So, there is a dramatic difference between the "rain pattern" in a region and the "available humidity". This is because of the effect of temperature on evaporation as discussed above.

So I used this key (already mentioned in Azelor's "sister thread")

Lower precipitation than evaporation (ARID seasons)
Very Hot + Moderate/Low/Dry
Hot + Low/Dry
Warm + Dry
Mild + Dry

Roughly equal precipitation to evaporation (SEMI-ARID seasons)
Very Hot + Wet
Hot + Moderate
Warm + Low
Mild + Low
Cold + Dry
Very Cold + Dry

Higher precipitation than evaporation (HUMID seasons)
All remaining combos

Palamb's humidity maps result in this (yellow is arid, lime-green is semi-arid and blue is humid):

january_humidity.gif - january

july_humidity.gif - july

This definitely changes things, but the process to work it out is still blurry in my head. So far, my only plan is a huge table, covering all possible combinations - not ideal.