This is not the weather forecast that Scott Jones and Daniel Kedraika wanted to hear at the Lismore Turf Club for the Lismore Cup race day today but punters will be looking for horses that can handle the wet.

The Bureau of Meteorology has issued a severe weather warning for large areas of the Northern Rivers.

A deep low pressure system over northern NSW is causing areas of rain across the Mid North Coast and Northern Rivers districts. Isolated thunderstorms are expected to develop over the Northern Rivers district afternoon. The low pressure system along with the rain areas are expected to move offshore during Friday.

This morning the BOM say "HEAVY RAINFALL which may lead to FLASH FLOODING are expected to develop about the Northern Rivers districts during the day, and continue into Friday morning.

"Six-hourly rainfall totals of 60 to 80mm, with isolated falls to 120mm are possible.

"Twenty-four-hour rainfall totals of 80-120mm, with isolated falls to 200mm are possible.

"Severe thunderstorms with localised heavier rainfall are expected over the Northern Rivers district in the afternoon and evening. Separate severe thunderstorm warnings would be issued when this occurs.

Locations that may be affected include Lismore, Grafton, Coffs Harbour, Port Macquarie, Taree and Dorrigo.

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.

The next Severe Weather Warning will be issued by 11:00 am AEST Thursday.