I have a Houdini and it is only water resistant and not water proof. Last year on the JMT I had a couple of thunderstorms that were real down pours - while short in nature, the deluge would most likely soak thru the Houdini and as Mat mentions - leave you cold. Additionally I had hail and that would be worse as you would be really cold. Coming down from Mt. Whitney it rained for about 5 hours straight - so you cannot depend on storms to be short.
I have a North Face Diad that weights only ~7oz and it protected me perfectly as it it waterproof. Kind of expensive at $200 - however you may be able to find them on sale.
Driducks are also fine - however I don’t like the papery feel they have and find they don’t breath well - although others may differ.
Enjoy the JMT - it is truly wonderful!
OregonBeerMan