A beautiful day quickly turned sour for western parts of Sydney yesterday afternoon. Sydney-siders were lucky enough to enjoy a warm spring day yesterday with maximum temperatures reaching 28 degrees in the city and 32 degrees in the west. However, the blue skies promptly turned a much darker shade of blue as a low pressure trough triggered some slow moving thunderstorms. These thunderstorms sparked up over the Blue Mountains just after lunch delivering 25 millimetres to Medlow Bath and 21 millimetres to Blackheath before edging into western parts of the Sydney Basin. Northwesterly winds rapidly strengthened with gusts up to 69 km/h at Penrith as the change moved through. Soon after, the temperature fell 14.8 degrees in the space of two hours. In fact, between 3:30pm and 3:58pm, the temperature fell 7.5 degrees. Showers were also heavy at times in Penrith, with 12 millimetres falling in just 14 minutes. Despite easing slightly, showers continued until around 8pm with 16 millimetres collecting in the bottom of the rain gauge. Showers and thunderstorms are a significant risk of developing each afternoon throughout the weekend, especially about western suburbs, as a slow moving low pressure trough continues to amble across the state.