Why does my façade only leak in some storms?
Wind-driven rain changes the pressure across the façade and pushes water through gaps that stay dry in still rain. In Singapore, Sumatra squalls bring gusts of 40 to 80 km/h, and wind direction varies between the monsoon seasons. A leak that appears only in certain storms usually points to an opening that needs both water and pressure, such as a joint, gasket corner or window sill, and suggests a pressure test may be needed to reproduce it.
From the RRD Field Guide, checked against the sources cited in Façade and Curtain Wall Water Leakage: How to Diagnose and Fix the Root Cause. General information, not advice for your project.
The fuller picture
Façade leaks are diagnosed by mapping when and where water appears, ruling out non-façade sources, then reproducing the leak with controlled water tests: AAMA 501.2 nozzle testing for curtain walls and ASTM E1105 pressure-chamber testing for windows and walls. Test from the bottom up, isolate one joint at a time, and repair the proven path rather than re-sealing everything.