The wind is accelerated as it is squeezed into the narrow entrance of the Bristol Channel - a classic Venturi effect that is also reinforced with the land mass in the upper reaches of the Severne and South of Bristol heating up. This creates rising air leaving a void that accelerates the colder wind along the channel over the sea toward Somerset and Gloucester in a general Westerly direction to fill the vacuum left by the rising warm air. 

This is typical summer sea breeze pattern found on many beaches in the UK but it is considerably exaggerated by the narrow Bristol Channel. A moderate westerly breeze will often strengthen to 20-25 knots in the spring or summer particularly when the air over the sea is considerably cooler than that over the land. 

The only forecast that seems to predict this effect is windfinder's super forecast. See the map for tomorrow's forecast - high hopes...