SYDNEY (Reuters) - Australia is bracing for the first major heat wave of the bush fire season this weekend with temperatures forecast to hit well above 40 degrees Celsius (104 degrees Fahrenheit), prompting authorities to urge people to stay indoors. Last summer’s bush fires, which Prime Minister Scott Morrison called Australia’s “black summer”, killed 33 people and billions of native animals. The country’s weather bureau attributed a slight weakening in the current La Nina weather phenomenon as one of the reasons for intense spring heat this year, as scant rainfall in November led to a rise in temperatures in the inland regions. “November has been quite unusual in many ways. We have only seen about half our normal rainfall and it is quite possible November will be one of our hottest Novembers on record,” said Andrew Watkins, the Bureau of Meteorology’s (BoM) Head of Climate Operations. BoM still expects a wetter than usual end to the year from La Nina, which is typically associated with greater rainfall and cooler than average temperatures in the equatorial Pacific Ocean. The heat wave sweeping in from Australia’s outback interior is forecast to move across the southeast of the country and by early next week will shift to the northeast. Most of Sydney is expected to experience sweltering temperatures above 40 degree Celsius over the weekend. Last summer’s prolonged bush fire season was fuelled by three years of drought but this year the risk is from grasslands after the prolific growth of vegetation helped by good rains in early spring. “Grass might be greener in the area where you’re right now, but it won’t take long for it to dry out once the heat of summer starts to appear,” said Richard Thornton, Chief Executive of Bushfire and Natural Hazard Cooperative Research Centre. Australia’s fire season usually runs from late southern hemisphere spring through summer. Reporting by Renju Jose; Editing by Michael Perry