Feb 5 (Reuters) - Moody's Investors Service on Tuesday raised the City and County of San Francisco, California's general obligation bond rating to Aa1 from Aa2 with a stable outlook. The upgrade, which affects about $2.2 billion of debt, "recognizes the quality of the city's tax base and economy, which have exhibited notable strengths through the recession and are solidly accelerating from the downturn," Moody's said in a statement. The upgrade also reflects Moody's "changed view of the likelihood of default on a California local government's GO bond relative to obligations secured by its general resources," the rating agency said. "This new view has generally resulted in a widening of our rating distinction between GO ratings and lease-backed obligation ratings," Moody's said. Previously rated A1 real property lease-backed obligation bonds were upgraded to Aa3, while existing Aa3 bonds have been affirmed, the rating agency said. The stable long-term rating outlook underscores the city's large, diverse tax base which is fundamentally solid and its financial position will likely remain "narrower than typical for the rating level but stable."