Philly may experience frost and frost, followed by a balmy Halloween

What will almost certainly be a historic week of weather in Philadelphia will start with a noticeable chill.

Temperatures outside the city are expected to fall well into the 30s early Sunday, although the region will likely be spared from widespread frost, said Ryan Adamson, meteorologist with AccuWeather Inc. But that would be a warmup for Monday morning.

The National Weather Service has placed all of Philly's neighboring counties, except Delco, under a freeze watch from 2 a.m. to 6 a.m. Monday as readings drop to 32 or lower. It was once so cold that ice would form on puddles, only now there are almost no puddles in the region.

” READ MORE: First freezes tended to occur later in Philly

Urbanized heating that slows nighttime cooling and a gentle northerly wind would keep the city from subzero temperatures, but frost – which could hit temperatures as low as 36°C – is possible in Philly and Delco early Monday, the weather service says.

Monday would also be the day when the region is likely to set a record for most consecutive days without rain – 30 – surpassing the 29-day streak that ended on Nov. 8, 1874.

And that frost and freezing potential would be heavily dependent on drought, forecasters say.

The extremely dry air and beautiful skies allow the day's heat to be radiated quickly into space, with rapid cooling occurring as soon as the sun fades.

Additionally, there will be fairly light winds early Monday, allowing the cool air to settle peacefully on the surface.

But winter will wait its turn.

Daytime high temperatures will approach 70 degrees next Tuesday, and Thursday could be one of the warmest Halloween days on record.

There's also a good chance that by 12:01 a.m. on November 1, Philadelphia will have experienced its driest month on record. The current record is held by two Octobers – 0.09 inches in 1924 and 1963. This boils down to 0, except for an immeasurable “trail” observed while most people were still in bed on Thursday.

” READ MORE: The drought here will soon make history

October is usually one of the driest months of the year; Normally it is not dry.

Adamson said an approaching front could ignite a 0.01 to 0.02 inch flood before midnight Thursday.

While trick-or-treating is safe from the elements, don't be surprised if you get sweaty. AccuWeather is calling for a high of 80 degrees, and with southerly winds ahead of the front, it will remain warm even after sunset, Adamson said.

“Sometimes you have to wrap yourself up under the costume,” he said. “You don’t have to this year.”