BAMOS March 2017
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Figure 3 . Selected rainfall radar images from 29 January 2016 . Images : Bureau of Meteorology / The Weather Chaser .
system was centred in the Western Bight ( Figure 1 ). A marked upper level trough extending across eastern Australia with an associated upper low extending to 250 hPa was located west of Tasmania ( Figure 2 ). Instability east of the trough resulted in showers and thunderstorms over eastern New South Wales and the Australian Capital Territory ( ACT ). The surface trough had been semi stationary west of the ACT for several days prior to the outbreak of severe storms on the afternoon of the 25th of January . With the northeast movement of the upper low and associated cold pool , atmospheric instability increased during the day .
Local Observations
The morning of January 29 was overcast with eight octas nimbostratus at 2200 feet above ground level and 6 octas Stratus at 1800 feet . Temperature at 9:00 a . m . was 17.9 º C , dew point temperature 15.4 º C with relative humidity 85 %. The wind was a light north-easterly at 3 km hr -1 .
By 3:00 p . m . it was cloudy . The cloud groups observed were ; total six oktas consisting of three oktas cirrostratus and five oktas cumulus / stratocumulus at 2500 feet moving from the southwest . The temperature was 24.9 º C , dew point temperature 17.9 º C , relative humidity 65 %. A light easterly breeze at 16km hr -1 had sprung up during the afternoon . While the dew point was well above what we normally experience in Canberra in January , there was no indication from the 1500 observations of the impending storm event .
Alerted by the forecast , I was watching the developing weather from 3:00 p . m . onwards . I first observed the developing storm cells on the Canberra radar around 5:00 p . m ( Figure 3 ). At this time the system was located well south of Gilmore . By 6:00 p . m ., I observed a line of storms extending roughly east-west to our south , beyond the southernmost suburbs of Tuggeranong . The most intense precipitation was in the Mount Tennant / Tharwa area , with only virga further east along the line . This was confirmed by the radar . The cells had been moving north during the period . By 7:00 p . m . it was obvious the Tharwa cell had changed course to the northeast and had greatly intensified , confirmed by the radar and visually .
Onset of the storm event and its impacts on the local area .
Around 7:15 p . m . I could clearly hear a roar which I soon realised was the sound of the oncoming storm . I realised I was hearing hail falling on the roofs in Chisholm to the southwest of the station . Initially the wind was from the east at around 13 km hr -1 , but changed to westerly at 31 km hr -1 gusting to 54 km hr -1 by 7:30 p . m . The noise of the hail increased to a loud roar , particularly in the back veranda with its iron roof . Large hailstones began to fall , soon carpeting the lawn and then the gardens and lastly the brick pavers in the courtyard . Even inside the house , the noise was incredible . The property has “ sarking ” beneath the roof tiles and “ pink batts ” in the ceiling . I normally only hear rain if it is torrential .
The roof gutters soon overflowed as the hail , now the size of golf balls , continued to fall and blocked the outlets . I estimate the event lasted about 10 minutes . The temperature had peaked at 26.8 º C at 4:50 p . m . By 7:15 p . m . it was down to 19.7 º C , dew point temperature 16.1 º C , relative humidity 80 %. At the height of the storm the temperature fell to 14.1 º C between 8:00 p . m . and 8.15 p . m . Rainfall rate as measured by the Davis Vintage Pro 2 PWS peaked at 256.0 mm / hour between 7:30 p . m . and 7:35 p . m . As the rain gauge funnel is quite large , this may represent the true rate even though the hailstones may have obstructed the outlet somewhat . The total rainfall between 7:30 and 8:30 p . m . when the rain ceased was 16.3 mm . The depth of hail on level surfaces was at least 7.5 cm with hail banked up against west facing walls to half a metre . There were banks of unmelted hail as drifts located in sheltered spaces in the shade as late as 11 am the following morning .
A second storm without hail crossed the station between 8:30 and 9:35 p . m . with a maximum rainfall rate of 24.4 mm hr -1 at 9:00 p . m , adding a further 5.6 mm of rain to the total of 22.5 mm . With hail still blocking the roof gutters again there was considerable overflows .
I had water come through the ceiling through a join in my office roof as the hail banked up on the flat galvanised corrugated iron , not helped by hail sliding down off the main house roof . At one stage after the storm , the depth of hail on the back veranda roof