We had a fairly typical "Sumatra" on 15 Aug. "Sumatra" is a nickname for a squall line that often forms over the Indonesian land mass of Sumatra & then moves over the Malacca Straits to affect Singapore and Malaysia.

This particular Sumatra seems to have started organising itself just before affecting Singapore.

"Sumatras" often affect us during the night, between 3am and 6am; one can see a line of echoes on the radar steadily making its way eastward. If there's no bad weather around we can usually get a bit of rest during night shifts between 11pm and 3am, but if you're expecting a "Sumatra" then you can forget about getting any rest because you have to keep monitoring it.

Sometimes the squall line weakens and dissipates upon moving over land, but the stronger ones will continue moving eastward out into the South China Sea.

This one has reorganised into quite a distinct line but looks as if it's weakening as it moves out to the sea area.

Sumatras are one of our more intense weather systems & usually occur during the Southwest Monsoon, but we can actually get them any time the steering level winds change to southwesterlies. They bring strong gusts & heavy rain, & spectacular lightning & thunder. They can move in during the day instead, & we usually refer to these as "late Sumatras". Once you know one is coming in, then forecasting is standard. You can usually estimate what time it'll arrive & get as much of your routine work done as possible plus get all the warnings ready.

I remember nights at home hearing the onset of one. You can hear the wind picking up & the tinkle of flowerpots falling & doors slamming. One colleague joked that it sounded like a typhoon coming. It can often be seen approaching in the early morning too, a line of cumulonimbus moving in from the west, the cloud tops pink or gold in early morning sunlight.

A schoolteacher called one morning, sounding worried because she & her students had seen one. She quavered, "Oh ... is it ... is it a front approaching?" She was probably a geography teacher & her students must have been asking her about it.

Of course the squall line often just misses Singapore, passing to the north over Malaysia or to the south instead. Sometimes if the steering winds are northwesterlies, the radar will show a line of echoes moving down the Malacca Straits, only affecting the western or southern parts of the island.

A colleague was once annoyed because his Sumatra seemed to be heading for Singapore dead on, but just before hitting the island, suddenly divided into two. One part went north, while the other passed south of us, & after skipping the island it actually merged back into one and moved out into the South China Sea! He couldn't believe his eyes; he had gotten all his warnings ready & been forecasting strong winds & rain for nothing. He was convinced the squall line had done it on purpose to spite him! Well, that's the nature of forecasting.