I know players can basically skip this whole encounter with 4 hours of environmental protection, which is really cool because it rewards resource management. But lets say they don't have 4 hours of environmental protection left and they have to build shelter. I know they get a bonus to fortitude check equal to survival check -10, but how many checks do they actually need to roll?
I feel like one per hour(8 checks) feels right, as lower than that the storm doesn't really feel deadly, but more than that means it's 99% a TPK as they are bound to fail some checks.
ENDURE SEVERE WEATHERYou can use Survival to better withstand the effects of severe weather. Typically, one check determines the success of your efforts for a single day. If you succeed at the check, you gain a +2 bonus to Fortitude saving throws against severe weather while moving up to half your overland speed, or a +4 bonus if you remain stationary and build a temporary shelter. You can grant this bonus to one other creature for every 1 point by which your result exceeds the DC. You can’t take 20 on Survival checks to endure severe weather.
I think you're expected to use the endure severe weather quoted above. So the pcs would decide to move halfspeed or stay still. Then they'd make survival checks to determine if they can build shelters/how many people to a shelter. They'd also only make 1 check each since the skill says efforts for a day.
If they beat the severe weather DC by 10, then they don't make fort saves against mold like it says in AP
Remember it's expected dc 15 for a +2 or +4 to saves, dc 25 to bypass it all, those can be tall orders depending on the party.
Oh you're totally right i don't know where i got my calculation, must had been tired when i read that part, but anyhow my group managed to keep enough environmental protection for the Moldstorm to be nothing more than a mention, so in the end i reasearched all this for nothing. :P
Still thanks for your help!