* Nikkei may reach 15,000 this week - broker * With USD/JPY assumption at 100, forecasts will rise 28 pct - analyst By Ayai Tomisawa TOKYO, May 13 (Reuters) - Japan's Nikkei share average surged to a fresh 5-1/2-year high on Monday as the weakening yen further bolstered exporters on hopes that they will raise their forecasts for the current year. Strength in Wall Street on Friday also spurred buying after some rosy earnings buoyed investor sentiment. The Nikkei rose 1.4 percent to 14,810.92 in mid-morning trade after climbing as high as 14,828.14, the highest level since January 2008. "There is increasing demand for Japanese exporters from foreign investors," said Kyoya Okazawa, head of global equities at BNP Paribas. With the earnings season in full swing and exporters are reporting overly conservative forecasts for the current year through March, many investors expect that the companies will raise their forecasts at some point this fiscal year. "Conservative guidance is the same old Japanese managerial judgment. The market doesn't mind conservative forecasts as they are pretty sure that the companies will revise up the already-announced figures," Okazawa said. Helped by gains in such exporters as automakers and electronics, he added that the Nikkei will probably reach 15,000 as soon as this week. The Topix gained 1.7 percent to 1,230.82. The yen last traded below 102 against the dollar. The likes of Panasonic Corp, Toyota Motor Corp and Sony Corp based their foreign exchange assumptions at 90 yen-95 yen to the dollar. Of about 1,394 companies reporting their full-year earnings, they based their dollar-yen assumptions at an average 92 yen and forecast an average of 21 percent growth in their operating profits for the year through March 2014, said Tomochika Kitaoka, strategist at Mizuho Securities. "But if the dollar-yen assumption is raised to 100 yen, a gap between companies' estimates and analysts' figures will be filled...their operating profits are expected to rise about 28 percent on year," Kitaoka said. On Wednesday, exporters led the gains, with Nissan Motor Co jumping 5.4 percent, Toyota Motor Corp adding 3.5 percent and Sony Corp gaining 5.1 percent. The weakening yen allows Japanese exporters to cash in overseas profits at more favourable rates. Analysts also said the market should stay upbeat after Japan avoided criticism about recent weakness in the yen at a meeting of Group of Seven finance officials on Saturday. "If international peers criticise the yen's weakness, investors who are on the nervous side could stop chasing the market higher. Now, such concerns are receding," said Kenichi Hirano, a strategist at Tachibana Securities.