Record
rain brings flooding to Perth and SW WANEWSBITS: Storms in central Victoria, NSW and SE Qld; further moderate rain in Far North Queensland
A slow-moving, complex low pressure system, the remains of Tropical Cyclone Diane, gave widespread falls of 50 to 75mm across southwestern WA today, breaking a number of April one-day rainfall records in the process. In the Perth metropolitan area, Perth Metro raingauge recorded 69.6mm for the 24 hours to 9am Wednesday, breaking the previous April record of 66.5mm set in 1904. The Perth Metro gauge has been moved several times in its 123 year history, so there is some doubt as to the validity of the record; however Perth Airport, Armadale, Swan View, Serpentine, Wanneroo and Gosnells City all set new April records in the Perth area (see details in the records section for Wednesday). Armadale's registration of 69.4mm is significant, in that its previous record in 97 years of measurement was only 51.6mm. Elsewhere, Marradong, Gordon River and Tamaru on the South Coast, and Kilburnie, Williams, Kojonolokan Hills and Cranham in the South Central set records. Williams and Marradong have records stretching back 117 and 105 years respectively, underlining the exceptional nature of this rain event.
The low that brought the rain had its genesis on 7 April when Tropical Cyclone Diane formed in the Indian Ocean some 4000km NW of Perth. The cyclone moved away to the WSW, but recurved on 10 April to the south then SE. It dropped below cyclone strength, and was captured by the westerly flow on 13 April, still moving slowly to the ESE. On Sunday 14 April, it arrived near the southwest WA coast, where it became multi-centred, with the whole complex virtually stationary until today under the stagnating influence of an upper ridge. The catalyst for today's heavy rain was the arrival above the moisture-laden surface low of significantly colder air along with a closed upper air low.
Increased instability and lines of convergence developed around the low, and a major rain area moved in from the northwest, crossing the coast near Lancelin around 7.30pm last night and spreading across the southwest during the evening to give falls from a few to around 20mm by 9 this morning as well as scattered thunderstorms and even overnight hail at Paynes Find. A second rainband with scattered embedded thunderstorms crossed the Central Coast around 8 this morning and moved only slowly southeast giving around 5 hours of rain which was at its heaviest in the Perth area between about 10am and 1pm. During these 3 hours the Airport gauge registered 50.6mm and the Perth Metro gauge 47.0mm. Details of the heavy short-duration falls are given below. This rainband moved slowly southeast, and was responsible for the bulk of the rain registered across the region in the 24 hours to 9am Wednesday. Further weak rainbands crossed the coast during the evening as the low span up offshore near Bunbury. Overnight, the low deepened slightly and moved NNE to cross the coast near Perth mid Wednesday morning with a central pressure of 1000hPa, but with precipitation now reduced to light showers. It continued to move away to the southeast, passing Albany late Wednesday afternoon.
Because the Southwest has recorded only around 10 to 30mm since the beginning of the year, the rain fell on parched soil and flooding was limited. However, roads around Kojonup, Williams, Katanning, Cranbrook and Denmark were closed briefly by local flooding, while roads around Lake Grace and Mount Barker were closed to heavy traffic until late Wednesday. A section of the Cranbrook to Frankland road was partly washed away.
NEWSBITS: Relatively unproductive but electrically active storms continued throughout the 24 hours in a belt running from central Victoria to southeastern Queensland. There were isolated moderate falls (see short duration rainfall below). The Innisfail area of Far North Queensland again saw some substantial overnight falls as onshore showers were enhanced by an upper cool trough and the orographic effect of the Bartle Frere Range. Innisfail recorded 91mm between 3pm yesterday and 9 this morning. Other heavy falls are listed below.