The Met Office has issued a yellow weather warning over parts of Powys from 10pm on Saturday, December 23, until 6pm on Sunday, December 24.
The weather warning, for rain, covers Llanwrtyd Wells, Builth Wells, Llandrindod Wells, and Rhayader, but has not been issued over Brecon, Presteigne, Crickhowell, or Ystradgynlais.
A spokesperson for the Met Office has said: "A band of rain will slowly move south across Wales on Saturday evening and through Sunday, probably clearing to the south later in the day on Sunday.
"20-40 mm of rain may accumulate quite widely within the warning area, but west-facing high ground could see 60-80 mm of rain during this period. Strong and gusty winds will accompany the rainfall."
Flooding of a few businesses and homes is possible, as well as interruptions to power supplies and other services.
Bus services may be affected with longer journey times taking place.
Spray and possible flooding on roads will make journey times longer.