Inverleigh Tornado, 21-12-03

Just south of Inverleigh, on "Murroon" property (approx 20 km west of Geelong) on Sunday 21st between 6.00pm and 6.20pm (see 6.10pm radar image below) there was severe wind damage to farm buildings during the thunderstorms which passed through NW-SE with a frontal system which crossed the whole state. There were reports of other wind damage in the Geelong district at the time with recorded BoM gust speeds up to 80kph, and even greater damage elsewhere in western Victoria.

The MSL pressure analysis chart for 4pm (AEDST) shows a low pressure system centred SW of the Victorian coast and moving eastward at about 30 knots. A vigorous cold front has passed Portland and is well into western Victoria.

This radar image shows several of the bands of rain which crossed the district. Since the damage path was so narrow and short it is not expected to display any tornadic signature on this scale. At 9am on the next morning (22nd) BoM rainfall recorded at Inverleigh was 16mm and at Winchelsea, 11mm.

Pressure dropped to 994hPa at 6pm at Geelong (at Belmont, 991hPa by 8pm) and a narrow damage path 30m wide for 1km has been traced. The map below shows the damage path.

The impressive damage included: 30cm wide cypress trunks snapped, 150mm thick steel girders bent, concrete blocks 1metre deep by 450cms were uprooted and blown up to 10m. Small sheds were blown up to 50m and a 1-tonne trailer was ripped from a tractor and deposited 10m away. Photos of this damage appear below.

 

The debris was spread over approx 500m and some was found 50m into the direction of the storm path. Unfortunately there are no eye-witnesses, but locals can confirm the storm noise, dark sky and wind shear as seen by cloud movements.

The damage is consistent with that of a brief F2 tornado, and I have estimated the gusts at around 200-250kph. The local "Geelong Advertiser" has called it a "freak twister". I don’t know about the twister part just yet, but it was certainly no freak for this district. In the last 3 years we have had at least 6 funnels including two touchdown tornadoes, as well as 3 waterspouts and a lot of other straight wind damage.

See 1 on the map above. Facing east from the Inverleigh-Winchelsea Road towards Geelong, this shows the narrow 30m gap created with numerous cypress branches down, including at least two 30cm trunks snapped around 8m above the ground and a thinner one on the right which has snapped back into the oncoming blast.

The damage for the next 400m is negligible over a crop paddock, then farm buildings bore the brunt of the storm. See 2 to 6 on the map.

Looking to the west-northwest from the demolished hay shed to the gap in the cypress trees, the intervening crop paddock is unharmed.

Looking southwest from the hayshed. Large sheets of corrugated iron twisted and mangled when they were blown up against the wire fence. Photo taken near 3.

Photo taken near 6. Looking southeast from the hayshed. In the foreground is some wreckage including a bent steel girder, the broken fence line and torn down trees. In the background a large tank on the left has been blown 500m. The green paddock had been strewn with debris but has been cleaned up.

Looking southeast from point 6. The large bucket of a tractor is in right foreground, having been removed from its host. Other heavy materials can be seen deposited, and a one metre long concrete lump used as a foundation for an upright pole of the hay shed can be seen after having been uprooted from its place about ten metres behind the camera position.

Several bent steel girders, 150mm across, lie amongst the hay shed wreckage.

In this photo, facing northeast, an undisturbed truck laden with tyres is only metres away from the scene of destruction in the foreground where the fence line and 4m high trees have been flattened.

The only visible damage sustained by the feed silos are the large dents and the loss of the roof from the one on the left.

Facing north from point 5. The silos in the background are undisturbed. In the foreground lies a concrete foundation block. The truck in the background has been loaded with debris salvaged over a radius of 500m in the next paddock.

WAS IT A TORNADO?

There are no known eye-witnesses to this event. Several neighbours have verified that a thunderstorm occurred in the area between 6pm and 6.30pm and this is confirmed by radar which shows moderate to heavy rain in small blocks crossing the district at that time.

However, because of the obvious speed of movement of the strong wind event, and the short damage path, it would probably not be reasonable to expect any sort of radar signature to be evident. The Paraparap Tornado, photographed and well-documented (9-1-2000) has demonstrated that a small, relatively weak tornado can form within the context of an "ordinary" thunderstorm cell, although there was little damage on the ground.

This event however, left certain fingerprints to suggest tornadic behaviour, and it has been suggested that a gustnado was responsible. However there are several factors which may also have to be considered:

  1. The damage to some farm buildings was considerable. On the Fujita Damage Scale, the category most appropriate is F2: "roofs torn off frame houses" – the hay shed roof was well-constructed and steel beams were lifted, bent and torn from their uprights. The whole shed was demolished. "Rail carriages pushed over" – a very heavy tractor with loading bucket and trailer were overturned and the 1-tonne trailer was rolled several metres. "Large trees snapped or uprooted" – Several mature cypress tree trunks at least 30cm wide were snapped about 8m above the ground, many smaller trees were uprooted. "Light-object missiles generated" – large corrugated iron sheets were mangled and blown across a paddock up to several hundred metres, and dents and gouge-marks are evident in trees, fences, silos and the ground. All this suggests that winds between 180-250kph were responsible, too great for a gustnado.
  2. Some debris was found lying on the windward side of the storm’s approach, in other words, to the northwest. For example, some tree branches (see first photo) are snapped and facing northwest. Several corrugated iron sheets were found some 50 metres west of the hayshed from which they were torn. (Point 2 on map). The remains of a toilet shed were found 30 metres to the south as seen on the map at point 3. This indicates the likelihood of a rotating wind shaft.

The issue of definition of this event will probably be disputed, however the degree of damage is obvious for all to see.

SKEW-T DIAGRAM

The most relevant diagram available is from Mt Gambier, some 300km to the west at 10am on the day in question. The eastward movement of this airmass at an estimated 30 knots (see MSL analysis chart above) guaranteed that the very moist air shown at most levels would be over the Inverleigh district later in the afternoon. This coupled with the very obvious wind shear pattern shown indicates the context in which the events took place.

 

END OF REPORT

Lindsay Smail

Geelong Weather Services, 2004

 

 

 

 

 

 

 

 

 

1 "A brief, intense vortex that forms on the leading edge of gust fronts . . . lasting from a few seconds to several minutes and are strong enough to cause minor damage." Definition from Storm Spotters Handbook, BoM, 1995.

2 See Storm Spotters Handbook, p.20.

Acknowledgment: Many thanks to Jane ONeill for formatting and providing the link to this report