It is too soon to know if a downward trend may be taking hold. But the data raise questions about whether problems in city schools and the rising cost of housing are affecting residents' wherewithal to remain in Philadelphia, said Temple University researcher and demographer David Elesh.
Both pocketbook issues were high on residents' minds in 2013 as officials grappled with a severe school-funding crisis and, separately, imposed sweeping property-tax changes that sent tax bills soaring in many neighborhoods. (The Census data are a snapshot of July 1, 2013, compared with July 1, 2012.)
"It is one year, and one year does not qualify as a trend, but it is worth looking at," Elesh said.
More detailed data expected at year's end may shed light on this. Until then, Elesh said, Center City and its adjacent neighborhoods continue to draw private capital - a positive sign.
"It's still an attractive place," Elesh said. "We still see lots of investment coming into Philadelphia from developers. I wouldn't be quick to call this a change in direction."
Census estimates tabulated on July 1, 2013, showed that 10,563 more people left Philadelphia than moved in during the prior 12 months. In 2012, that gap was only 5,670; in the year before that, 5,929.
It is a significant uptick compared with 2008-12, when outmigration fell to as low 3,849 one year and reached no higher than 5,929 in another.
Yet, even at roughly 10,600, the 2013 exodus remains well below levels reached during the 1990s, when it was common for more than 30,000 people to pack up and leave in a single year (July 1991-92, 1994-95, 1995-96).
Elesh was curious about the spike because he said the recent recovery appeared tenuous.
"It has never been that strong," he said. "So I've been wondering how long it's going to continue."
In Pew Charitable Trusts surveys this year and last, many Philadelphians expressed pessimism about remaining in the city for more than five years, largely because of the condition of schools.
Elesh said housing affordability was another major factor in people's decisions about where to live.
In July 2013, City Council completed a sweeping property revaluation that caused tax bills to soar in some once-forlorn neighborhoods, such as gentrified Graduate Hospital and South Philadelphia.
The revaluation lowered taxes in poorer neighborhoods but had the net effect of making housing in resurgent neighborhoods near Center City more expensive than it had been through higher tax bills - and all against the backdrop of a public school system in distress.
"I've met young couples with children . . . who have called my office saying, Kenyatta, we're going to have to move," Councilman Kenyatta Johnson, whose district includes hard-hit Graduate, said Wednesday upon hearing about the latest Census data.
Last year, Johnson publicly warned that middle-class homeowners might leave the city if faced with rising tax bills.
On Wednesday, he said the true impact of the tax changes would become clearer in the months to come as pending tax appeals were processed and homeowners decided whether to remain or, as they have told his office, leave the city.