it says: Artillery cards require a 6 when establishing contact. Do I still need to roll a 6 even if I have the card increased priority. (The card say: add 1 to the die rolled when establishing contact, so... I am not sure.

What do you think?

The thunderstorm makes all target numbers a 6. The increased priority adds one (or two if you have them both in play) to your roll, so in that case you would need at least a 5 (or 4 if you had two). The one effect doesn't over-rule or cancel the other.