Bureau of Meteorology meteorologist David Wilkie is warning locals to batten down the hatches and be prepared as widespread rain and thunderstorms are expected to form across the state Thursday.
"Thursday is expected to be the day of highest impact with rain and thunderstorms expected across large parts of NSW and the ACT, with severe thunderstorms possible in multiple areas," Mr Wilkie said.
Mr Wilkie believes the thunderstorms will begin to develop in the west of NSW from Wednesday afternoon and into Thursday morning.
"Thunderstorms will begin to form across the west of the state from this afternoon and continue into Thursday morning, before extending to more eastern parts of the region through the day and into Thursday night," he said.
"Severe thunderstorms are possible during Wednesday, but are more likely on Thursday, with conditions across many areas more favourable for their development."
Mr Wilkie wanted to stress that though there is a large area that will be in the warning issue area, storms are usually intense even if they do only form in a smaller area.
He has also said that areas in the South West Slopes, Riverina and Central West should be prepared for both storms and/or rain on Thursday.
"However, it is important to understand thunderstorms are often small area events, which means some communities may experience quite intense storm activity while locations nearby are not impacted," Mr Wilkie said.
"Hazards may include damaging winds, heavy rain, and large hail.
"Flash flooding is also possible in some locations.
"Rainfall from thunderstorms can vary significantly over short distances, though moderate to locally heavy totals are expected about southern parts of the western slopes, including the Central West, South West Slopes, eastern Riverina and Alpine."
Though the storms are expected to ease after Thursday Mr Wilkie is encouraging locals to keep an eye on their rain radar to make sure they are not taken by surprise if there is another event.
"While conditions are expected to begin easing after tomorrow, there is still the possibility of thunderstorms on Friday," Mr Wilkie said.
"Given the dynamic nature of the situation people should watch for changes to warnings and updates, and monitor their local radars."