The yellow weather warning is in place from 5pm today through to Friday 10am with much of north east Wales and the north west of England affected.
The second highest level of warning advises that there will be very poor visibility in "many inland areas" this afternoon and evening and it is expected to worsen later this evening and tonight.
"This fog will become more extensive overnight and we are expecting frosts in the morning with temperatures expected to drop to below freezing".
It will be cold tonight, with dense fog and some frost for a time especially in midland, northern and eastern areas with overnight lows of -1° to +3°.
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Up to four inches could fall over those two days, with forecasters warning of blizzards and frequent lightning strikes, which could cause power cuts and mass travel disruption. It will be mainly dry at first with a few bright spells in eastern areas, but rather cloudy overall, with patches of drizzle and mist scattered about. Top temperatures 7 to 10 C. The rain will peter out after dark and it will become misty.
Friday is set to be breezy followed by heavy rain in Munster or Connacht. Lowest temperatures are expected to be 4° to 8°.
"Fog will be quite stubborn to shift and could still be an issue into tomorrow afternoon, there are some places where it won't clear all day".
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