NORWAY — The National Weather Service has issued a flood warning for Oxford County and much of New Hampshire for late Wednesday through Thursday afternoon.

The warning says that the combination of rainfall and snow melt on frozen ground causes ice to break up on rivers. As the ice breaks up, it can get lodged at bridges, bends and islands and cause an ice jam, flooding the river upstream.

Water can quickly back up behind a jam, causing flooding with little or no advance warning, the weather service says. Snow and ice can also block drains and culverts, causing smaller localized flooding.

Those living in areas prone to flooding are urged to monitor forecasts and be prepared to take action if waters rise. The NWS warning includes the towns of Andover, Roxbury, Upton, Rumford, Norway, Oxford and Fryeburg.

The weather service says dense fog is also possible Wednesday night.

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