Although cold drops are a regular feature of Mediterranean autumns, we need to go back to October 2008 to find a storm of similar intensity to the one forecast for the next few days.
The cause of this particular storm is the conjunction of two areas of low pressure. By themselves, both areas of low pressure are capable of producing heavy rain, but by "interacting and joining", they will create a very pronounced instability in the upper layers.
A cold drop is a normal feature of the Mediterranean climate in autumn However, in the past few years, cold drops have been conspicuous by their absence.
Just to remind you, cold drops are caused by Arctic air meeting the humid wind that runs along the length of the Mediterranean Sea
According to AEMET, there could be long periods of intense rain totalling over 100 litres per square metre in 12 hours, particularly in Castellon.