If you’ve been putting off buying that window air conditioner because you think this heatwave just couldn’t last, you may want to reconsider.

“At least early in the week” it will be cooler, National Weather Service meteorologist Bill Leatham told the Herald Saturday afternoon, before destroying that fairytale promise of relief with his next line. “And then it warms back up.”

Leatham didn’t have much time to elucidate the week’s weather ahead on Saturday because he and the other team members in the NWS’ Norton office were monitoring some severe thunderstorms about to unleash some havoc from Worcester County to all parts west in Massachusetts.

As Leatham spoke with the Herald, NWS Boston issued a severe thunderstorm warning for the western part of the state — including Northampton, Amherst and North Amherst — set to last until 5:45 p.m.

And Leatham said flooding could come with it, as he predicted an inch and a half to three inches of water coming down in Western Mass. The flood watch will last from Sunday night into early Tuesday.

The rain will help with the extreme humidity, Leatham said, “but not too much.” The dew point — that “temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity,” the NWS explains — will be in the 60s into the middle of the week, while temperatures will continue to grow, simmering into a hot and sticky balance in the air.

As for Sunday, Leatham said residents of Greater Boston should expect temperatures in the mid-70s and possible highs in the mid-80s to be followed by a possible shower or thunderstorm in late afternoon or evening.

That’s a relief to many who probably checked their phones over the past week and had to groan when highs were reporting in the high-80s or even low-90s and temperatures — and that humidity — didn’t drop by much when it came time for sleep.

But that’s what Leatham predicts for Wednesday, the hottest and yet driest day of the week, though it will stay plenty hot from Thursday into the weekend, but with higher humidity.

Saturday was also the 343rd anniversary of what the NWS said could be “the first written record of an actual tornado in what is now the United States,” a storm that ripped through Cambridge and took the life of John Robbins, who worked as a servant in the town and is now believed to be the first recorded tornado victim.

The NWS posted that eyewitnesses to the storm had written that “a thick black cloud in continuous circular motion produced a great noise in the process of tearing down trees and picking up bushes, trees and large stones.”