1. Unlikely to take place, to be true, or to happen: It is improbable that the temperature will be 30°C. and hot the next day taking into consideration that it is presently -10°C. and snowing heavily at the moment!
3. Too far-fetched to be considered believable: It is really rather improbable that Tom's parents are millionaires considering that they live in a disreputable part of town!