90mm of rain now predicted by BOM this weekend
UPDATE, 5pm: THE BUREAU of Meteorology has upped its rain forecast again for Sunday, with heavy showers of up to 90mm now expected to fall across the Northern Rivers.
At this stage the heaviest downpours of up to 90mm are predicted to strike Tweed Shire, while Byron, Ballina, and Lismore are expected to cop up to 80mm.
It is expected to be a fast-moving event which by Monday will have moved out of the region, eventually giving way to sunny skies mid-week.
But Sunday is expected to be a write-off for all outdoor activities.
Higgins Storm Chasing has also issued predictions of up to 150mm of rain across 24 hours possible in some areas of Northern NSW.
Further south, Sydney and the south coast could be in the firing line of a potential East Coast Low developing late Sunday and early Monday.
UPDATE, 11.45am: THE Richmond-Tweed SES will have all hands on deck this weekend to respond to any emergencies both in the Northern Rivers and elsewhere after a warning order was issued for a potential East Coast Low.
SES region controller Andrew McPhee said the warning order had triggered a protocol to ensure it had extra staff on call.
"At the moment we're looking at the rain starting on Saturday, so we would certainly be having additional staff available over the weekend," Mr McPhee said.
"We're not tracking it at the moment as a severe as event as (the previous ECL on June 4) but that doesn't mean we're not taking it seriously.
"The picture from the BOM is it's not expected to impact the entire coast as last week's did, and it will be a shorter duration.
"But these systems are pretty dynamic."
"We obviously do our preparation for the worst case scenario. An East Coast Low is inherently unpredictable, it's a dynamic system - it could have an impact anywhere along NSW."
Mr McPhee also said any intense rainfall event into already wet catchments was a concern.
"We've been tracking the system for a couple of days already and our flood intel guys are looking at where the potential hotspots are should the current regime of rainfall eventuate," he said.
"If it falls over a very short period of time as it did last time we expect to see greater runoff because the catchment is already pre-loaded.
"High intensity rainfall falling over a short period of time (also) has the potential to cause flash flooding."
UPDATE, 10.10am: WEATHER forecasters Higgins Storm Chasing (HSC) says "heads up" flood watches could be issued for parts of north eastern NSW in the coming days.
"A majority of computer model forecast guidance is in agreement for a low pressure system to develop over southern inland Queensland on Saturday before moving south east into NSW on Sunday," the group posted on its website.
"Areas of heavy rain are also likely to develop ... widespread rain areas of 50mm likely with 24hr totals up to 150mm possible ... this low pressure system is also likely to further develop into an East Coast Low near Sydney late on Sunday or early Monday."
"HSC heads up flood watches may be issued for other parts of eastern NSW in coming days.
"Our detailed and updated NSW forecast will be issued by noon on Thursday. The next flood watch will be updated on Thursday night."
THURSDAY 7.40am: THE Bureau of Meteorology have upgraded the weather event that is set to hit the region this weekend.
A strong high pressure system over the Tasman Sea extends a ridge to the northeastern New South Wales, the BoM said.
A trough approaching from the west will bring some rain to far western districts today and the remainder of the inland tomorrow.
Over the weekend, the trough is expected to deepen over the northern inland and it is likely that a low pressure system will form within the trough and move southeast towards the south coast.
This system is likely to bring widespread rainfall to all but far western parts of the state from Saturday through to Monday.
Forecast for the rest of Thursday:
Patchy fog early this morning.
20% chance of a shower about the coastal fringe, mainly in the north.
Daytime maximum temperatures in the low 20s.
WEDNESDAY 3.25pm: Parts of the Northern Rivers can now expect up to 75mm on rain on Sunday with the potential for even heavier falls of more than 100mm in some places.
The Northern NSW Severe Weather group had this to say on the subject on their Facebook page:
"A strong upper trough is forecast to move into NSW Friday and develop a surface low pressure system.
"This will deliver widespread rainfall to eastern Australia with some moderate to heavy accumulations likely from Friday into Monday."
"For NE NSW we're going to see the best/worst of it on Sunday.
"Widespread 50-75mm is possible and some areas may see over 100mm. Currently the highest totals are favouring the Tweed (again).
The surface low is most likely going to end up on the southern NSW coast by Monday and may become a stronger East Coast Low system. This trajectory will contact the rain away from NE NSW pretty quickly.
This map from BoM shows rainfall accumulations for Thursday to Monday averaged from various weather models.
Higgins Storm Chasing has issued an bolder prediction, with its map showing widespread falls of up to 100mm right through the Northern Rivers.
However, Northern NSW Severe Weather's Michael Bath said where exactly the low formed would play a crucial role in how much rain the Northern Rivers would get.
Mr Bath said 50-75mm was a "conservative" estimate, but although there talk among some forecasters of 150mm, he believed that was overstating it - at least at this early stage with four days still to go.
The crucial difference between this event, and the previous heavy rain event on Saturday June 4 - which saw moderate flooding in Lismore - is the low pressure system generating the rain is forecast to form further south, and also move very quickly away to the South Coast.
There is also no king tides involved.