BAMOS
Jun 2020
23
Figure 2: Estimated
tornado path and damage
indicators used to assess
the tornado intensity
rating. Source: Bureau of
Meteorology
Tornado Damage Assessment
A damage assessment was conducted by the Bureau of
Meteorology from 14:00 AEST on Wednesday 20 May 2020, 13
hours after the tornado event and found 21 damage indicators,
of which 10 damage indicators were used to estimate the
intensity of the tornado using the Enhanced Fujita Scale
(EF‐scale) and associated damage assessment process.
Damage largely consisting of roof tiles blown off, particularly
along the ridge lines, trees and fences blown over, sheds
and trampolines blown and twisted, and evaporative air
conditioning condensation towers being blown up to two
houses away was observed with a width of approximately
60–80 metres. The more significant damage appeared to have
occurred on the northern side of the damage path with a width
of approximately 20–30 metres which consisted of increased
roof coverage being removed, windows and doors being blown
and smashed inwards, garage doors blown in or collapsed,
the complete collapse and destruction of pergolas, patios
and garages, as well as collapsed interior roofs which were
associated with rain ingress resulting from the loss of roofing
material. Several hardwood gum trees in Mount Duneed had
their branches snapped aloft, some of reasonable size >10 cm
in diameter, defoliated and in a handful of cases, trunks were
snapped 1–3 metres above the ground. The most extensive
damage was observed to be between 20 and 30 metres
wide which was consistent with that observed with property
damage in Waurn Ponds suggesting that the track of most
intense damage remained narrow over an extended distance
and did not broaden as would be expected with thunderstorm
downbursts and thus, providing increased evidence for a
tornado.
Approximate Tornado Path
The path of the tornado illustrated in Figure 2 was approximated
from a combination of radar data, SES RFA data, damage
assessment, witness statements and CCTV footage. The
observed tornado track during the damage assessment was
approximately 3.8 km in length.
The tornado was estimated to have tracked towards the
southeast at approximately 90 km/h derived from both the
damage assessment and confirmed by radar data, covering the
total estimated tornado path in just under 3 minutes.
Damage Assessment Process
Tornado damage assessment estimates were conducted by
identifying representative Damage Indicators (DI) and assigning
a Degree of Damage (DOD) based on visible damage as per
McDonald and Mehta (2004). The ‘expected’ value of wind speed
provides an estimate of the 3‐second wind gust speed based on
a set of “normal” conditions, whilst the upper bound and lower
bound estimates of wind speed account for circumstances that
may cause the actual wind speed associated with the damage
to deviate from the expected wind speed.
The ‘expected’ wind speed estimate for each DI is then
categorised by the EF‐scale and assigned a tornado intensity
rating, e.g. EF1, EF2.
Rating
Enhanced Fujita Scale
Estimated 3‐second Wind Gust Speed (km/h)
EF0 105–137
EF1 138–178
EF2 179–218
Table 1: Enhanced Fujita Scale tornado intensity ratings
and associated estimated 3‐second wind gust speeds
adapted from McDonald and Mehta (2004) Table 6 and
converted to metric units.
EF3 219–266
EF4 267–322
EF5 >322