Greetings L.J. Privara,
Depending on the brand of windows installed, and how they where they installed, the mullion (the area where the windows are joined together) may not be flashed.
Does it ever rain on the west side of the building, and if so, what happens?
Flash and silicone caulk would be my first line of defense!
I would use a product like "Geocell" silicone. This product will last for an extreamly long time and can be applied in damp conditions.
Think about gravity and the path of least resistance, that is what water does. (Well it does not really think but, you know what I mean.)
I hope this helps and if you have any other questions, please post back.
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