confirming the weather

As the past weather records show that both Beijing and Shanghai would be warm then, sometimes a little hot.
Beijing's temperature will vary from 15C to 30C during May 5-9. Little rain may fall sometimes.

Temperature of Shanghai will range from 18C to 27C and there may be unexpected thunderstorm.

Therefore, both long-sleeved and short-sleeved shirts are necessary. Shorts may help in hot afternoon, especially in Beijing. Though, I also suggest you take along a light coat to keep warm in early morning or rainy night.