
Another situation for caution is an abrupt increase in speed
of northerly winds with little change in wind direction. These
cases generally occur in the morning or early afternoon. An
approaching front may be well to the west and there may be
no clouds to indicate the presence of a front. Wind speeds can
increase suddenly under a blue sky and boaties may be lulled
into a false sense of security by clear blue skies and light winds.
If strong northerly winds are forecast, boaties should take heed,
as the onset of such northerly winds over the waters of northern
Port Phillip can be more abrupt than for adjacent land stations.
Summary and messages for boaties
Sudden wind speed changes can occur on Port Phillip especially
during the warmer months when boaties are more likely to be
on the water. There are many variations of the wind around the
bay and each area will have its own peculiarities. The analysis
above suggests several messages for the northern part of Port
Phillip.
Most abrupt wind speed increases are associated with: a backing
change from northerly (generally strong) to southerly, often
squally, and associated with thunderstorms. These changes
are generally well forecast and should be taken seriously. There
Acknowledgements
The data for the study were obtained from the Bureau of
Meteorology. We also made extensive use of the archive of
Port Phillip time series graphs developed by Andrew Watkins,
and accessible through the website he maintains http://www.
baywx.com.au/. We are also grateful to Ted Williams (Bureau of
Meteorology) for his insights into the June 2018 case.