Flood watches and severe weather warnings have been issued in four states and territories as a showers and thunderstorms start to intensify over southeastern Australia. A pool of cold air surging north from the Bight is clashing into a mass of warm and humid air sitting over Australia's southeast. This interaction is fuelling a multi-day outbreak of rain and thunderstorms. During the 24 hours to 9am on Friday, the heaviest falls from this system occurred over central and eastern districts of SA, far western NSW and western Victoria. In SA, Appila's 51mm was close to double the Mid North site's long-term monthly average for December. Murray Bridge picked up 28mm, which is their heaviest rain in a year and more than a month's worth of rain. Adelaide's 13mm up to 9am today was the city's heaviest fall in three months. Showers and thunderstorms have also been moving through western and southern NSW since Thursday afternoon. Menindee's 24mm during the 24 hours to 9am today was its heaviest rain in 15 months. Widespread falls of 20-40mm were recorded across western Victoria during the last 24 hours. Apsley's 42mm up to 9am was its heaviest rain since 2011 and more than the town's entire monthly average for December. Rain and storms will continue to affect a large swathe of southeastern Australia throughout Friday and Saturday. The heaviest falls are likely to occur over northeast Victoria and in southern and southeastern NSW. As of midday AEST today, flood watches were in place for a number of river catchments in southern NSW, Victoria and northeast Tasmania. Severe weather warnings for heavy rain and flash flooding were also in force for almost every district in Victoria and the southern inland of NSW, including the ACT. A flood watch that was in place for parts of SA this morning has been finalised, with the recent rainfall no longer expected to produce any significant rises in rivers and creeks.