I trust my broody hens to manage rain, other weather, and everything else. Mine hatch in the main coop with the other chickens. Sometimes if my coop isn’t crowded I leave everything to Mama, let her bring the chicks off the nest when she want to and deciding when to take them outside. If the main coop is crowded I lock her and her chicks in a shelter in my electric-netting enclosed run for about three days and nights, then let them roam with the flock. She takes them back to that shelter at night so I can lock them away from predators. This shelter does not have a ramp.
When the weather is wet she still takes them out into the wet grass but they spend a lot more time than normal under her staying warm and drying out.
My hens manage weather very well but your situation is different. Isolating the hen and chicks from the flock won’t make a difference as far as weather goes but that ramp worries me. It’s quite possible the chicks will have trouble managing that ramp the first time or two the hen wants them to go back to the coop. Chickens don’t understand the concept of gate. I’ve seen a broody hen go through a gate and walk along the fence. If all her chicks don’t make it out of the gate she winds up with some on the wrong side. Neither the hen nor the chicks know to go to the gate so they are on opposite sides of the fence desperate to get back together. On more than one occasion I’ve seen my rooster leave the rest of the hens and go take care of the stranded chicks until the hen finally got back through the gate. One time he looked at her with an expression that said “What is this woman, not again”! Most of my broody hens don’t have that problem and most of my mature dominant roosters won’t take care of the chicks like that, but some are more talented than others.
What I’d envision with that ramp is that the hen might go to the top of the ramp and call her chicks. She might walk up, she might fly. Instead of the chicks following her up the ramp they may gather under her on the ground and not know to go to the end of the ramp to get upstairs. In that situation most of my broodies would probably go back down to the chicks but with some I would not be surprised if she just kept calling them. I think if I were in your situation I’d wait until the rain stopped before I let them out so they can learn to use the ramp in the dry. I usually don’t recommend stuff like that. By two weeks of age they should be able to fly up to the top of the ramp if they will.
How well is that coop ventilated, especially up high? Does it stay dry? Wet is what causes smell problems. Good ventilation can help with that a lot but if rain is getting in to make things wet ventilation won’t solve the problem. High humidity sure doesn’t help either. I don’t know what that coop looks like or how much overhang you might have with the roof. A good way to provide more ventilation is to open the top of the walls under the overhang so it stay dry. If you don’t have overhang you might need to put a sloped roof over an opening or use shutters to keep rain out.
