A SYNOPTIC DESCRIPTION OF THE FIRST NATIONAL AUSTRALIAN STORM CHASE
Clyve Herbert |
| 1st October 1998 |
| A large high pressure system off the N.S.W. coast extends a strong ridge along the Q.L.D coast (this high pressure area had suppressed activity over northern N.S.W. the preceding 4 days). A weakening cold front was moving steadily across the Australian Bight and was accompanied by a pre-frontal trough and a small low near Mt Gambler. The prefrontal trough passed over my location of Leopold (Geelong), at 1700 hrs EST, producing a wind gust of 85 kph and about one hour of light rain. The cold front followed at about 1900 hrs, bringing much cooler air. This system brought an end to a five day warm spell which was somewhat unusual for late September. The 300 hPa analysis shows a jet with maximum winds above l00 knots extending from the head of the Australian Bight to the south island of New Zealand. |
| 2nd October 1998 |
| By early on the 2nd October the southern portion of the pre-frontal trough has moved quickly to the southern Tasman sea, however the trough has started to slow down appreciably across northern Victoria and south western N.S.W. The surface cold front is undergoing frontolysis across northern Victoria and a second cold front is approaching from the western Bight area. At 300 hPa a jet maximum extends across southern Victoria although the strongest winds are receding southeastward towards New Zealand. This has an effect of shearing the southern portion of the trough rather quickly to the east, leaving its northern portion stalling over the Australian mainland. |
| 3rd October 1998 |
| A second cold front is now crossing Victoria. The previous day's cold front has sheared away to the east and is crossing New Zealand. The former pre-frontal trough is still stalled over western N.S.W. The second cold front is amalgamating with the inland trough over the south west slopes of N.S.W. The anticyclone off northeast N.S.W. is weakening with a new ridge pushing into the northern Bight. At 300 hPa wind speeds have declined over south eastern Australia but note the strengthening of the 300 hPa flow across south western WA showing the early stages of cyclogenesis that will effect eastern Australia on 6th October. The Sydney storm chasers were on Rooty Hill at the time of this synoptic chart viewing patches of "lee high cirrus" and weak cumulus with the temperature registering at 32.6C and humidity values around 30 percent. |
| 4th October 1998 |
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| Enhanced satellite image 0802Z 4th October 1998 (NSW coast) |
| A new closed anticyclone has moved to western Victoria and the cold front has joined with the inland trough over central N.S.W. and separates a weakening high pressure ridge extending from northern New Zealand from the southern high pressure area over Victoria. The trailing clearing edge of the trough was marked by a line of altocumulus castellatus that drifted slowly northward from Sydney during the early morning. The storm chase team intersected the first thunderstorm development near the Liverpool Range, which later developed into a strong multicell. This development coincided with maximum day heating and the passage of the surface trough. At 300 hPa a marked trough is apparent across north eastern NSW. Also note the continued 300 hPa strengthening of the jet across southern WA |
| 5th October 1998 |
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| The high pressure system over Victoria the previous day has moved to the Tasman sea and is combining with a high east of New Zealand, but is still separated by an intersecting cold front that extends to the northeast tip of N.S.W. The former inland trough which incidentally has survived as a feature since the 1st October can still be seen extending westward to the northwest slopes of northern N.S.W. Again, as the trough lumbered northward its clearing edge was marked by a line of altocumulus castellatus. The chase team positioned themselves at Tenterfield to watch strong growth developing again along the surface trough The team intersected a major development just south of Stanthorpe near Storm King Dam which became a severe multicell, with over shooting tops, backsheared anvil and 2cm hail around 5pm At 300 hPa a major change is occurring over virtually all of southern Australia with considerable strengthening of the 300hpa flow strength in response to marked cyclogenesis west of Tasmania. Also note the increase of the westerly 300 hPa flow over northern N.S.W. and southeast QLD. which coincided with a phase of more intense thunderstorm activity in these regions late on the 5th. |
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| Inflow column to severe multicell - Storm King Dam - 5pm Photo - C Herbert (ASWA archive) |
| 6th October 1998 |
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| A rapid change has occurred since the previous day. The Sydney chasers were at Stanthorpe. An active cold front has pushed into NSW and a deep low is near Tasmania with a central pressure below 975 hPa and a strong cold airmass follows the cold front. An extensive cloud band is associated with this system. A pre frontal trough is evident over southwest QLD. and a small low is analysed over far north N.S.W. At 300 hPa the jet maximum has advanced northward to the southern QLD border. Note the joining branch over north central NSW which is occurring over the small surface low in that area. The Sydney chase team viewed the passage of the cold front at Stanthorpe. The frontal passage produced about 15mm of rain and slight thunder, and a very pronounced wind change from the northwest to southwest direction . Falls of rain reached 40mm over the northern tablelands of N.S.W. with the passage of this frontal system. |
| 7th October 1998 |
| The cold front has moved offshore from eastern Australia except for the trailing northwest tip which is extending into central east of QLD. A large area of cold air has spread through all of N.S.W. and southern QLD. The deep low has moved rapidly to the south of New Zealand. Note the orographic ridge along the west coast of south island of New Zealand with a corresponding lee trough along the east coast of the island. This interesting feature can often be found as the pre-frontal northerly airstream is blocked by the high mountains of the south island. At 300 hPa the jet maximum has aligned almost southwest to northeast across south Australia and south west QLD and has weakened substantially from the previous day. Another feature is the presence of an upper cold pool over the Tasman sea to the west of the surface cold front. |
| ACKNOWLEDGEMENTS |
| Ray Kollmorgen of Bureau of Meteorology,
Melbourne, for provision of the hand drawn 300 hPa analysis charts. MSL Analysis charts courtesy of the Bureau of Meteorology |
