AGL Energy Ltd (ASX:AGK) has sliced its earnings forecast for the 2011 financial year as a result of recent extreme weather events in Queensland, New South Wales, Victoria and South Australia.

The estimated cumulative impact to underlying NPAT for the financial year ending 30 June 2011 is $30 million to $35 million.

AGL said recent weather events which have affected the national electricity market include co-incident very high temperatures in South Australia, Victoria and New South Wales; a record heat wave in Sydney; Cyclone Yasi; and major flooding in Queensland.

These events have resulted in Several days of record electricity demand in New South Wales and the highest recorded electricity demand in South Australia, coupled with extended periods of high and volatile electricity prices; the shut down, ahead of Cyclone Yasi, of the Yabulu power station (242MW) over which AGL has dispatch rights; and disruption of gas supplies and transmission constraints to the Oakey power station (282MW) over which AGL has dispatch rights.

These events exposed AGL to increased customer demand for electricity during a period of higher underlying pool prices of up to $12,500 per MW hour, compared with recent wholesale electricity pool prices of below $40 per MW hour.

AGL has previously advised that underlying NPAT for FY2011 would be in the range of $450 million to $480 million. In light of the above events, and a relatively flat preliminary first half result, 2011 underlying NPAT earnings guidance has been revised to between $415 million and $440 million.