Malaysia Meteorological Department stated that Northern states of Malaysia will be having a wet weather & possibility of flood on certain areas.
Not sure on how will it be effected Langkawi however, i suggest all tourist & visitors need be aware. The weather in Malaysia is unpredictable. At one part it might be wet & after 5-10 minute drive the weather suddenly different.
There's expected to be an increased of wave height & wind speed. Wave expected to be from 1.0m to 1.5m. If you wish to go for island hopping tour make sure all the safety life jacket provided. Don't be hesitate to go back if you see the weather condition beginning to change from good to bad. Most of the island hopping operator aware about this & if the operator decide to stop the tour, don't object because they know the weather well.
Anyway, for those who's been planning to come & already in Langkawi during wet season. Make an alternative plan to round the island by going to dry places such as Underwater World, Craft Center, Buffalo Park, Crocodile Farm etc2.
Avoid slippery waterfall & if you still wish to go there, be cautious. DONT let yourself be in one of the statistic of falling from the waterfall especially in Telaga Tujuh (Seven Well). Have fun & be cautious during this wet season.
From time to time I suggest you closely monitor the weather condition here & plan your activity accordingly.