East Coast Low 19-22 April, 2015 Wrap-up A/Prof Ian D. Goodwin, Macquarie University

1. ORIGIN AND TYPE OF STORM The East Coast Low (ECL) storm that occurred over the western Tasman Sea and along the Illawarra to Mid North Coasts from the 20th to 22nd April, 2015 began to form on the 19th April. This type of ECL is a Southern Secondary Low (definition in Browning and Goodwin, 20151) and it evolved (18-20 April, 2015) from the confluence of an Inland Trough over eastern Australia and a surface and mid-tropospheric extratropical low cut-off from the main westerly airstream over the Southern Tasman Sea. The SSL storm continued to evolve into a deepening trough oriented parallel to the east Australian coast, and sandwiched between two strong anticyclones centred on ~40°S and situated in the Great Australian Bight and the other situated over New Zealand. A classic ECL formed by 10pm on the 20th when the two anticyclones merged over the Southern Tasman and a closed low pressure system formed over the NSW coast. The ECL was semi-stationary and deepened during the 21st April. The system weakened from 10 am EST on the 22nd April and is forecast to slowly migrate to the south-east during the 23rd April.

1 Browning, S.A., & Goodwin, I. D. (2013). Large-scale influences on the evolution of winter subtropical maritime cyclones affecting Australia’s east coast. Monthly Weather Review, 141(7), 2416-2431.

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2. METEOROLOGICAL IMPACTS The ECL produced intense rainfall and significant flooding from Sydney to the Hunter and Lower North Coast Regions during the evening of the 20th and continuing during the 21st and into the morning of the 22nd. The flooding was most extensive in the Hunter River Valley, particularly in the northern section near Dungog. Flooding at Hawkes Nest is considered to be the most extreme for over 50 years. High winds and extreme ocean waves occurred.

2.1 Rainfall

Intense rainfall was experienced over the Illawarra to Hunter Regions. In the Hunter Valley, up to 300 mm of rain was recorded in 24 hours to 9 am on the 21st April, and a further 100-150 mm to 9 am on the 22nd April. NSW rainfall maps are below.

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2.2 Winds

The maximum recorded winds during the 21st and 22nd April are listed below for coastal stations. The strongest winds were recorded from Pittwater to the Port Stephens regions. Wind damage was widespread particularly sheared tree limbs and trunks rather than fallen trees from sodden roots. The trees in the Pittwater, Central Coast and Hunter regions displayed failure through tortional twisting and shear during wind gusts, particularly during the evening of the 21st through to midday on the 22nd.

Location Date/Time EST

Mean Wind Speed km/h

Max Wind Speed Gusts km/h

Mean Wind Direction °

Williamtown 210415/2.50 am

82 113 SSE

Nobbys Head/Newcastle

210415/5 am

83 135 SSE

Norah Head 210415/6.52 am

96 135 S

Gosford 210415/2.50 43 80 S

Terrey Hills 210415/00 40 76 S

North Head 200415/8 pm

210415/630 am

67

70

102

104

SSE

SSE

Sydney Airport 72 96 SSE

Bellambi 210415/7.30 am

76 91 S

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2.3 Ocean Waves

Ocean waves began to build from 12pm on the 20th and reached a peak on the Sydney Buoy at 3 pm on the 21st with an average significant wave height Hsig of 7.5 m, a maximum wave height Hmax of 14.9 m, a peak period of 12.1 secs and a mean wave direction of 149 °. The mid-shelf wave direction was SSE throughout the storm. The maximum wave height is a new record since waverider buoy records commenced in 1974. The recent storm waves now rank as the second largest event in the instrumental record for Sydney (see table below). The storm duration (Hsig waves above 3 m and Hmax above 6 m) was ~74 hours.

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Sydney

Rank

Duration

Type

Storm Peak Storm Mean Total Power

(MW/m)

Storm rms Hsig Peak Hours Hsig Hmax Tp Dirn Hsig Tp Dirn

1 11/05/97 303 ETL 8.4 13.7 12.8 151 3.1 11.6 138 21.2 3.5

2 21/04/15 74 SSL 7.5 14.9 12.1 149 NA NA NA NA NA

3 3/08/90 157 ETL 7.2 11.8 10.2 2.9 10.2 7.2 3.0

4 28/07/01 29 ETL 7.0 11.7 13.5 169 5.2 12.3 161 5.5 5.4

5 9/06/07 491 ETL 6.9 14.1 10.8 135 3.2 11.2 153 29.5 3.3

After Shand et al, 2011.

2.4 Wind Setup and Storm Surge

Elevated sea level of + 0.23 m was observed at Fort Denison for the duration of the storm. This combined with wave setup during the spring high tide (predicted 1.94 m) at 9.15pm on the 20th, caused wave runup and swash to overtop the foredune crest. The extent of sand and pumice indicates that inundation extended up to 50 m landward of the foredune crest. Similar overtopping occurred during the spring high tide on the 21st.

2.5 Beach and Dune Erosion

Open coasts

Observations of beach erosion and storm response were made for the Sydney Northern Beaches and Newcastle Beaches. All displayed minor scarping of the foredune and overall beach lowering (sand loss offshore). The most widespread impact was the storm surge inundation and backshore sand transport. Major dune scarping and beach/dune erosion occurred in the Narrabeen embayment as an adjustment to artificial sand nourishment sites. Overall, the minor coastal erosion was due to the conditioning of the shoreface and surf zone to higher wave energy over the past 6 weeks. Sand bars were present offshore that resulted in incident wave energy being dissipated offshore, and restricting the shoreline impacts to be mainly due to the wind setup and wave runup.

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Estuarine coasts

South facing shores within the estuaries, including: Manly Cove, Sandy Point, Palm Beach, Pittwater, Gosford, Brisbane Waters, northern Lake Macquarie, and Winda Woppa, Jimmy’s Beach all experienced severe erosion. This was caused by the anomalously SSW to SSE gale force winds and long fetch within each estuary. This estuarine erosion is significant and anomalous. There are a significant number of waterfront properties at Palm Beach and at Jimmy’s Beach (Winda Whoppa) that are now threatened by ongoing erosion.

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Shoreline erosion at Sandy Point Palm Beach, Pittwater

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Wrecked motor boat at Manly Cove

Major shoreline erosion at Jimmys Beach, Winda Whoppa, Port Stephens

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3. SEASONAL OUTLOOK FOR ECL OCCURRENCE There is a heightened risk of more extreme SSL events this autumn as the large scale atmospheric circulation and anomalously warm SST along the east coast and in the NE Indian Ocean are precursors for SSL formation. The Browning and Goodwin 2013 paper identified the drivers of SSL formation in the figure below.

The mean sea level pressure anomaly chart (below) for the previous month 15/3/15 to 15/4/15 shows the distribution of similar large scale SLP anomalies.

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East Coast Low 19-22 April, 2015 Wrap-up