Taichung is generally regarded as having one of the most agreeable climates all throughout the year. It's not as muggy and hot as the south in summer and is entirely different from Taipei with its seemingly eternal drizzle, its bone chilling winters and oppressively hot summers. Besides that, Taichung is rarely hit by typhoons and there are comparatively few rainy days.
Annual average temperature: 22.8°C (73.04°F) Annual average rainfall: about 1600 mm Average humidity: 79%
For weekly forecasts, you can check the Central Weather Bureau's website.