The local weather in July is typically warm in the daytime (up into the 90's) and cool at night (down into the 60's). Most years, "cool" requires a fan in your room to keep the air circulating. In 2000 and 2001 there were several nights where it was so cool, blankets were desirable. There is usually a rainstorm or two during the week of SUUSI - and of course you can now check the forecast online.
See also what to wear and the current SUUSI weather.