If it’s been a while since you checked your car’s battery and block heater, today might be a good time to do it.

Environment Canada says starting Sunday, the mercury is going to “plunge,” dropping overnight temperatures in Thunder Bay down to below -20 C.

“Polar air that’s sweeping through the prairies will come into Northwestern Ontario as well,” meteorologist Gerald Cheng warned Friday.

Cheng said it won’t warm up much during the day, a trend that will continue through to the middle of next week.

The normal low in Thunder Bay at this time of year is -14 C. The daily high is -4 C.

The deep-freeze will likely come as a shock after a stretch of above-normal temperatures that dominated the second half of November. The first half of that month was unseasonably cold.

November was also unusually dry. Thunder Bay received only 34 millimetres of rain, compared to the normal amount for that month of 56 mm.

Cheng said slightly warmer temperatures by the end of next week may be accompanied by an Albert Clipper that is expected to bring some snow to the Northwest, although not in large amounts.

After that, below-normal temperatures are forecast to return to the region, Cheng said.

Meanwhile, provincial police are advising sledders to exercise caution on waterways as ice thickness remains thin in many areas.

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