1999 lesabre a/c freezes

[ggoguen]

My wife pointed out that her a/c has been leaving a lot more water out of the evaperator drain. I then drove it and noticed the air flow out of the vents gets real low. I checked under the hood and the low side line and the accumulator are iceing up-I assume the evaperator is also. This system doesn't have a press cycling switch, that I can find. What could cause the a/c to freeze up?

[LastOne]

It is low on freon.