With the forecast calling for the coolest overnight stays in the region in six months, the National Weather Service says isolated frost is possible away from the heat island of Philly on both Thursday and Friday mornings.

It will likely be close, said Ray Martin, a senior meteorologist with the National Weather Service in Mount Holly. In addition to cooler temperatures, frost would require clear skies and near-calm winds.

A frost warning is in effect for counties to the north and west of the region, and the weather service says areas of frost are possible Thursday morning outside the city in Bucks, Burlington, Chester and Montgomery counties, as well as the Pine Barrens.

Another round will be played Friday morning in the outer reaches of those counties and the Pine Barrens.

On Thursday morning, temperatures in Philly could drop below 40 degrees for the first time since April, and as high as 34 degrees in northern Montgomery County.

If temperatures stay above freezing in the Philly area, what explains the frost?

Temperatures do not have to drop to freezing point for frost to form.

The formation of frost is inextricably linked to the microclimate and the characteristics of the terrain. Although there are no strict temperature criteria, frost can generally develop at temperatures around 36 degrees, said Martin's colleague Lee Robertson.

Two important conditions are clear skies and little to no wind. A clear sky allows the day's heat to be efficiently radiated into the atmosphere.

When the wind does not move the atmosphere, thin layers of cool air form on the metal surfaceS and in areas close to the ground, they can be several degrees cooler than “ambient” air temperatures measured several feet above the ground.

One factor that could counteract this week's frost, Martin said, is the general lack of moisture in the atmosphere and soil.

In terms of rainfall, the region is at 0 so far in October. According to the weather service's Middle Atlantic Forecast Center, over the past 30 days, Philadelphia received 8% of normal precipitation and Camden County received 7%.

And Martin said there was no rain in sight.

What does the cooling spell mean for winter in Philadelphia?

Most likely nothing.

Long-range forecasters are looking at trends around the globe in October, but not so much at what's happening in Philly.

October's Siberian snowpack could have an impact on the North American winter, according to a study by Judah Cohen, a polar scientist at Atmospheric and Environmental Research in Massachusetts. But so far the snow has been nothing special and hasn't sent any clear signals, he says.

Seasonal forecasters are taking a close look at the cooling of sea surface temperatures in the tropical Pacific, which they expect will become a long-lasting La Niña event that will impact the entire United States throughout the winter.

The government's Climate Prediction Center puts there a 60% chance of La Niña developing, but that's not a done deal yet, and it's unclear what the exact impact will be on Philly and the rest of the Northeast would.

In any case, La Niña will be a factor in the winter outlook, such as AccuWeather Inc.'s forecast released Monday. Another mild winter with below-average snowfall was expected here.

The National Oceanic and Atmospheric Administration will announce its outlook on Thursday.