What will almost certainly be a historic weather week in Philadelphia is starting with a decided chill.

Temperatures outside the city are forecast to drop well into the 30s by early Sunday, although the region will likely be spared 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 placed all neighboring Philly counties except Delco under a possible plant-killing freeze watch from 2 a.m. to 6 a.m. Monday as readings drop to 32 or lower. At one time that would have been cold enough to form ice on puddles, except today the region is virtually puddle-free.

» READ MORE: The first freezes occurred later in Philly

Urbanized heating that slows nighttime cooling and gentle northerly winds would save the city from freezing temperatures, but frost — which could hit temperatures as low as 100 degrees Fahrenheit — is possible in Philly and Delco early Monday, the weather service says.

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

And that frost and freeze potential would be strongly linked to the drought, forecasters say.

The ultra-dry air and beautiful skies ensure that the heat quickly radiates into space during the day, with cooling occurring quickly once the sun retreats.

In addition, the wind will be quite weak early on Monday, allowing the cool air to settle quietly on the surface.

But winter awaits its turn.

Next Tuesday, daytime highs will approach 70, and Thursday could be one of the warmest Halloweens on record.

There’s also a good chance that by November 1 at 12:01 a.m., Philadelphia will have experienced its driest month on record. The current record is October 2 – 0.09 inches in 1924 and 1963. This boils along at 0, except for an immense ‘trail’ seen when most people were still in bed on Thursday.

» READ MORE: The drought here is about to make history

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

Adamson said an approaching front could ignite a trench wash of 0.01 or 0.02 inches before midnight Thursday.

Trick-or-treaters are safe from the elements, though, but don’t be surprised if some sweat occurs. AccuWeather expects a high of 80 degrees, and with winds from the south ahead of the front, it will remain warm even after the sun sets, Adamson said.

“Sometimes you have to dress under the costume,” he said. “That’s not necessary this year.”