The Bureau of Meteorology says rain is about to arrive in the weather districts of the North West Slopes and Plains and Northern Tablelands.
Forecaster, Olenka Rudewych, says a cold front is moving across the region on Friday and Saturday.
Rain will develop over the North West Slopes and Plains early Friday, with rain extending over the Tablelands by Friday afternoon.
She says reasonably good rain is expected.
"There'll be widespread falls of around 10mm to 20mm over the North West Slopes and Plains, with locally heavier falls around the 30mm to 40mm mark in thunderstorms, in particular," she said.
"The Northern Tablelands should see around 5mm to 10mm for Friday, but mainly in the Western parts of the district."
Ms Rudewych says the system will be short-lived.
"The cold front will move through fairly quickly, bringing a sudden drop in temperatures," she said.
"But the weather will start to clear on Sunday."
© ABC 2013
01:24 EST Parts of Sydney and the Blue Mountains were blanketed by large hailstones after a severe thunderstorm moved across the area this afternoon, causing five warehouses to collapse.