  The Probabilistic Solar Particle Event foRecasting (PROSPER) model predicts
the probability of occurrence and the expected peak flux of Solar Energetic
Particle (SEP) events. Predictions are derived for a set of integral proton
energies (i.e. E$>$10, $>$30 and $>$100 MeV) from characteristics of solar
flares (longitude, magnitude), coronal mass ejections (width, speed) and
combinations of both. Herein the PROSPER model methodology for deriving the SEP
event forecasts is described and the validation of the model, based on archived
data, is presented for a set of case studies. The PROSPER model has been
incorporated into the new operational Advanced Solar Particle Event Casting
System (ASPECS) tool to provide nowcasting (short term forecasting) of SEP
events as part of ESA's future SEP Advanced Warning System (SAWS). ASPECS also
provides the capability to interrogate PROSPER for historical cases via a run
on demand functionality.
