|A photo of the tornado from a local resident. It was accompanied by hail. Photo Source: Stawell Times|
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A radar loop of the event can be found by clicking below, note the cell near Stawell and Ararat between 2pm and 5pm local time.
Based on radar analysis, it looks to me to be a left moving type supercell storm ( intially I had my doubts and thought it was right moving intially), but if you look carefully between 2pm EST, and 4pm you can see the cell actually move left of the mean flow from Stawell to Ararat. It seems like the cell initiates around 2pm, and disapates/gusts out around 5pm after generating a right split. Three factors therefore convince me this was a supercellular tornado - longevity, deviant motion from mean flow and splitting behaviour. There is also some indication from the eyewitness that the storm was very much tornadic, with the tornado not being that short lived : "Then suddenly it deviated away to the right, broke down and formed another small twister in a neighbour's paddock.", this would suggest an unstable vortex phase or possibly a second tornadic circulation. From the lifting/damage described (particularly trees ripped out and moving the toolbox) I would estimate it was probably an F1. There is also some indication that largish hail was also present within the storm given the mentioning by the resident, and this was likely a result of the supercellular processes at play (which with extremely cold uppers can overcome the relatively low instability).
The maximum temperature on the day was 13.9, but a 3pm observation at Stawell aerodrome gives a 11.9 degree temperature with 85% RH and southeasterly surface winds and a surface pressure of 1011.7hPa. A quick look at the synoptics suggests an incipient low pressure system located in NW Victoria as of 00UTC, with a trough to the southeast, forming in the preferential post-frontal air mass (typically the unstable airmass following a frontal passage on the equatorial side will lead to secondary cyclogenesis).
By 06UTC the surface low had deepened to 1009hPa suggesting cyclone intensification and a slight move to the Southeast.
Checking the radar we also can identify that the cells death was preceded by a shift of wind direction at steering level to northeasterly while prior to this motion was front the north. Based on the trough observation from analysis, and this behaviour we can conclude that the cell was initiated by the trough, and developed in a favourable pre-trough regime (almost warm-front like from the moisture observations).
Looking at the upper air information at 00UTC:
The shear characteristics are very hard to work out without an upper air sounding, but the radar and surface observations give some clue. Surface observations were southeasterly that fits with the presence of the trough as the initiating boundary, and is also the direction the storm ended up moving, and hence the cell likely enhanced its storm relative helicity (tendency to spin) by doing so. General motion of cells without deviant behaviour was to the south, suggesting overlying northerly flow. Motion of cells not deviantly moving was of the order of 25-30 knots which would suggest at storm steering level (around 700mb) that we had 25-30 knot northerlies at least, possibly more. Overlying shear was probably much stronger given the type of environment, and a quick look at 300mb yields:
A strong northerly jet on the lee side of the developing cyclone. Based on this information, we therefore know we had an environment highly conducive to left moving supercells, with turning from SE to Northerly shear vectors producing useful helicity to any storm that developed.
This leads me to my final analysis: This event was a cold core tornado that was produced by a reasonably long lived, left moving, low-topped supercell in a low instability, highly sheared environment. The environment for the cells lifetime was characterised by high relative humidity (85%), low instability (sub 200 J/kg CAPE, but -2 LIs in the 0-700hPa lift), and reasonably high cyclonic shear (likely in excess of 50 knots). These conditions are perfect for the development of low-topped supercells, and given the relatively low LCLs, turning within the cell along with helicity increases due to motion into the surface inflow, and low level instability associated with high lapse rates, it is little wonder that it produced an F1 tornado. These systems are relatively typical for postfrontal airmasses or in the vicinity of low centres in southwestern Victoria (See Coloquhuon et al. 2008 and other studies).
If you are interested to know more, please leave a comment or question.
Acknowledgements: The Bureau of Meteorology for synoptic analyses, The Weather Chaser for radar loop and the Stawell Times for the photo.