Thunderstorms are developing in Western Australia and are about to spread across the state and become heavy, leading to soaking falls but also potential damage. A low pressure trough has been gaining heat and moisture for the past week and is now deepening and broadening, causing showers and storms to become increasingly severe and more widespread. This trough will be at its most effective this weekend before it weakens and moves east on Monday. During the next few days some storms have potential to produce damaging winds in excess of 100km/h, large hail and heavy downpours, leading to flash flooding. Western and southern parts of the state are most prone to wind and hail damage while southern parts are most prone to flash flooding. The slow-moving nature of the low pressure trough gives the opportunity for some places to be hit by more than one storm. On the positive side, some of the rain will fall independent of storms and will be very welcome, particularly in the far south of the state. All up rainfall is likely to add up to 50-to-100 millimetres in the Goldfields, Southeast Coastal and Southern Coastal. Some places may see their biggest rain in two-to-five years with more than a month's worth of rain falling in a day or two. So far this year Esperance is running more than 100mm below average, Salmon Gums more than 50mm below average and could do with a good soaking in order to boost grain crops. The coming rainfall should be lighter further west, with less than 20mm likely along the west coast, including Perth.