Trans slips only when it is cold outside

Many transmissions have fluids that are cooled using the same process that your engine fluids use - the radiator system. Transmission fluid is special in many ways. When you first start your car, the fluid is not yet warm. Many transmission systems share the cooling system with your car, so if it is running hot, your transmission fluid will suffer.

It could also simply be that your transmission is low in fluids.

In any case, your fluids are probably the source of the problem. Whether low or just getting hot for one reason or another. They need to be replaced and the transmission fluid and cooling systems need to be checked.

This usually means that the fluids, filters, seals and possibly other components inside the transmission need to be replaced.

Check tranny fluid make sure you have the correct amount and fluid in it
I also would start by replacing tranny fluid and filter