This weather event consisted of a one in fifty year extreme low pressure system that spawned up to 7 tornadoes. The storm resulted in a state wide blackout on Wednesday 28th September at 4.30 pm and extreme winds, waves and coastal flooding in the afternoon of Thursday 29th September.
Sea level in upper Spencer Gulf normally varies by 1 - 2 m or so every 12 hours due to tides. In addition, westerly and southerly winds can pile water up near the head of the Gulf. On the afternoon of Thursday September 29th, the intense atmospheric low pressure system drove extreme southerly winds (> 80 km/hr), that, in association with a high tide led to a storm surge of about 3.5 m at Port Pirie in the late afternoon of Thursday 29th September.
The SAROM and TGM models were run for this period and the latter used to make a two-day forecast of sea level from 6 am on September 28th. The TGM results for sea level and winds are illustrated by the “still” (Figure 1) below.
The animation of sea level and winds begins prior to the storm on the 25th September 2016 and runs through to September 30th after the storm.
Chronology of the extreme storm surge in upper Spencer Gulf
- 26-27th September: weak (10 – 20 km/hr) westerly winds and sea level dominated by tides.
- 28th September (Wednesday 2.30 pm): the northerly (southward) winds associated with the eastern side of the low begin with speeds of 10-30 km/hr. Sea level is lowered at the head of the Gulf by up to a meter.
- 28th September (4.30 pm): The front comes through with strong 40 -50 km/hr westerly (eastward) winds and tornadoes: a state-wide power blackout occurs. Sea level is yet to respond.
- 29th September (Thursday morning): The westerly winds intensify and turn to the south and sea level on the eastern side of Spencer Gulf is raised by a meter or so.
- 29th September (late afternoon-early evening): the winds have now swung around to blow to the north-east (up Gulf) with amplitudes of up to 80 km/hr. The winds now pile water up the Gulf and in conjunction with a high tide of a meter or so lead to the storm surge of about 3.5 m at Port Pirie and the upper Gulf.
A comparison is made in the Figure 2 below of the SAROM now-casts with the observed sea level at Port Pirie (the green circles). The black curve denotes the SAROM now-cast and black dashed the SAROM tides alone. As can be seen, the SAROM now-cast models the total sea level at Port Pirie very well.
A more stringent test is for the predictive skill of the TGM, it was run from the 28th September 6 am (ACST) in forecast mode alone (the red curve). It provides a very good prediction of the timing and magnitude of the 3.4 m storm surge.
Prior to the storm event, the results in Figure 2 also show that both SAROM and the TWG reproduce the observed tidal variations quite well.
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