re: What's the rule for pouring concrete when there is a chance of rain?

Unless it's still raining. If you run out of time, the concrete will set before you can finish it.

From a structural standpoint there's nothing wrong with placing (you don't "pour" concrete, you place it) concrete in the rain. This is true if you don't care what the finished surface will look like.

If it rains onto plastic (unhardened) concrete, the rain action washes out the mortar paste on the top of the surface leaving exposed aggregate as the surface. This is not the right way to do an exposed aggregate surface and generally the gravel used in generic residential patio concrete is different than what would be used in the exposed aggregate concrete. Different as in not aesthetically uniform or attractive when exposed to view.

Summary: Don't knowingly place patio or sidewalk or driveway concrete when there's a high rain probability.

Slippy... wait you fricking dipshit.

The people that say pour in the rain don't know shite.

The rain will not prevent the concrete from curing, concrete can be poured underwater and still cure.

However, the finish will look like shite if it rains any appreciable amount during and after placement. The finishers will do their best to try and finish and cover it but it will still look like crap unless you can guarantee the rain stops for 4-5 hrs after placement (and nobody can).

Based on the forecast for now for Friday I would not schedule a pour for an outdoor patio. The rain we will likely get tonight and tomorrow will probably make the sub grade too wet for placement much less getting the truck to the area where they can pour out.

Everybody wants to rush the pour when you have to live with the crappy job afterward...Today would have been the day to pour to beat the rain in BR.

Let the rain pass, dewater, then place in dry conditions.