A southern low pressure system and a La Niña ALERT are not warmly welcomed news for the NSW North Coast, as it's possible some heavy rain events could occur between now and January 2022

 

Ch 9 News on 13 October 2021 reported; Meteorologists warn a "beast from the south" will produce a dangerous mix of heavy rain, severe thunderstorms, damaging winds, hail and snow over Australia's south-east as a week of spring wet weather continues.

Coastal catchments with much of their historical and ongoing development situated on floodplains are particularly vulnerable when a climate driver triggers prolonged or frequent heavy rainfall events, as widespread or flash flooding can result. Property damage, stock & crop losses may occur and, sometimes the deaths of people caught in flood waters. 

Similarly low pressure systems can batter coastal catchments.

WeatherZone tweeted this.....

Here is the forecast accumulated rain over the next four days. 🌧

Flood watches have been issued in TAS and VIC, where some places could see more than 100mm by Sunday. Localised flooding is also likely in NSW and QLD from severe storms, especially on Thursday. pic.twitter.com/sVfxdgoCNU

— Ben Domensino (@Ben_Domensino) October 13, 2021

This was the Australian Bureau of Meteorology (BOM), on 12 October 2021:

The Bureau's ENSO Outlook has been raised to La Niña ALERT. This is due to continued cooling in the tropical Pacific Ocean and an increase in the number of climate models showing sustained La Niña conditions over summer. Historically, when La Niña ALERT criteria have been met, La Niña has subsequently developed around 70% of the time. A 70% chance of an event is approximately triple the normal likelihood. La Niña events increase the chances of above-average rainfall for northern and eastern Australia during spring and summer.

Most oceanic and atmospheric indicators of ENSO are currently within the ENSO-neutral range, but some have shifted towards a La Niña-like state. Sea surface temperatures in the central tropical Pacific Ocean are neutral, but have cooled over the past three months and are supported by cooler than average waters beneath the surface. Some atmospheric indicators, such as the Southern Oscillation Index (SOI) and cloudiness near the Date Line, are approaching La Niña levels. Six of the seven international climate models surveyed by the Bureau meet La Niña criteria from November.

A weak negative Indian Ocean Dipole (IOD) event continues. Most models suggest the negative IOD event will ease to neutral levels in late spring. A negative IOD increases the chances of above-average spring rainfall for much of southern and eastern Australia, while a neutral IOD has little influence on Australian climate.

The Madden–Julian Oscillation (MJO) has been active over the Maritime Continent since late September. The MJO is forecast to progress eastwards over the coming week and weaken as it approaches the western Pacific. While the MJO is over the Maritime Continent region, it encourages enhanced rainfall over the tropics to the north of Australia.

The Southern Annular Mode (SAM) index has been neutral for the past week after 5 to 6 weeks at positive levels. While it is forecast to remain neutral for the coming week, it is expected to return to generally positive levels from October to December. A positive SAM during spring typically brings wetter weather to eastern parts of Australia, but drier than average conditions for western Tasmania.

Climate change continues to influence Australian and global climate. Australia's climate has warmed by around 1.44 °C for the 1910–2019 period. Rainfall across northern Australia during its wet season (October–April) has increased since the late 1990s. In recent decades there has been a trend towards a greater proportion of rainfall from high intensity short duration rainfall events, especially across northern Australia.......

The El Niño–Southern Oscillation (ENSO) is currently neutral.

However, all of the seven international climate models surveyed by the Bureau anticipate further cooling of tropical Pacific sea surface temperatures. All models surveyed indicate La Niña thresholds may be met during November. Six of the seven models indicate this cooling will be sustained at La Niña levels until at least January 2022—long enough to meet minimum La Niña event criteria (i.e. at least three months). Four models continue the event into February, but by March only two models continue to meet the threshold.

La Niña typically enhances spring rainfall in northern and eastern Australia.

BACKGROUND

“Major floods frequently isolate towns, and can cause death, major disruptions to road and rail links, the evacuation of many houses and business premises, and the widespread flooding of farmland…..

As we are specifically concerned with major flooding near the coast, we consider a flood to be major if (i) it causes inundation of a river within approximately 50 km of the coast or (ii) if there is non-riverine flooding overland near the coast, from the active part of a weather system, that extends at least 20 km along the coast. In (i) extreme rainfall extends well into the hinterland and the upper reaches of the river catchments, causing a flood that drains down the river systems to coastal areas. In (ii) extreme rainfall is confined to the coast and floods form directly over the coastal area rather than propagating down the river systems. At major coastal centres there are official river height records that determine whether a flood meets the major level or not, and we have used this data available.” [ResearhGate, Callaghan J & Scott, S in Australian Meteorological and Oceanographic Journal 64(3):183-21 DOI:10.22499/2.6403.002]

On Australia’s eastern shores East Coast Lows are also often a trigger for heavy rainfall and widespread flooding. Sea surface temperature gradients associated with the warm eddies of the East Australian Current are an important contributor to the development of the lows.