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Nikkei jumps on weaker yen, Wall St gains; Nissan, Sharp soar

* Volume key for whether gains to continue - analyst * Sharp soars as forecast seen as too conservative * Nissan jumps as investors reconsider cheap valuations - fund manager By Ayai Tomisawa TOKYO, May 13 (Reuters) - Japanese stocks jumped on Tuesday as a weaker yen and a record close on Wall Street boosted investor sentiment, while Nissan Motor and Sharp gained on hopes that their perceived conservative forecasts suggested more upside to their earnings.

The benchmark Nikkei share average was up 1.8 percent at 14,405.06 in midmorning trade, trading above its 25-day moving average for the first time since April 25.

The Dow and S&P 500 hit record closing highs on Monday, as strong corporate results and an improving economic outlook spurred a broad rally on Wall Street.

"The Japanese market's gain should be temporary as it's merely benefiting from strong U.S. shares," said Masashi Oda, chief investment officer at equity investment department of Sumitomo Mitsui Trust Bank. "If volume is high at the end of the day, we may see more rallies in days ahead." Nissan surged 5.4 percent to a one-month high of 916 yen, after forecasting 405 billion yen ($4 billion) in net profit for this financial year. Although it missed the average forecast of 425.4 billion yen in a Thomson Reuters I/B/E/S poll of analysts, traders said the company's guidance was too modest.

"Investors sold off Nissan last year after the company cut its forecasts," said a fund manager at a Japanese asset management firm. "The stock is trading below its book value, and they realized it's overly sold. An appropriate level should be around 1,000 yen." Similarly, Sharp soared 7.7 percent to a three-week high of 279 yen after the company issued forecasts that traders said were too conservative given the expected strength in Sharp's solar and LCD businesses.

Exporters gained after the yen slipped to a one-week low against the dollar. Dollar/yen last traded at 102.22.

Toyota Motor Corp gained 1.6 percent and Sony Corp added 2.5 percent.

The broader Topix rose 1.6 percent to 1,175, while the new JPX-Nikkei Index 400 advanced 1.6 percent to 10,710.45.

(Editing by Chris Gallagher)