Residents in southwestern parts of Western Australia could be forgiven for thinking summer hasn't started yet, after an unusually cool start to the season. Conditions this December have been more typical of November, with both days and nights running about 2 degrees below the long-term average. Daytime temperatures in Perth are averaging just shy of 27 degrees so far this December, around two degrees below the long term average of 29 degrees. Geraldton is averaging around 27 degrees during the day, when typically it is about 28 or 29 degrees. Most notably, there has been a lack of days exceeding 30 degrees over the region. Perth and Geraldton have only cracked the 30 degree mark twice, halfway through the month. The cause of the cooler than average days has been frequent cold fronts, preventing heat from pooling over WA's southwest. However, most computer models predict the second half of December to be significantly warmer than the first half. From Thursday, a run of hot days are on the cards as a low pressure troughs drags heat down from the interior and stalls. Many parts of the state's southwest, including Perth, Geraldton and Northam will experience a run of 35 degree heat. Friday and Saturday will be the hottest of these days, reaching into the mid 30s. There are indications that the heat will even stick around late into December as the trough barely moves as cold fronts stop affecting the region. This would mean Perth will enjoy some of the warmest, sunniest weather across the nation on Christmas.