There is currently a proposal for a mathematical modeling stackexchange on Area51:
This is intended to have no overlap with physicsSE; questions that are on-topic on one site should be off topic on the other. With that said, the site is still in the definition phase, so it needs people to post suggested mathematical modeling questions that they would like to see answered in the future. These questions can then be voted on by others (a 5 vote limit) to decide what the site should be about.
This should help with any questions that are too experience based for MathSE and too mathematical or too localized for PhysicsSE