An area of low pressure in the northern Bay of Bengal is already bringing huge amounts of rain to the western part of Myanmar between Sittwe in the north, and Yangon in the south.
Some 228 millimetres of rain fell in Maubin in the 24 hours ending at about 00:00 GMT on Tuesday, and 77mm fell in Yangon.
The circulation may make landfall before it has time to be classified as a cyclone, but the rain will continue regardless.
The main focus of the rain will transfer northeastwards across the country, but in its wake, there will be further heavy and prolonged showers.
This may mark the real onset of the monsoon across Bangladesh and Myanmar. Rainfall across the region doubles between May and June.
Flooding from monsoon rain causes scores of deaths each monsoon season. As recently as 2015, 100 people died in the floods, and in 2016 more than 400,000 people were displaced by floodwaters.
The heaviest of the rain is not expected to affect the Rohingya refugees in Bangladesh in the immediate future. For the hundreds of thousands of Rohingya who have fled Myanmar to Bangladesh, the monsoon season is a particularly worrying time.
The camps are often in the most flood-prone areas, and flimsy plastic sheeting and tents are vulnerable, both to heavy rain and strong winds.
In recent weeks, the UN Refugee Agency (UNHCR) has been supporting Bangladesh’s largest non-governmental organisation BRAC in the training of 20,000 community outreach members in emergency preparedness.
“As heavy rains and strong monsoon winds approach, it is critical that we stand firmly with the Rohingya refugees we serve,” said Kevin J Allen, UNHCR Head of Operations based in Cox’s Bazar.
“It is equally important that we empower and train refugees to play their role in the response, working hand-in-hand with the national authorities, sister UN agencies and civil society.”