My specialty is the volcanoes of the Central American arc in Guatemala. Dr. Chuck Wood asked me if I would have a go at your excellent question on eruption forecasting. In the past several decades, our short-term forecasting of large volcanic eruptions has improved by leaps and bounds. But in reference to explosive volcanic behavior, short-term usually means hours to days, and rarely a week or more. Volcanologists and seismologists who monitor active volcanoes have integrated several methods to track the state of an active volcano.
Seismic monitoring consists of deploying a network of portable seismometers around the volcano.
The seismometers are capable of detecting rock movement in the Earth's crust. Some rock movements may be associated with the rise of magma beneath an awakening volcano. An increase in SO2 may indicate an increase in magma near the Earth's surface. When used together, these methods are capable of providing precise short-term information on volcanic behavior. The science of monitoring volcanoes is still growing.