Weather with Malcolm RileyFeatures of the Winter Weather Map

High Pressure Systems

High pressure systems (anticylones) are identified by the letter ‘H’ along with the central pressure of the high on the weather map. The highs move slowly from west to east but do like to ‘sit’ over cold areas such as a winter continent. As summer turns to winter the high pressures are more prevalent over the Australian continent.
The winter high is generally of a higher pressure than its summertime counterpart. The strength of the high is often the driver that will affect maritime weather conditions around the Australian coastline.
Wintertime highs will be generally centred around 30 degrees south. This position will bring periods of light and variable winds to this latitude, south easterly trade winds to the tropics and the westerlies (roaring forties) to Australia’s southern coastlines.
If the high is strong and centred over NSW it is likely to increase the strength of the southeasterly/easterly trade winds from Cape Yorke to Darwin with the winds being lighter over the Kimberly coasts.

Low Pressure Systems

Low pressure systems are identified by the letter ‘L’ along with the central pressure of the low on the weather map. As the maximum solar heating moves to the northern hemisphere during autumn and winter, the sea ice around Antarctica increases northward about 1,000 kilometres. This effectively increases the size of the Antarctic continent.
The deep low pressure systems around the continent tend to remain over the warmer sea. To some extent this pushes low pressure systems further north. This inturn will increase the northern ‘reach’ of cold fronts.
Low pressure system, can form off the east coast (generally south of Brisbane) at any time of the year but are more frequent during the winter months. The weather equator, the area of maximum heating on the globe that causes the air to rise and low pressure to form, has moved into the northern hemisphere. Tropical lows and cyclones would not be generated in the Australian tropics.

Fronts

Fronts are borders between warm and cold air masses.

Cold Fronts

Cold fronts are surges of cooler, denser air from higher latitudes that push in under warm air lifting it, releasing energy and generating rainfall. They are shown on the chart as triangles spaced along a line.
With Southern Ocean lows further north during winter the frequency and strength of cold fronts through southern coastal areas is increased. Winter cold fronts are frequent – sometimes a cold front will pass through an area on a daily basis.
Winter cold frontal influence is usually south of 30 degrees south but given the right conditions can extend briefly to latitudes around Brisbane.
Fronts and/or their troughs that do reach up into lower latitudes will cause a relaxing of the trade wind system across northern Australia and the wind speeds will reduce in tropical areas.

Warm Fronts

Warm fronts are surges of warm air that ‘ride over’ cooler denser air releasing energy and generating widespread rainfall. Warm fronts are infrequent and usually confined to the southern most parts of Australia and are marked on the chart with semicircle bumps spaced along a line.

Troughs

Troughs are identified by a dashed line and indicate areas of lower pressure. They are not as frequent over northern Australia as during summer when they are semi-permanent. Troughs along the east and west coasts are often the extensions of stronger fronts to the south.

General

Sea breezes can occur in winter in southern Australia but are usually weak and short lived. In east coast tropical areas the sea breeze is likely to be a slight enhancement of the south east trades, perhaps changing the direction briefly to more easterly.
Features of the Winter Weather MapAlong the north coasts of tropical Australia sea breezes only form when the high to the south is weak and a trough has extended into the area. In the Kimberlies winter sea breezes can be stronger and more frequent than other tropical coastal areas.

Forecasts

The chart shown is for 10pm on the 17 of August 2009, below are the forecasts for the following morning for some selected areas around the country.
Tasmania’s southwest (in the westerlies):
Northwest winds 20 to 30 knots. Seas 2 to 4 metres.
West to southwest swell 3.5 to 4.5 metres.
Off Sydney:
Wind: SW 10/15 knots. Sea: 1 to 1.5 metres.
Swell: S 1.5 to 2 metres.
Off Fraser Island (in the vicinity of the trough):
SW/NW winds 15/20 knots, ahead of a S/SE change 25/30 knots early morning.
Seas to 1.6 metres, increasing to 2.5 metres.
A 1.5 to 2 metre S’ly swell developing.
North Queensland Cooktown to Thursday Island:
SE winds 20/25 knots.
Seas to 2.2 metres outside the reef, on a 1.5 to 2 metre SE swell. Seas to 1.4 metres inside the reef.
Off Arnhem Land:
East/southeast winds 15/25 knots. Late afternoon inshore seabreezes 15 knots. Seas 2 to 2.5m.
Off Broome:
NW/SW winds 5/10 knots increasing to 10/15 knots in the afternoon. Seas to 1.0m. Swell to 0.5m. (the afternoon increase is sea breeze).
South of Cape Leeuwin (ahead of the cold front):
W/NW winds 20/30 knots, reaching 30/40 knots for a period early morning. Seas 2.0 to 3.0m. Swell rising to 4.0m.
South of Port Lincoln:
Wind: NW/N 13/18 knots, increasing to 18/23 knots west and south of Kangaroo Island by midday. Sea: 1.5 to 2m. Swell: W/SW 2.5 to 3.5m, decreasing to 1.5 to 2.5m by evening.

*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.