There isn't a GM command, but....
Setup a GM Menu in the 'D' Menu.
Then its just a few clicks to change the weather on any map you want.
Here is one I setup with the default option being the map I am on. This is part of a GM Menu that I can access from anywhere, setup so that I can clear mobs, items, give exp, drop items etc on any map.
Just a note, if you want to clear the weather, use ResetMapAttr command.
Code:
[@AdminGiveMapExp]
#ACT
Mov A3 %USERMAP
Mov A1 "SNOW"
#SAY
This will change the attributes of 1 Map\
Please Choose Which Map and Attr\\
{FCOLOR/10}Map : {FCOLOR/1}<$OUTPUT(A3)>\
{FCOLOR/10}Attr : {FCOLOR/1}<$OUTPUT(A1)>\\
<[ Enter Map Name ]/@@AdminChangeMapAttr1>\
<[ Enter Attr Value ]/@@AdminChangeMapAttr2>\
<[ Change Selected Item ]/@AdminChangeMapAttr3>\\
<[ Back ]/@AdminAdminScript>
[@@AdminChangeMapAttr1]
#IF
#ACT
Mov A3 %INPUTSTR
Goto @AdminChgMapAttr
[@@AdminChangeMapAttr2]
#IF
#ACT
Mov A1 %INPUTSTR
Goto @AdminChgMapAttr
[@AdminChangeMapAttr3]
#IF
#ACT
ChangeMapAttr "%A3" "%A1"
#SAY
Map '<$OUTPUT(A3)>' attribute changed to {FCOLOR\8} <$OUTPUT(A1)>\