Freezes and frosts are possible in the Philly region Monday, with a very balmy Halloween to follow

What almost certainly is going to be a historic weather week in Philadelphia is going to begin with a decided chill.

Temperatures are forecast to fall well into the 30s outside the city early Sunday, although the region is likely to be spared any widespread frost, said Ryan Adamson, a meteorologist with AccuWeather Inc. But that would be a warm-up for Monday morning.

The National Weather Service has all of Philly’s neighboring counties, except Delco, under a potential plant-killing freeze watch from 2 to 6 a.m. Monday, for readings dropping to 32 or below. Once upon a time that would be cold enough for ice to form on puddles, except that these days the region is pretty much puddle-less.

» READ MORE: First freezes have tended to show up later in Philly

Urbanized heating that retards nighttime cooling and a gentle north wind would spare the city from freezing temperatures, but frost — which can occur at temperatures as high as 36 — is possible early Monday in Philly and Delco, the weather service says.

Monday also would be the day that the region in all likelihood is going to set a record for the most consecutive days without rain — 30 — besting the 29-day streak that ended on Nov. 8 way back in 1874.

And that frost and freeze potential would be very much tied to the dryness, forecasters say.

The ultradry air and fair skies allow daytime heat to radiate rapidly into space, with cooldowns occurring quickly once the sun backs off.

In addition, winds will be quite light early Monday, and that will enable the cool air to settle peacefully near the surface.

But winter is going to wait its turn.

Come Tuesday, daytime highs will approach 70, and Thursday may mark one of the warmest Halloweens on record.

Chances also are excellent that by 12:01 a.m. Nov. 1 Philadelphia will have experienced its driest month ever. The current record is held by two Octobers — 0.09 inches in 1924 and 1963. This one is cooking along at 0, save for an immeasurable “trace” observed when most people still were in bed on Thursday.

» READ MORE: The dryness around here is about to make history

October usually is one of the driest months of the year; usually, it’s not arid.

Adamson said that an approaching front could ignite a gully-washing 0.01 or 0.02 inches before midnight Thursday.

However, trick-or-treaters will be safe from the elements, but don’t be surprised if some sweat develops. AccuWeather is calling for a high of 80, and with winds from the south in advance of the front, it will stay warm even after the sun sets, Adamson said.

“Sometimes you have to bundle up under the costume,” he said. “You won’t have to this year.”