Thursday 31 MAR to Saturday 2 APR 2005
National Daily Weather Summary
Major Event
Surface pressure and the thickness of the atmosphere between the 1000 and 500hPa pressure levels Wind, temperature and humidity at the 850hPa pressure level (about 1.4km)
Vorticity at, and height of, the 500hPa pressure level (about 5.5km) Total precipitable water in the atmosphere (in mm) and instability index (CAPE)
Jetstream analysis at 200hPa (about 12km) Vertical air motion at 700hPa (about 3km)

This set of charts for 10am EST/8am WST helps make sense of the torrential rain in southern WA and the record-breaking hot and cold conditions across the country. COLA

AUS: Why temperature and rainfall records tumbled across the nation

A rare and extreme kink in the global heat transport system in late March/early April brought extreme mid-Autumn heat to most of Australia and extreme cold to the southwest. The interaction of hot and cold airmasses with remnants of tropical moisture also gave record rainfall to parts of the WA South Coast. Further details of individual weather events and records are given in the Daily Weather Summaries for 31 MAR, 1 APR and 2 APR.

Heat from the sun-drenched tropics normally mixes southwards to the poles as a succession of modest waves in the atmosphere drag warm air south and cool air north. These show up on the familiar weather map of Australia as the northerlies ahead of and southerlies behind each cold front that sweeps across the country's southern flank.

This event was rare in the strength of the hot and cold airmasses and the extreme temperature gradient set up between them. The charts at left give a snapshot of the event at 10am EST/8am WST on 1 April, when the event was at its most intense. Each chart slices through a different part of the atmosphere, giving an insight into the major thermodynamic forces behind this event. By opening the six charts in different windows and making sure they are aligned, you can click between them to make comparisons.

The 850hPa chart , about 1.4km above sea level, is a good place to start as it shows airmass properties above any local effects caused by the ground or sea. A broad tongue of very great heat for mid-Autumn is sweeping south across SA and twisting east into Bass Strait. Balancing this southward movement is an invasion of cold southern air over SW WA. Temperatures reach 26° in the Head of the Bight, but are down to 2° over SW WA. By comparison, average temperatures at this elevation at the beginning of April are around 9 to 10° along the coast between Albany and Adelaide. Note the temperature gradient of 28° between the Head of the Bight (26°) and a point south of Albany (-2°), a distance of only 1100km.

This huge temperature imbalance is both causing and caused by the strong jet stream visible in chart , with core winds over WA in excess of 150km/h. This in turn is producing uplift across southern WA, shown by the strong negative (upwards) vertical velocity at 3km in chart . The uplift reduces surface pressure, resulting in the complex low pressure area at the surface in chart .

Chart , at 500hPa or about 5.5km, shows that the maturing deep middle-atmosphere trough is cutting off from its broader parent trough to the south. An upper low is forming on the coast to the west of the surface low in response to both the cold air and the uplift. The spin imparted to this by the upper jet is shown by the twirl of deep blue, which represents vorticity -- in this case cyclonic rotation around the low --which enhances uplift.

Chart shows high relative humidity levels in SW WA as the lower temperatures there reduce the amount of moisture the air can carry. Much of the area is close to 100%, where condensation would be occurring. Chart shows precipitable water -- the actual amount of moisture available through the whole atmosphere -- expressed as the number of millimetres of rain you would theoretically get if you squeezed all the moisture out of the air above any point. A tongue of moist air entered WA from the northwest last Saturday. Its remnants are now wrapping around the low, arriving in the Albany region from the east, condensing in the colder air moving in from the southwest, and rising in the area of increasing uplift to give record rain to the Albany area.