In the WRF user manual, or maybe it's the online FAQ, it says that you should chose the number of processors so that you domain is decomposed into squares (north_south * east_west) of no less that 15 * 15. So if your domain is 20*20 and you use 16 processors, you've got squares of 5*5 for each processor, which is much too low. The extra communication between the processors will slow the model down much more than any gain from increasing the number of CPUs. Personally I usually go for squares of about 20*20 per processor, as the scaling in WRF is certainly not linear. So I would only use one processor for you domain. As to why you get higher temperatures with more processors, i can't explain that, but would be interested to hear from anyone who can.