Clark County’s economy showed continued strength in September, adding 400 jobs over the month in seasonally adjusted numbers and bringing total growth for the past 12 months to 5,900 jobs.The official job growth rate of 4.2 percent is likely a low estimate, said Scott Bailey, regional economist for the state Employment Security Department. An unofficial tally shows an increase of 6,400 jobs over 12 months, for a 4.6 percent growth rate. That compares to 1.9 percent job growth for the U.S., 2.5 percent for Washington; 2.2 percent for Oregon; and 2.4 percent for the Portland metro area.Jobs increased in every employment sector, led by trade, transportation and utilities, up 1,400 jobs; professional and business services and construction, both with 1,000 new jobs; and education and health services, which showed an increase of 900 jobs.The preliminary August unemployment rate of 7 percent was revised upward to 7.5 percent. The preliminary September rate of 5.9 percent also is likely to rise by a half-percent, Bailey said, while noting in his monthly report the significance of even a fleeting rate that starts with the number “5.”“We haven’t had one of those since April 2008,” Bailey wrote. “Unemployment hit double-digits in December of that year, and didn’t drop down to single digits for another three years.”While initial unemployment claims increased slightly for the month, they remained low, Bailey noted. “Continued unemployment claims continued their long decline from recession highs,” he wrote.