Ottawa and the rest of eastern Ontario can expect to be plunged into the deep freeze as people head back to work and school.

Frozen tree branches

A winter storm in Ottawa on the first weekend of 2015 was followed by a quick freeze as temperatures dropped across southern and eastern Ontario. (Michel Aspirot/CBC)

After an unseasonably warm Christmas holidays, Environment Canada says a frigid air mass could bring temperatures as low as -30 C on early Thursday morning.

On Monday, as the air mass moved in, the temperature was expected to hold at about -15 C, but with the windchill it will feel like -25 C.

The City of Ottawa has issued a frostbite advisory beginning on Monday until further notice.

The temperature Monday overnight is expected to drop to -20 C and rise Tuesday to a high of -10 C, with flurries possible.

Frozen trash can Ottawa

Icicles form on a downtown Ottawa garbage bin as temperatures dropped suddenly overnight on Jan. 5. (Matthew Kupfer/CBC)

On Wednesday, there is expected to be a mix of sun and cloud and possible flurries, with a high of -16 C.

Wednesday overnight and early Thursday morning is when the temperature really drops to an expected -30 C in Ottawa, and -31 C in towns like Pembroke and Petawawa.

The temperature should warm on Thursday to a high of -11 C.