Fog will develop in the early hours of Saturday and will become widespread across the whole of the east of England by mid morning.
The “be aware” alert is valid from 1am until 11am and is for the whole of Dorset.
Motorists in Herts and Essex can expect hard driving conditions on Sunday morning – with visibility possibly as low as 50m in places – as the Met Office issued its second fog warning of the weekend.
Similar traffic disruption can be expected on Thursday morning and the PSNI has urged drivers to use their fog lights, as appropriate for the conditions.
Saturday will see sunny intervals throughout the day with highs of 16C and Sunday will be cloudier with similar temperatures.
The added dangers posed by fog is that a collision involving one vehicle can quickly involve many others, especially if driving too fast and too close to one another. “Vulnerable road users should avoid, cycling or walking in fog unless it is a necessity”.