The 1950s saw enormous changes in Australia. The Menzies Government had just been returned to power, the nation was still recovering from the austerities of a wartime economy, and war would be declared on the Korean Peninsula on the 26^th of June 1950.

These were times of change in the weather, too. January 1950 in eastern Australia marked the beginning of what appeared to be a significant change in climate patterns especially rainfall. The early 1940s had been marked by severe drought. The 1950s in general were marked by a number of heavy rain events, notably February 1954 and 1956. However, probably no events in our weather history have approached the duration and extent of those that occurred at the beginning of the winter of 1950.

Rainfall that year had started promisingly; substantial rains fell across much of Queensland and central and northern inland NSW during the first three months, with a good start also to the monsoon season in the north. The monthly Southern Oscillation Index during 1950 remained positive for the entire year and reached its highest value, 22.6, in June.

May 1950 had given some indication of what was to follow, with two low-pressure systems bringing heavy rains to the southern half of the coast and the adjacent Gippsland area of Victoria. But no-one could have predicted that by the end of June, a further 5 cyclonic storms would affect the NSW coast and nearby ranges with storm force winds, extensive flooding, thunderstorms with hail and squalls, shipping losses and even a tornado on the mid north coast.

The southeast coast of Australia and adjacent oceans are favoured breeding-grounds for what are known as 'east coast lows', similar to the 'northeaster' of the eastern United States. They occur at any time of the year, but June is particularly prone to their development. They usually form when the subtropical jetstream develops a dip to the south. Cold air aloft becomes cut off from the main upper flow while high pressure often moves over Tasmania and intensifies. As low pressure develops to the north and large quantities of moisture are drawn in from the sea, there often follows a self-sustaining mechanism for rain and gales that may last several days.

On this occasion, the result was a spectacular demonstration of just about every possible synoptic situation of which this type of storm could be a major feature.

The breakdown in the normal westerly flow that had commenced towards the end of May became more pronounced as a cold front moved across Tasmania on the 1^st. This was followed by a high-pressure system moving southeast and a cut-off trough forming in the easterlies to its north on the 3^rd. A weakening high over the NSW central coast collapsed as a front moved northeast into the Tasman Sea, and by the 6^th a depression had formed near Lord Howe Island with further troughs along the central and northern ranges and slopes and over central Queensland (Fig.1).

On the 7^th, a double-eyed complex depression formed over the NSW/Qld border. The effect of this was to draw in very moist air from the Coral Sea. By this time a closed upper air low had moved into NSW with the 500mb level down to 5400 metres. This very cold air interacted with the surface moisture to bring widespread heavy rain from the NSW mid north coast to the Illawarra (Fig 2.)

On 10^th, a secondary upper trough triggered the formation of a small low just near Sydney by the 11^th, bringing further heavy rain to southern and eastern parts of Sydney and the adjacent Illawarra (Fig.3).

This had barely cleared when yet another trough formed over inland Queensland and the northeast inland of NSW. With the previous depression still over the central Tasman Sea, a moist southeast to northeast flow was maintained into this new system.

On 14^th, a low formed over inland Queensland and moved rapidly southeast to be centred near Sydney on the 15^th. This system developed twin centres over the Hunter district with a third centre off the south coast (Fig.4).

By the 16^th, a complex depression with 3 separate centres was located off the Hunter and mid-north coasts, stretching to Lord Howe Island. The 3 centres merged into one on 17^th before moving northeast by the next day. At this stage, high pressure continued in an almost stationary position near Tasmania and another cyclonic depression formed to the northwest of the other system, moving southwest to be located off the Queensland/NSW border on 19^th (Fig.5).

This system oscillated back and forth for the next 2 days before finally retreating into the northern Tasman Sea ahead of another trough and embedded low-pressure system over central Queensland.

On 22^nd June a depression moved across northern NSW to be centred off the north coast on 25^th. At this point it attained a central pressure of 996mb (Fig.6) before moving fairly rapidly southeast into the Tasman Sea.

As it did so, it deepened further to 986mb with a huge circulation covering most of the Tasman (Fig.7).

By now, a more normal winter pattern was beginning to assert itself over southeastern Australia. With a succession of cold fronts and the low becoming slow moving in the far southern Tasman, a typical cold southwesterly flow was established with very windy conditions for a time over the southern half of the NSW coast and ranges (Fig.8).

. What was obvious for the duration of this event, was the substantial split in the polar jet stream south of the continent which resulted in a series of cold pools of air becoming cut off over our latitudes from the main upper flow further south (Fig.10)

Fig.10. The 500 mb constant pressure chart for 9am, 25.6.1950 (Courtesy NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado USA)

Probably the most noticeable feature on first looking at the rainfall data for these events, was the persistence of the rainfall rather than individual daily amounts, though there were numerous and widespread high daily totals just the same. For many northern and central coastal places, rain fell without stopping for up to 36 hours at one stretch. Of 536 rainfall reporting stations stretching from the Illawarra coast and eastern ranges in NSW to the Gold Coast and adjacent ranges in Queensland, 94% of them had over 100mm or more for the month, 54% had 250mm or more, and 28% had over 500mm.

Figs.10 and 11 show the rainfall and departures from normal over Australia for June 1950.

From 7^th to 9^th of June, 74% of observing stations down to the Illawarra received over 75mm, while 50% of them received over 100mm and 8% received over 150mm. Most of Sydney received 100-150mm with locally higher totals, notably Killara (170) and Pymble (154). Gosford on the central coast recorded 212mm in this time.

From 10^th to 13^th falls were lighter than in the previous event with a few totals exceeding 100mm. Rainfall totals from 14^th to 17^th were much heavier especially in the Hunter, Sydney and Blue Mountains areas. Raymond Terrace (Hunter) received 337.7mm during this time, most of it in 72 hours. 30% of NSW stations received over 100mm.

From 14^th to 17^th rainfalls totalled over 100mm at 40% of stations, most of these being in the Hunter and Sydney areas, with many totals from 150 to 250mm and more in the 3 days to 17^th, notably Concord Golf Club (337), Gosford (285.2) and Pymble and Turramurra (293).

From 18th to 21^st falls were lighter, but nearly half the stations received a further 50mm with widespread heavy falls of 100 to nearly 300mm over the mid north coast of NSW.

From 22^nd to 29^th, after which the weather had basically cleared, 61% of stations received a further 150mm or more. Heaviest falls were on the NSW mid north coast again with Comboyne receiving 600mm in 5 days, on top of the 424mm it had already had for the month up to 21^st.

The following table shows the 20 highest monthly station rainfall totals, all of which occurred in NSW:

The following table shows the top 15 monthly totals in southeast Queensland:

It's worth mentioning that Springbrook Forestry Camp fared relatively poorly for this month compared to June rain events that have occurred there in other years. In 1967 it received 1631.2mm, of which 621mm fell in 24 hours.

Extensive flooding was an inevitable outcome of these weather patterns, especially from mid-month. Soils, saturated from heavy rains in the earlier storms, were unable to sustain further falls. This was particularly so in the Hunter Valley. The floods wreaked havoc everywhere up and down the north and central coasts with substantial crop, livestock and property losses that in today's terms would probably exceed 30 million dollars.

In Sydney, widespread thunderstorms with torrential rain, hail and squalls accompanied the passage of a complex low-pressure system on the 7^th with falls of 60 to 95mm over many northern and eastern suburbs. Two upper troughs on 8^th and 10^th brought further widespread thunderstorms and hail to southern and eastern parts of Sydney and the nearby Illawarra coast.

Severe storm force winds drove at least 1 ship aground on the mid north coast and there were reports of a tornado in that district. Storm warnings were issued frequently for shipping in the Tasman Sea with wind gusts close to the centres of the depressions exceeding 70 knots. In Sydney, from the 16^th to the 29^th, maximum wind gusts exceeded gale force strength (63km/hr) on 9 days. The highest gust was 78km/hr from the east at 8:05pm on 23^rd.

What is even more remarkable is the total wind run for the month, which came to 12552 kilometres.

At the end of the month, with the breakdown of the rain-bearing systems and a return to a normal winter pattern, boisterous gales over southeastern Australia caused further property damage, as the last of the coastal depressions moved into the southern Tasman Sea and deepened before being absorbed into the westerly flow.

Events such as this have become less frequent over the last 30 years or so. There were a number of similar events through the 1950s to 1970s, notably June 1967 which was similar in terms of the frequency of coastal depressions. Why they occur less often now, I don't think anyone can say with certainty.

This event delivered probably the most sustained and concentrated rainfalls to this part of Australia on record. Now that 55 years have passed, are we in for a repeat in our time?

I am most grateful to Mike De Salis of the Sydney Bureau of Meteorology who retrieved the double A4-sized surface weather maps for the month from the Bureau's archives, all hand-drawn (some in pencil) with station plots, and patiently scanned and saved them in graphic format. He also provided extensive rainfall data as well as a summary of Sydney's day-by-day weather for the month.