The chilliest air in more than four months will follow a cold front passing across the Philadelphia region sometime Tuesday, accompanied by another sure sign of autumn — vast hosts of migrating birds.

A strong storm rumbled through parts of Chester and Montgomery County at midmorning Tuesday, and the front could set off more of them into the evening. The Storm Prediction Center had the region under a “slight risk” for severe weather — but “certainly not as high a risk as we’ve seen lately,” said Sarah Johnson, lead meteorologist at the National Weather Service Office in Mount Holly. No widespread flooding was expected.

Of more interest to the birding community was the forecast for Wednesday: Sunny with a near 100% chance of birds on their way to their wintering grounds in the South and in Central and South America.

This is the peak period for migrators, and birders love cold fronts, says Keith Russell, a program manager for Audubon Mid-Atlantic who has described Philadelphia as a “migration central.”

One of the biggest complaints we hear from bird watchers, is ‘Why aren’t we getting any cold fronts?’

Brett Ewald, New Jersey Audubon's Cape May Bird Observatory director

The birds who have spent the summer breeding in the massive boreal forests of southern Canada and their fledglings are apt to hitch rides on atmospheric waves.

“One of the biggest complaints we hear from bird watchers, is ‘Why aren’t we getting any cold fronts,’ ” said Brett Ewald, director of New Jersey Audubon’s Cape May Bird Observatory. “We’re always looking for that next cold front.”

» READ MORE: Why bird-watchers flock to Cape May and Philly: ‘We are migration central’ (from October 2019)

For migration purposes, this particular front evidently was strategically positioned late Monday, extending from Quebec to central Michigan, Johnson said. Along with the thunderstorm threat in advance of the front, after it clears the region temperatures could drop into the 40s, even in the city, by early Friday.

Winds will blow gently, under 10 mph from the north and northwest, which should make for excellent flight conditions, and the night-migrating songbirds, who use the stars and topography as navigation aides, are likely to partake in stopovers across Pennsylvania and New Jersey.

Birdcast.net, affiliated with Cornell University and other organizations, has the entire region in or near the high zone for migrators on Thursday night, with 320 million forecast to fill the night skies across the country based on radar observations that detect nightly liftoffs.

The Cape May observatory, ranked among the nation’s top birding sights by Audubon magazine, is in a fortuitous position, as those post-frontal winds from the northwest “push birds toward the coast,” said Ewald. It is situated on a finger of land separating Delaware Bay and the Atlantic Ocean, an ideal jumping-off point for heading south.

Birders might sight a rarity such as the Connecticut warbler after a frontal passage, but at Cape May, “You should be able to find 50 species, and 100 on a good day, several hundred after a cold front,” and maybe 10,000 songbirds.

» READ MORE: Up to 1,500 birds flew into some of Philly’s tallest skyscrapers in one day. The slaughter shook bird-watchers. (from October 2020)

He said that after a frontal passage, “you can potentially get 3,000 to 5,000 raptors.” He recalled that on a mid-October day a few years back, 5,400 American Kestrals were spotted.

The Heinz Center for Environmental Education near Philadelphia International Airport is hosting a brisk traffic, refuge manager Lama Gore said Tuesday. “Right now we’ve got a lot of songbirds,” he said. “The warblers are passing through. That really gets the attention of the bird watchers.”

The fronts not only are only commuting aids, said Ewald, but they also signal the autumnal chill that begins to thin out those insect buffets upon which some birds are so dependent.

Ewald cautions that all fronts play out differently, and it isn’t possible to predict precisely how many migrators appear in their wakes.

» READ MORE: Here’s what to know as Philly’s days shorten before our eyes in autumn

“You don’t know how many you’re going to see,” he said, “and how many different kinds you’re going to see.”

But if the front is strong enough, he said, “you can get multiple good days in a row.”