Forecasters for the Bureau of Meteorology are tracking a line of storms moving toward parts of the Upper- and Lower Hunter as moisture from the tropics meets a low pressure system moving across NSW, expected to bring heavy rain to the region on Wednesday and into the coming days.

Though less than one millimetre of rain fell over parts of the Lower Hunter on Wednesday morning, forecasters say a second low pressure trough moving up from South Australia, expected to reach the Hunter tomorrow (Thursday), could bring heavy rain and damaging winds to the region as the wet weather ramps up again toward the end of the week.

There was a very high chance of showers and possible thunderstorms, particularly across the Upper Hunter Wednesday afternoon forecasters have said, as storm fronts move from Mudgee across much of the Central Coast and into the Hunter.

Meanwhile, senior meteorologist Jane Golding said some areas of the state could receive a month's worth of rain before the end of the week.

"We're seeing rainfall records broken that we haven't seen in 10 years ... over the course of three days many locations will exceed what they'd usually see in November," Ms Golding said on Wednesday.

"The other feature is that it's being followed by some really cold air, some really gusty winds and some really unseasonable snow to some low levels in the south."

The Bureau has warned moderate to major floods are likely around the Northern Slopes, with the Upper Macintyre, Gwydir, Peel, Namoi, Macquarie, Bell, Lachlan and Belubula rivers likely to flood.

SES Assistant Commissioner Dean Storey said the service had gone into a "heightened state of readiness" and extra resources were being deployed to flood-prone areas.

A severe weather warning has been issued for parts of the Hunter, Central Tablelands and Sydney, among other regions, for severe thunderstorms producing heavy rain and possible flash flooding, damaging winds and the potential for hail over the next several hours.