Happy October! Break out your swimsuits and crank up the A/C.

As promised by the weather wizards late last week, a surge of warmth is ushering in a brief but mighty heat wave across Southern Ontario with temperatures expected to hit 30 C by Tuesday afternoon.

Humidex values will spike as high as 38 C in Toronto, if meteorologists are correct, making it feel more like mid-summer than the second week of fall.

Enjoy it while you can.

Mid summer-like (& record breaking) heat & humidity for October 1st! Highs near 30C & humidex in the upper 30s! Increasing risk for thunderstorms this afternoon & evening w/ strong winds & torrential downpours; Periods of rain tonight & Wednesday; Autumnal late week & weekend pic.twitter.com/E1mlo5Reoh

— Doug Gillham (@gtaweather1) October 1, 2019

"Tuesday will be the last hot day of the year in Ontario," says the Weather Network, which is calling today's blast of heat "summer's grand finale."

 "As toasty as it looks set to get, the heat will be short-lived, with temperatures dropping beneath a cold front passing through later Tuesday that will also raise the risk of storms producing torrential downpours."

The heat wave should peak at around 30 C around 2 p.m. on Tuesday before temperatures start to plummet and rainy weather takes hold.

A more than 20-degree temperature swing is expected between now and Wednesday night, according to Environment Canada, with a forecasted overnight high of just 7 C.