The Bureau of Meteorology has issued a Severe Weather Warning for parts of the Northern Rivers today.

A trough of low pressure with a moist and unstable airmass has triggered deep and persistent showers in the Northern Rivers this morning.

HEAVY RAINFALL, which may lead to FLASH FLOODING is forecast for the Northern Rivers. Six-hourly rainfall totals between 40 to 80 mm are likely with isolated totals up to 150 mm possible.

Currently, rain is occurring across the Northern Rivers and is expected to persist until this afternoon.

(The BOM radar at 8:44am)

Locations that may be affected include Lismore, Byron Bay, Ballina, Evans Head, Yamba and Maclean.

The Bureau's Helen Read said several places received over 100mm to 9am this morning; they included Alstonville, 116mm and Upper Crabbe Creek near Mullumbimby, 120mm, while Lismore Airport recorded 85.6mm.

"Moderate to heavy totals are expected along the northern coast and adjacent ranges, with widespread falls of 50 to 70 millimetres forecast north of Coffs Harbour and localised falls exceeding 120 millimetres are possible. Isolated thunderstorms are also possible in the area with the risk of flash flooding, Helen said.

"The upper trough is expected to move slowly south down the Queensland coast today and be offshore on Wednesday. The Surface trough will develop during today and weaken on Thursday."

A Flood Watch still exists for the Wilsons and Richmond River.

"This is a very dynamic situation, with high levels of uncertainty in the timing and location of the heaviest falls. The Bureau is continuing to monitor the situation and will provide updated advice as required. Catchment specific warnings will be issued if and when required," The Bureau said on its website.

The State Emergency Service advises that people should:

* Don't drive, ride or walk through flood water.

* Keep clear of creeks and storm drains.

* If you are trapped by flash flooding, seek refuge in the highest available place and ring 000 if you need rescue.

* Be aware that run-off from rainfall in fire affected areas may behave differently and be more rapid. It may also contain debris such as ash, soil, trees and rocks.

* After bushfires, heavy rain and the loss of foliage can make the ground soft and heavy, leading to a greater chance of landslides.

* Stay vigilant and monitor conditions. Note that the landscape may have changed following bushfires.

* For emergency help in floods and storms, ring your local SES Unit on 132 500.

You can stay up to date with the weather situation on the BOM website through the Lismore App Weather and Weather Radar.