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Tropical Storm GREG (43W), 21-27 Dec 1996

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MonsoonMaiden

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Tropical Storm "Greg" (43W) was an unusual weather system in that tropical cyclones (TCs) seldom occur at latitudes low enough to affect Malaysia. The country is divided into West Malaysia (Peninsular Malaysia) and East Malaysia (consisting of the states of Sabah & Sarawak).

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GMS IR image at 240400Z. 2 hours later, the JTWC upgraded the cloud cluster to Tropical Depression status

"Greg" was first observed as a cloud cluster that developed on the night of 20 Dec 1996, over the South China Sea to the northeast of East Malaysia. Cloud clusters are a common feature in this area during the Northeast Monsoon, but this one proved to be unusually long-lived, with convective development enhanced by low-level convergence between westerly winds and northeasterly monsoonal flow. On 240600Z the Joint Typhoon Warning Centre (JTWC) assigned it tropical depression status. Winds to the north of the system continued to be enhanced by the northeasterly monsoon, so that by 250000Z tropical storm intensity had been reached. The storm's movement was unusual : it began tracking east-southeast toward the East Malaysian coast.

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"Greg" affecting Sabah on Christmas night.

“Greg” moved into Sabah at around 251600Z (Christmas night), depositing heavy rains that triggered floods and caused rivers to overflow their banks. Powerlines were downed, water supplies disrupted and road and communication links washed out. The storm affected a total of 17000 people from 226 villages along Sabah’s west coast. At least 163 people died, most of them migrant workers. The sad thing about these workers is that many had entered the country illegally from Indonesia & were never accounted for. Another 3000 were left homeless.

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Post-analysis from JTWC 1996 Pacific Typhoon Season Summary :

Greg's east-southeastward motion was very unusual. TCs which form within (or move into) the South China Sea late in the year are often blocked from moving west by well-established northeasterly monsoon flow. Such TCs often remain quasi-stationary or move southwestward and dissipate.

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242330Z Dec GMS imagery showing five tropical cyclones : Greg (G), Fern (FN),

Ophelia (OP), Phil (P) and Fergus (FG) lying within twin monsoon troughs

Greg formed in the South China Sea when an unusual large-scale wind pattern dominated the region : During the second half of Dec 1996, twin low-latitude monsoon troughs became established between approx 100 deg E and 170 deg E, one trough north and the other south of the equator, and a belt of low-level westerly winds existed in equatorial latitudes between them. A total of five tropical cyclones - two in the northern hemisphere and three in the southern hemisphere - were formed within these monsoon troughs.

It is hypothesized that the strong westerly winds to the south of Greg provided the flow asymmetry responsible for its eastward motion. This factor plus the existence of the large circulation of Fern (42W) to Greg's northeast were cited as possible sources of the east-southeast movement of Greg.

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