Looking at the current Belchertown skin, I see no mention of "showerswn" in the code.
./skins/Belchertown/js/belchertown.js.tmpl: "showers": "rain",
./skins/Belchertown/js/belchertown.js.tmpl: "showersn": "rain",
./skins/Belchertown/js/belchertown.js.tmpl: "showersw": "rain",
./skins/Belchertown/js/belchertown.js.tmpl: "showersw": "rain",
./skins/Belchertown/js/belchertown.js.tmpl: "snowshowers": "snow",
./skins/Belchertown/js/belchertown.js.tmpl: "snowshowersn": "snow",
./skins/Belchertown/js/belchertown.js.tmpl: "snowshowersw": "snow",
./skins/Belchertown/js/belchertown.js.tmpl: "snowshowerswn": "snow",
./bin/user/belchertown.py: "showers": "rain",
./bin/user/belchertown.py: "showersn": "rain",
./bin/user/belchertown.py: "showersw": "rain",
./bin/user/belchertown.py: "snowshowers": "snow",
./bin/user/belchertown.py: "snowshowersn": "snow",
./bin/user/belchertown.py: "snowshowersw": "snow",
./bin/user/belchertown.py: "snowshowerswn": "snow",
You might want to check your installation to see if it matches.