Heavy rain and thunderstorms will soak parched parts of eastern NSW this week, causing flash and riverine flooding. The impending drenching will be caused by a slow-moving low pressure trough combined with moisture-laden winds from the Tasman Sea. Persistent rain and periodic thunderstorms will affect the coast and adjacent ranges from about the Illawarra up to the Mid North, including the Hunter Valley, between Tuesday and Friday. The heaviest falls are likely to occur between the Central Coast and Lower Mid North Coast from Wednesday, near and to the south of the trough. Parts of the Central Coast and Lower Hunter, including Newcastle, could receive 100-200mm of rain in 24 hours from Wednesday, with isolated falls exceeding 200mm. While this type of weather event is not unusual in eastern NSW at time of year, it follows a prolonged period of notably dry weather in some areas. Rainfall during the nine months between July and March was 300-400mm below average in parts of the Hunter. Williamtown's 397mm during this time was only about half of its long term average and the driest July to March period in 38 years. Parts of the Hunter could see more rain during the next few days than they have so far this year. A flood watch and severe weather warning for heavy rainfall have been issued in parts of the the Mid North Coast and Hunter Districts, where flash and riverine flooding are likely from Wednesday. Due to the recent lack of rain in river catchments across the region, mainly minor river flooding is expected to occur from this event. Rainfall should ease significantly by the weekend as the low pressure trough weakens and moves offshore.