After a few wet, cool and miserable days so far in January, Sydneysiders will be treated to a return of summer next week. A slow moving low pressure trough will direct an extremely hot airmass over much of the southeast in the coming days, including the Sydney Basin. The heat will start building from Sunday and on Monday the mercury will reach the mid-to-high thirties in the western suburbs and low thirties in the east. Tuesday will be a touch warmer and a stark contrast to the previous Tuesday when the temperature failed to get out of teens throughout Sydney and was the coldest January day in 37 years. Some suburbs may even be up to 20 degrees warmer than last Tuesday. Sea breezes will be a significant feature this week as well, limiting the heat along the coast but also reaching a lot of western suburbs during the afternoon. This will be most noticeable on Wednesday when a stronger sea breeze will bring a brief reprieve from the heat. Thursday is shaping up to be a scorcher. The airmass will be at it's hottest and sea breezes will really struggle against the stronger westerly winds. The western suburbs are likely to warm to the high thirties and even into the low forties. For those closer the coast the sea breeze may bring a little relief but not as much as previous days. Tops in the city are expected to reach the mid- to-high thirties. Afternoon and early evening thunderstorms are a risk each day, especially for those in the western suburbs which may bring some brief cooling. Widespread relief will come late on Thursday night and into Friday morning as a cold front sweeps through the Harbour City bringing a much cooler end to the working week and, unfortunately, weekend.