When travelling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Newcastle is roughly greater than that of Cartagena. You can assume this to be the case over the next 19 days.
You can imagine temperatures to be in the 32 °C range in May. The weather in Cartagena is generally hotter than Newcastle's. As flight availability is subject to become more limited as peak season approaches it is ideal to book in a timely manner.