I will go against the grain here a bit. I do not find the message that bad, but the tone is a bit patronizing to me.
Now, if I knew the manager in question, and this manager usually has this informal way of communicating to staff, I would give her a pass. It 't the thought that counts and all.
But from some-one I do not know well, this message would rub me the wrong way too. I myself would chose a more formal style of message, for instance:
"Due to the weather the normal office-hours will be more flexible tomorrow."
I do not mean to imply that this is better, and I am sure this makes me come across as cold and uncaring to some.