What does SWES mean in SNOWTAM?
Snowtams follow the ICAO Format found in ICAO Annex 15, Appendix 2

This says that the abbreviated heading starts with SW** where ** is the ICAO location identifier. The location identifier document, ICAO 7910 is a \$245 document so I can't link to it here, but all the Swedish airports start with location identifers that are "ES", hence any SNOWTAM you find from Sweden will be SWES reports. Reports for the United States typically start with "K" so they would be SWK*. Canada is "C", so SWC*.