It depends on where you are standing, if you calculate that you can make a U-turn and go back to the last town you passed is a good idea for me.
The last time I got caught in a storm like this I was arriving to Picun Leufu in Neuquen, as I was almost there I went to the town, I rented a house and spent the night there, the storm stopped just when I arrived to the gate of that house, luckily it had a covered garage.
For those trips I have a watertight bag for things like the notebook or something that can not get wet, for the cell phone and wallet I have the rain suit in the saddlebags of the bike.
The rest of the things I carry can get wet but never drive when the first drops start to fall, it loosens the dirt from the street and ends up creating a very slippery surface, if you are on the road and you can't wait for it to finish, brake for a few minutes so that the water washes the asphalt.