Weather by Malcolm RileySoutherly Busters

Southerly Busters





Afloat reader Mr Jon Neeves has asked the question: “What are Southerly Busters, how do they form? And can there be some idea given of the timing of the front?”
Southerly Busters are generally thought to be uniquely southern NSW phenomena. This is not the case. They occur on the east coast of Tasmania, in New Zealand, and on the coast of Argentina. They are known as ‘backdoor fronts’ in North America, and ‘Spanish plumes’ in Europe.
The phenomenon of a southerly buster usually occurs in spring and summer. Three basic ‘ingredients’ are thought to be needed to generate a southerly buster type phenomenon.
    1.   A mountain range running near an eastern coastline.
    2.   An existing cold front that is moving perpendicular to the mountain range.
    3.   Heating or warm air moving in ahead of the front.
The part of the cold front that is over the mountain range will slow down due to friction caused by the ranges. The part of the cold front over the sea is not affected by this friction and continues to move up the coast. This distorts the cold front into an ‘S’ shape instead of a straight line.
The areas ahead of the cold front are warmer with lower pressure and the areas behind the front are cooler with higher pressure. This heat and pressure differential enhances the strength of the cold front, and acts similarly to that of a sea breeze formation (see Afloat Jan’10).
The temperature enhancement of the cold front is greatest if the front moves up the coast during the afternoon near the time of maximum heating. On the far south coast of NSW the enhancement of the front due to the temperature difference effect is not great. This effect reaches a maximum intensity between Nowra and Newcastle and rarely has much effect north of Port Macquarie.
Southerly busters like any other front can generate thunderstorms that can in themselves, bring hazardous weather to the mariner; strong wind gusts, hail, lightning, heavy rainfall and possible waterspouts.
There are probably 20 to 30 southerly buster type fronts along the coast of NSW each year. However, only a few are strong enough and arrive during the afternoon (maximum heating) so as to become the classic southerly change with strong and gusty winds and a dramatic drop in temperature (up to 15°C).
The graph at right shows the wind changes with a southerly buster in 2001 at Sydney Airport. The wind change went from to north-easterly at 10 knots to a southerly at 30 knots with gusts above 40 knots within a few minutes.
The Bureau of Meteorology can and do predict the arrival of changes:
COASTAL SOUTHERLY CHANGE EXPECTED SYDNEY AROUND 08Z AND WILLIAMTOWN 11Z THEN WEAKENING NEAR TAREE/SCONE AFTER 14Z.
And
METARAWS YSSY 0530Z 02015G19KT CAVOK 33.4/20.1 Q1003.7 RMK TTF:FM0730 18028G42KT 9999 SCT035.
These are from forecasts that are provided for the aviation industry for the change shown on the graph. These aviation forecasts are written in a code that pilots train to learn. Due to the vast majority of aviation lodging flight plans, the forecasts and any changes can be communicated to individual pilots quickly.
In a nutshell the forecasts that start METARAWS is saying: the wind at 0530 UTC at Sydney airport, northeasterly 15 knots gusting 19 knots will change to a southerly at 28 knots gusting 42 at 0730 UTC.
Exact timings of wind changes such as sea breezes, troughs and fronts are not given in coastal waters forecasts for several reasons.
Timings of events are difficult and often several adjustments/amendments to the forecast may be necessary as the weather situation unfolds– this certainly can occur in aviation forecasts.
Photographer Carlo Borlenghi captures an image of a classic southerly buster which turned the Hobart race into a mid-race demolition derby.Coastal waters forecast areas are large (60nm out to sea) a change near the coast may take many hours to get to a ‘spot’ out to sea; timings would have to include latitudes and longitudes which makes for a complex forecast.
However, timings can be estimated by mariners from the graphical products provided at http://www.bom.gov.au/jsp/marine/wind/index.jsp and watch out for new graphical products for NSW during next year.
For those who look at the video links from these articles I have two this week of the same storm event, a southerly buster in Wellington (NZ) last year. The first shows a spectacular maelstrom of cloud ahead of the squalls and strong gusts http://www.youtube.com/watch?v=vc5hr8NzGXs.
This is the same event with a charity yacht race going on. You can just see the fleet off the point, gusts were recorded at 78 knots http://www.youtube.com/watch?v=oyVy0AfjbWw.
Southerly Buster chartIf the sky turns dark in one sector, it does not matter if it is from a front or a thunderstorm, whether it is forecast or not you may not have much time put your storm plan into action.
“And I would recommend to them to keep the halyards clear for running, and to remember that any fool can carry on but a wise man knows how to shorten sail in time …”
– Joseph Conrad, 2 June 1923. Letter to owner and crew of Tusitalia.

*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of Qld and is a Master V. He gives education courses on Marine Meteorology.

BoM in Hobart for Show

The Bureau of Meteorology will have a stand at this year’s Australian Wooden Boat Festival in Hobart from 11-14 February.
They will be located in Princess Wharf number 1 (The Taste of Tasmania Shed). Mal and his team will be available to answer your nautical or general weather questions.