Heavy rain and flooding – New South Wales South Coast – 4 to 7 May 2021

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Following an exceptionally wet March and a subsequent dry April 2021, the first week of May 2021 across the east coast of Australia has again been wet.

The initial Bureau of Meteorology rain models suggested that upwards of 50 mm of rain would fall across much of the east coast especially along the coastal strip of New South Wales south of Sydney. Models suggested slightly heavier falls of between 50 mm and 100 mm could fall in isolated locations. The event would not have been considered significant for the 4 day period.

However, rainfall totals in some locations south of Sydney especially within the Shoalhaven region, an area between Ulludulla and Batemans Bay and around Bega not only exceeded expectations but have been heavy leading to isolated flooding events.

Accumulative totals have reached between 200 mm and 300 mm for the event and even across Sydney rainfall totals exceeded expectations. As a result, moderate flooding has occurred at Menangle Bridge on the Nepean River south west of Sydney and minor flooding has occurred along the Shoalhaven River at Nowra.

Some of the daily rainfall totals have been considerable during this event although falls over 100 mm have been concentrated within small localities.

For the 24 hours to 9 am 5/5/2021, the heaviest rainfall totals include:-

Black Range (Near Bega) 156 mm.

Bega River at Kanoona 123 mm.

Bega - 122 mm.

Lake Tabourie - 100 mm.

Angledale - 119 mm.

All localities except Lake Tabourie are close to Bega on the New South Wales South Coast.

For the 24 hours to 9 am 6/5/2021 some rainfall totals include:-

Brogers No 2 (Brogers Creek) - 129 mm.

Fitzroy Falls Dam - 114 mm.

Nowra ASW - 113 mm.

Brooman (Directly north of Batemans Bay) - 104 mm.

Such falls were limited to small areas. Here in Sydney rainfall totals ranged from 8.2 mm at Penrith to 40 mm at Lucas Heights for the 24 hours to 9 am 6/5/2021. There were short bursts of heavy showers throughout the day but for the most part, most rainfall totals across much of Sydney fell between 20 mm and 26 mm.

For the 24 hours to 9 am 7/5/2021, even heavier rainfall totals occurred within small areas south west of Wollongong encompassing the Kangaroo Valley, parts of the Shoalhaven region and the eastern side of the Southern Highlands around Fitzroy Falls and Robertson. The heaviest totals include:-

Brogers No 2 (Brogers Creek) - 182 mm.

Fitroy Falls Dam - 171 mm.

Beaumont (The Cedars) - 162 mm.

East Kangaloon - 156 mm.

Robertson - 147 mm.

It is clear that Brogers No 2 (Brogers Creek) has had more than 300 mm during this rain event.

The weekly rainfall plot for New South Wales to May 7 2021 on the Water and the Land site clearly shows that large parts of the New South Wales South Coast has recorded between 200 mm and 300 mm of rain including an area south west of Wollongong, an area between Ulludulla and Batemans Bay and another small area close to Bega.

This event has impacted the Warragamba Dam catchment and it is likely that more water inflows will result in water spilling again over the dam wall.

Conditions are now clearing and further significant rainfall is not likely from this system. This will allow flood waters that have occurred to subside.

Typhoon Surigae – Philippines – 18 to 23 April 2021

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The first typhoon of the year across the north west Pacific has been menacing the coast of the Philippines especially Luzon Island however, the storm is not forecast to make landfall.

The storm formed east of the Philippines then tracked north west skirting the eastern coastline of Luzon Island. The storm is now forecast to track northwards then turn north east taking it away from population centres and back out to sea where the storm will decay.

This storm reached Category 5 on the Saffir Simpson Scale where it had peak wind gusts of 145 knots or approximately 268 km/h making it an intense system.

At the time of writing, CIMSS has the storm as being rated Category 4 on the Saffir Simpson Scale with peak wind gusts to 120 knots or 222 km/h but over the next 2 days, the storm should weaken as it passes over colder waters of below 26C.

The north west Pacific Ocean is a favourable location for the development of intense typhoons due to favourable ocean temperatures and ideal conditions and such intense storms occur most years. Often landfall occurs across the Philippines, Taiwan and or southern China.

However, it is expected that Typhoon Surigae, will remain well clear of population centres.

Recent satellite images show an intense storm with a well defined eye located directly east of Luzon Island. It is a compact storm with well defined convection occurring especially across its eastern side.

The storm will only be affecting shipping lanes and aircraft flight paths given its location.

Tropical Cyclone Seroja at landfall – How strong was the storm? 12/4/2021

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Tropical Cyclone Seroja has made landfall very close to the town of Kalbarri in Western Australia and has caused some significant damage to the town. In addition, the regional city of Geraldton was also impacted by gale force and damaging winds including a period of heavy rain as the storm passed over.

The storm was travelling unusually fast at landfall and reached a speed of at least 55 km/h. It had a relatively narrow track as it traversed over land towards the south east.  

Media reports have indicated that peak wind gusts reached 170 km/h and that the storm was a Category 3 system upon landfall.

While verified data is still being processed and updated, a number of weather stations within proximity of the storm have recorded the following peak wind gusts:-

Geraldton Airport - 120 km/h at 8.52 pm and 8.57 pm. However, not recorded in the detailed notes, a single peak gust of 121 km/h was recorded at 9 pm according to data taken from “Australian Weather News”.

Furthermore, significant peak wind gusts at Geraldton Airport occurred mainly between 8.23 pm and 9.08 pm on Sunday evening.

Morawa Airport - A single peak wind gust of 119 km/h occurred at 11.26 pm.

Shark Bay - Maximum peak wind gusts of 91 km/h occurred.

According to observations taken from the following sites (Taken from “The Australian Weather News” 12/4/2021)

Binnu Agricultural Station - A peak wind gust of 122 km/h occurred at 8.04 pm.

Morawa Agricultural Station - A peak wind gust of 132 km/h occurred at 11.21 pm.

At the present time, additional verified data is not available and the above represents the highest identified wind gusts known as at 8 pm 12/4/2021 (Sydney Time - New South Wales).

Tropical Cyclones across the world are categorized according to their location. For example, across the Atlantic Ocean, Eastern and Central Pacific, the Saffir Simpson Scale is used and CIMSS models use the scale. Generally the National Weather Service (USA) uses the scale to determine the strength of a tropical storm.

Tropical Cyclone Seroja was determined to peak as a Category 1 system at landfall on the CIMSS models. A Category 1 storm has sustained peak wind gusts of between 119 km/h and 153 km/h and based on available data, this storm would fit the Category 1 storm system. If verified peak gusts of 170 km/h did occur, then it is possible that the storm reached Category 2 on the Saffir Simpson Scale.

Note - A Category 2 storm has maximum sustained peak wind gusts of between 154 km/h and 177 km/h on the Saffir Simpson Scale.

The Bureau of Meteorology uses a different scale for measuring the strength of tropical cyclones which makes a comparison somewhat confusing. The scale is called the “Australian Tropical Cyclone Intensity Scale” which was introduced in 1989/1990 and under this scale:-

A Category 1 storm has sustained winds of 63 to 88 km/h and with peak wind gusts of 91 to 125 km/h.

A Category 2 storm has sustained winds of 89-117 km/h with peak wind gusts of between 126 km/h and 166 km/h.

A Category 3 storm has sustained winds of 118-157 km/h with peak wind gusts of between 167 km/h and 225 km/h.

A Category 4 storm has sustained winds of 158-198 km/h with peak wind gusts of between 226 km/h and 280 km/h.

A Category 5 storm has sustained winds exceeding 198 km/h with peak wind gusts greater than 280 km/h.

Hence on the Saffir Simpson Scale, this storm appears to have rated Category 1, possibly Category 2 but under the “Australian Tropical Cyclone Intensity Scale”, the storm would have fit as a Category 3 system.

The Australian system for determining the strength of a tropical cyclone has lower thresholds than that of the Saffir Simpson Scale.

Hence, depending on what scale is used, the strength of Tropical Cyclone Seroja will vary and can be confusing.

The storm has all but decayed and for the 24 hours till 9 am 12/4/2021, a narrow band of rain of between 50 mm and 74 mm occurred along its path. The highest verified rainfall along its path for the is 24 hours is 76 mm at Binnu West.

Source

1 - Australian Weather News 12/4/2021.

2 - Bureau of Meteorology 12/4/2021 - Australian Tropical Cyclone Intensity Scale 1989/1990.

3 - National Weather Service - Saffir Simpson Hurricane Scale.

Addendum to topic - Peak wind gusts

Further to the above topic, further data has been identified in which a peak wind gust of 139 km/h occurred at Binnu West at 9.47 pm Sunday and the Bureau of Meteorology has identified a peak wind gust to 170 km/h at Meanarra Tower near Kalbarri at 7.03 pm on Sunday. They appear to be the higher wind gusts not appearing in the day to day data sets from surrounding weather stations.

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