Southern England and south Wales are on amber alert in anticipation of torrential rain and 80mph gales expected on Monday in what could be one of the worst storms since 1987.

"The storm is developing over the Atlantic and is likely to hit south-west England just after midnight on Monday morning and then track north-east across the country," Emma Compton, a Met Office forecaster, told the Observer. "We have three grades of warning – yellow, amber and red. Amber simply means 'be prepared'."

The Environment Agency has warned of widespread flooding caused by up to 25mm of rain in six hours. High winds could bring down trees, damage homes and buildings and bring power cuts and transport disruption during Monday's rush hour.

The exact timing and path of the storm is still uncertain. "It could sweep south into the Channel and miss us completely," added Compton. "Further satellite observations will make it clear when and where it will strike."

Frank Saunders, chief forecaster at the Met Office, told the Press Association on Saturday night: "We are confident that a severe storm will affect Britain on Sunday night and Monday. We are now looking at refining the details about which areas will see the strongest winds and the heaviest rain.

"This is a developing situation and we'd advise people to stay up to date with our forecasts and warnings over the weekend, and be prepared to change their plans if necessary. We'll continue to work closely with authorities and emergency services to ensure they are aware of the expected conditions."

Atlantic storms of this type usually develop further west across the ocean, losing strength by the time they reach the UK and Ireland.

But this is expected to appear much closer to land, potentially moving across the country while in its most powerful phase. A strong jet stream and warm air close to the UK are contributing to its development and strength.

Chris Burton, a forecaster with MeteoGroup, the weather division of the Press Association, said: "We are going to see some heavy and persistent rain heading north across England and Wales overnight from early this evening, from the south-west.

"As the rain pushes north the winds will pick up, and by midnight there should be gusts of about 60mph across south-west England.

"Through the early hours much of southern England will see winds of 60-80mph, maybe closer to 90mph in exposed areas on the south coast.

"Further north, in south Wales, the Midlands and East Anglia these will be between 50mph and 70mph.

"The storm is going to move through very quickly, and as we go into Monday morning the winds will ease off from the south-west, and by lunchtime most areas will start to see the winds ease off and die down."

The Environment Agency has teams working to minimise river flood risk, clearing debris from streams and unblocking culverts, and are closely monitoring water levels so they are ready to issue flood warnings if necessary.

A spokesman said: "We are supporting local authorities who will respond to any reports of surface water flooding.

"Seafronts, quaysides and jetties should be avoided due to the risk of overtopping by waves and wind-blown shingle."

Martin Hobbs, head of asset resilience at the Highways Agency, said: "Be aware of sudden gusts of wind and give high-sided vehicles, caravans, motorbikes and bicycles plenty of space."

Home insurers were bracing themselves for a large number of storm damage claims.

Ferry services have been cancelled for Sunday and Monday between Plymouth and Roscoff, and between Penzance and the Isles of Scilly.

The weather front has been christened the St Jude storm by forecasters. St Jude is the patron saint of lost causes, whose feast day falls on Monday.