A broad trough has combined with a surface low and tropical moisture across southwestern parts of Western Australia, bringing widespread thunderstorms and heavy rainfall to the region overnight. The heavy rain is very unusual for this time of year, with some locations receiving record daily totals for December.
The heaviest falls were on the coast between Bunbury and Perth, with widespread totals of 30-40mm. These rain amounts are far more typical of winter than summer, with some stations recording more than four times their December average.
Mandurah to the south of Perth recorded 74mm to 9am this morning, making it the town's heaviest December rainfall in over 100 years of records. This also eclipsed the previous record of 69mm set just last year, and was the town's heaviest fall for any month since 2005.
Bunbury recorded 43mm, its highest December total in 11 years. Harvey, halfway between Bunbury and Mandurah recorded the state's highest total, with 109mm. The state's capital managed to escape much of the deluge, with only 9mm recorded in the city. However, this is only just shy of the December average of 10mm.
The thunderstorms were also accompanied by intense lightning, with around 100,000 strikes recorded across WA across the last 24 hours, with at least half of these in the south if the state.
The unseasonal weather is set to continue for at least a couple of days, with flash flooding and damaging winds remaining a threat until around Thursday evening. Perth and surrounds should pick up at least another 10-15mm, with potential for much greater falls under slow-moving thunderstorms. Further east around the Goldfields, large hail is also a risk.
The region will return to more typical summer weather by the weekend, with mostly sunny skies and temperatures in the high 20's.
© Weatherzone 2012
17:34 EST The weather bureau's long-term forecast is predicting a drier than normal October to December for much of eastern Australia and north-west Western Australia.