TC ILSA REPORT

10th to 17th December 1999

By Carl Smith
carls@ace-net.com.au


Click here for TC ILSA Map Animation

REPORT

See notes at end for source information etc..

During the first week of December 1999, monsoonal activity was building in the region to the north of Australia, stretching across southern Indonesia, far northern Australia, southern Papua New Guinea, and the Solomon Islands. A small Tropical Low developed in the Coral Sea off Cape York peninsula resulting the BoM issuing a CYCLONE WATCH for the Cape York area, but the Low soon weakened and moved east, away from the coast. During the second week of December, intensifying activity, called a monsoonal burst by the BoM, began to concentrate on the Indonesian end of this range. From this monsoonal activity, 2 cyclones were born, TC ILSA (01S) and TC JOHN (02S). TC JOHN is the subject of a seperate report.

TC ILSA formed in the South Indian Ocean to the west of Christmas Island and the northeast of the Cocos Islands, with JTWC issuing a warning for TC 01S at 2100 UTC on the 10th of December noting that it was tracking east-southeastwards at 3 knots (KT) and that a 1439 UTC Special Sensor Microwave Imager (SSM/I) pass and a 1354 Tropical Rainfall Measuring Mission (TRMM) pass indicated a well defined Low Level Circulation Centre (LLCC) with a convective band to the north begining to wrap in towards the center. It was named by the BoM Perth at around noon WST (0200 UTC) on the 11th of December 1999 when a Tropical Cyclone WATCH for a Category 1 cyclone was issued for Christmas Island.

It was upgraded to Category 2 at noon WST (0400 UTC) on the 12th of December with central pressure of 985 hPa (hPa = millibars = MB) and maximum wind gusts of 130 km/hr (70 KT) when about 400 km (215 NM) westsouthwest of Christmas Island and 600 km (325 NM) east of the Cocos Islands moving southeast at 11 km/hr (6 KT). JTWC noted at 2100 UTC on the 11th that a SSM/I pass depicted deep convection continuing to build in towards the LLCC specifically from the northeast, and imagery indicated a convective band developing southeast of the LLCC, with synoptic data showing stronger winds in the northern half, and they forecast it to track generally southeastward within the mid-level steering flow of the subtropical ridge extending from northwest Australia.

ILSA was then downgraded to Category 1 at midnight WST on the 12th-13th (1600 UTC 12th) with central pressure having risen to 990 hPa (MB), whilst JTWC noted at 2100 UTC on the 12th that animated imagery depicted persistant deep convection particularly to the northeast of the LLCC, with imagery revealing improved organisation of the system over the previous 6 hours and enhanced infrared imagery indicating continued cloud top cooling, and water vapor imagery continuing to show good outflow aloft.

ILSA made it's way south-southeastwards, then eastwards, passing well to the south of Christmas Island, and the watch for Christmas Island was soon cancelled. It continued to move in an east-northeast to easterly direction over the next few days, with the central pressure varying between 985 and 990 hPa in the BoM shipping warnings, towards the fast developing and more dominant TC JOHN, which no doubt inhibited further development due to increasing vertical shear from it's outflow.

After SEVERE TC JOHN Category 5 made landfall over northwestern Australia on the morning of the 15th of December and began to weaken, TC ILSA was now moving into the area to the north of Western Australia, and was entering a more favourable environment for development, so causing some concern for communities that had just experienced TC JOHN. As JOHN weakened overland, so ILSA slowly intensified, with northwest Australian coastal communities under a cyclone WATCH on the 15th, and being Category 2 when the first WARNING was issued at 8:05 am WST (0005 UTC) on the 16th for communities between Cape Leveque and Pardoo, saying that central pressure was 985 hPa (MB) with maximum wind gusts of 150 km/hr (80 KT) near the center moving southeast at 25 km/hr (13 KT). JTWC noted at 0300 UTC on the 16th that visible satellite imagery depicted a partially exposed LLCC about 30 NM east of the deep convection, and that animated satellite imagery showed that the deep convection had decreased in areal extent and was symmetric, with animated water vapour imagery showing good outflow in the western quadrant.

During the afternoon, ILSA weakened to Category 1, with the central pressure increased to 990 hPa (MB), the maximum wind gusts decreased to to 110 km/hr (60 KT) and it was moving southeast towards the Eighty Mile beach at 30 km/hr. JTWC noted at 0900 on the 16th that animated visible satellite imagery revealed that the LLCC had become completely exposed with convection sheared about 70 NM to the west-southwest.

By 6am on the 17th (2200 UTC 16th), ILSA had weakened further to 995 hPa (MB) with maximum gusts to 90 km/hr (50 KT). JTWC at 2100 UTC noted that animated satellite imagery depicted that convection had increased in the previous 6 hours, and the LLCC had moved about 20 NM under the convection, with synoptic data indicating that the winds were symmetrical around the LLCC, and Port Hedland radar suggesting that that the convection was loosely organised and of overall moderate intensity.

At noon WST (0400 UTC), it crossed the coast on the Eighty Mile beach near the Sandfire Roadhouse on the edge of the Great Sandy Desert. More than 100 mm (4 inches) of rain was recorded in the Pardoo area, and no further BoM advices were issued. JTWC at 0300 UTC noted that Port Hedland radar suggested that the convection had decreased in areal extent and become poorly organised over the previous 6 hours.

TC ILSA degenerated into a rain depression over the Great Sandy Desert during the next 12 hours or so. This is a largely uninhabited area, and very few people would have directly experienced TC ILSA.


NOTES:

Note 1: Tropical Cyclone Advices and Shipping Warnings used as sources for meteorological information in this report were issued by the Australian Bureau of Meteorology (BoM) Perth Tropical Cyclone Warning Centre and are Copyright Commonwealth of Australia 1999, Bureau of Meteorology - http://www.bom.gov.au. BoM information is used unless otherwise noted.

Note 2: BoM Perth WA issues warnings to the Australian public using kilometers for distances, kilometers per hour for maximum gust wind speeds, hPa for central pressure, and times are WST, which is Australian Western Standard Time = UTC + 8 hours. I have converted all these and added in them brackets, so changing to nautical miles (nearest 5 NM), knots (nearest 5 KT), hPa = MB, and times are also given in UTC.

Note 3: Joint Typhoon Warning Centre (JTWC), Pearl Harbour, tropical cyclone warnings were also used for extra meteorological information where indicated, and that material is copyright JTWC - http://www.npmoc.navy.mil/jtwc.htm.

END REPORT


Maintained by Carl Smith.
carls@ace-net.com.au

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