The most powerful storm for a number of years, with gales of more than 80mph, could hit much of southern England and Wales, though it may miss Britain entirely.Gales of more than 80mph could bring down trees or cause damage to roads and buildings, potentially causing major transport disruption and power cuts.The Met Office has upgraded its yellow alert warning to amber, with forecasters predicting the storm will have a high impact and is more likely to happen.Eddy Carroll, chief forecaster at the Met Office, said: This storm doesnt exist at the moment, but our forecasts models predict it is likely to develop in the west Atlantic on Saturday. Then its likely to rapidly intensify just west of the UK late on Sunday before tracking across England and Wales early on Monday. There is still a chance this storm may take a more southerly track and miss the UK, bringing impacts elsewhere in northern Europe, but people should be aware there is a risk of severe weather and significant disruption. With that in mind, people should keep up to date with and act on the advice in our forecasts and warnings as the situation develops. The warning from the Met Office said: A very intense low pressure system is forecast to run northeastwards across the country early on Monday, bringing the potential for an exceptionally windy spell of weather for southern parts of the UK. At the same time, persistent, heavy rain could cause some surface water flooding. There is some continuing uncertainty in the timing, intensity and track of the low. However, the public should be prepared for the risk of falling trees as well as damage to buildings and other structures, bringing disruption to transport and power supplies.A developing storm is expected to reach the UK later on Sunday. This is expected to run northeastwards, probably across England and Wales, with very strong winds on its southern and western flanks. There is the potential for gusts of 60-80 mph quite widely and locally over 80 mph, especially on exposed coasts, both in southwesterly winds ahead of the low centre and west to northwesterly winds behind it.