Bulgaria's National Institute of Meteorology and Hydrology has issued a yellow warning for heavy rain on 1 October in the far southeast of Burgas region, where it expects around 30 litres of rain per square metre.

The first two days of October could bring the worst rain of the autumn so far on the southern Black Sea coast.

An independent weather source has shared more detailed forecast models that point to higher totals in some spots. The regional Swiss HD model suggests 50-70 litres per square metre in the mountain part of Burgas region, mainly near the Turkish border. By Friday, parts of the far south of the Tsarevo and Malko Tarnovo municipalities could see totals close to 150 litres per square metre, though not across the whole of either municipality.

These figures describe a possible scenario for limited areas, not a guaranteed total for the whole southern coast, and the model could still change a lot in later runs. A more moderate outlook points to 20-40 litres per square metre in the far south of the coast, with heavy rain, thunder and gusts of wind up to around 70 km/h.

Forecasters are watching Strandzha, Tsarevo, Ahtopol, Sinemorets, Lozenets and the area around Malko Tarnovo, and keeping an eye on Primorsko and low-lying parts of coastal towns too. If a strong thunderstorm passes through, the rain could be short but very heavy, with totals differing by tens of litres per square metre between two nearby spots.

The stronger rain near the coast comes down to warm sea water, which adds extra moisture and heat, moist air and an unstable atmosphere. The areas most at risk are ravines, small drainage basins and spots where storm drains can't cope, raising the chance of rivers rising fast and low ground flooding.

This is not a forecast for widespread flooding, but a warning of local risk if a storm sits right over a town or village. Meteorologists say people should stay alert at least until the end of 3 October. People in Strandzha and along the far southern coast should stay out of ravines and low-lying areas during heavy rain, and should not drive through flooded roads. If conditions worsen, people should follow updates from the National Institute of Meteorology and Hydrology, local authorities and disaster response services.

On 1 October from 11:00, Burgas region will run a technical check of the National Early Warning and Alert System. The sirens are for the drill only and have nothing to do with the rain.