Britain is about to heat up again although a near-200mph jet stream will sweep in a torrent of thundery downpours. Warm air will waft in from the Continent next week around a huge ‘Scandinavian High’ pressure system settled off the east coast. Temperatures will rise around 5C from the current chilly lows, although torrential downpours threaten half a month’s rain. The weather will be steered by high pressure to the east, low pressure to the west, and a powerful jet stream in between. Met Office meteorologist Alex Burkill said: “Currently, we have an active jet stream across the UK, and it is r…