Can a Muslim who lives in the area where he is praying i.e. not traveling join Maghrib and 'Ishaa prayers and pray 'Ishaa in advance with Maghrib. Nothing that it was raining, or snowing and the Imaam announced to all people that 'Ishaa will be prayed Qasr for travelers and those who are not traveling should complete 'Ishaa to four Raka's.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

There is no harm to combine Maghrib and 'Ishaa' in the mosque at the time of Maghrib because of rain. This is the opinion of the majority of scholars of the Shaafi'ee, Maaliki and Hanbali schools, although, they differed about some minor details. All of them did agree that it is permissible to combine Maghrib and 'Ishaa' prayers in the mosque at the time of Maghrib because of rain. If the Imaam leading the prayer is a traveller, he has to shorten the prayer but the people who pray behind him and who are residents have to complete their prayers. Rain is a reason for combining the prayer, but not for shortening it. 

Allaah knows best.