It said the depression was centred over Nan at 10am on Saturday at latitude 19.0 degrees North, longitude 100.5 degrees East.
With maximum sustained winds of about 55 km/h, the storm was moving west at a speed of 30 km/h, and was forecast to decline to a low pressure system and approach the North later on Saturday.
The depression was expected to unleash heavy downpours in the North and other parts of the country, the department added.