Severe Tropical Cyclone Jacob

15 - 28 February 1985


Early on I5 February, satellite and synoptic data indicated a weak low off the eastern tip of Timor. A day later the convective activity became more vigorous and extended over most of the Timor Sea. The low moved steadily in a westsouth-westerly direction along the monsoon trough and slowly intensified to reach tropical cyclone intensity by 1800 UTC 16 February. Jacob became the fifth tropical cyclone active in a monsoon trough which stretched from the north-west coast of Australia to the east coast of Africa. The tropical cyclones outside the Australian region were Feliska near 12.0°S, 47.0°E and Gerimena near 16.S°S, 56.0°E.

After attaining tropical cyclone intensity the rate of development of Jacob changed from slow to nor- mal according to the Dvorak satellite analysis scheme. Peak intensity of 950 hPa was estimated to have been reached at about 0600 UTC 20 February. An unusual feature of the time variation of intensity of Jacob was the long duration of near peak intensity. Estimated central pressure was below 960 hPa from 0600 UTC 19 February to 0001 UTC 24 February (4.75 days). During this period the surface circulation extended 800 km from the centre and gales were reported up to about 550 km from the centre. The highest wind speed reported was 93 km/h from a ship located 190 km from the centre at 0600 UTC 21 February. At 2300 UTC 23 February a buoy about 25 km south of the centre recorded a pressure of 961.6 hPa and at 0136 UTC 25 February a buoy about 110 km to the northeast of the centre reported a pressure of 988.7 hPa.

Entrainment of cool and relatively drier air from the south intermittently suppressed convection over the southern quadrants of the storm from 22 to 26 February. The inflow of cool air from the south, highlighted by the south to north orientated low stratocumulus cloud- band near 20°S, 101°E, has suppressed convection in the southwest quadrant of Jacob . During this period Jacob closely (to within about 100 km) paralleled the track of Isobel four to five days earlier. The mixing and cooling of the ocean surface in this area by Isobel very likely had a weakening effect on Jacob .

From 24 to 26 February Jacob decayed more rapidly over the cooler waters but maintained a banding structure with convection near the centre, until early on 26 February when the high-level circulation was sheared by upper-level northwester lies. Over nine days from 1200 UTC 17 February to 1200 UTC 26 February, while above cyclonic intensity, Jacob travelled about 4800 km at an average speed of 20 km/h.

There was no known impact associated with TC Jacob .

For more details see the TC Jacob Report (pdf)

Track and intensity

All times in WST - subtract 8 hours to convert to UTC.

Best Track of Severe Tropical Cyclone Jacob