Adelaide may see the first heatwave of the season next week when a scorching air mass leaves Perth and marches east. Broadly speaking, weather systems are driven from west to east across southern Australia by the earth's rotation. So when Perth is about to swelter through the hottest five-day spell this early in the season in over 70 years, those living in Adelaide should take note. Perth's heatwave will run from about Thursday to next Tuesday. A high pressure system centred over the Bight is responsible for holding a hot air mass over the nation's west coast during this period. However, as the high moves east and into the Tasman Sea from Monday, the heat will spill across the border into South Australia. Adelaide may be in line for five consecutive days over 30 degrees from next Monday, potentially the longest run of December heat in three years. The hottest days are likely to be mid-week, when the mercury should climb to the mid-to-high thirties, about 10 degrees above the December average. Those without a pool should start planning their escapes from the heat. Once it reaches Adelaide early next week, respite may not arrive until next Friday or Saturday.