The Sydney Basin's first 40C of the season will occur this week and the city could experience its warmest spell of December days in 17 years. A stagnant weather pattern gripping NSW is about to produce a prolonged spell of warm days and nights in Sydney. While eastern suburbs will be moderated by sea breezes, parts of western Sydney are forecast to reach 38-41 degrees on three out of the next five days. The warmest day of the week across Sydney is likely to be Thursday, when a number of western suburbs should exceed 40 for the first time this season. Maximum temperatures in the city are forecast to reach the high twenties to mid thirties for the next seven days and this run of warmth could be extend into the middle of next week. Such a prolonged spell of days this warm is unusual in Sydney at this time of year. The last time the city registered 10 consecutive days over 26 degrees during December was the same year it hosted the Olympic Games. Nights will also be warm, with the mercury struggling to drop below 20 degrees across most of Sydney during the next week.