Since fw 5.8x (also with 6.00 and 6.71beta) the "storm alert" seems not to work on my 5x. I have set to the lowest value (2 mb/3 hours) and alarm tone and vibtration are also enabled. The value of 2 mb/3 hours was exceeded for several times without an alarm (sound / vibration + pop up).
Is this working on your 5x?