The order of priorities is obvious and most golfers have it backwards. Dry hands come first, dry grips second, dry body third. If the club is slipping in your hands, no jacket in the world will save the shot.
So a towel belongs under the umbrella canopy or inside the bag, not clipped to an outside ring where it soaks through in five minutes. Two is better: one for hands and grips, one for clubheads. Wipe the grip before every shot; it takes three seconds.
Rain gloves are a category of their own and worth the money. They are made of a synthetic that grips better wet than dry, which sounds like nonsense and is nevertheless true. Wear them as a pair on both hands. A leather glove in rain goes hard and slick within three holes.
Umbrella: 130 cm minimum, a vented double canopy, fibreglass ribs. A petrol-station umbrella inverts on the first gust and breaks its ribs doing it. Mounting it on a trolley bracket beats holding it, because it frees both hands for the shot.
Jacket: under 10,000 mm of hydrostatic head is not enough for an hour of steady rain. Gore-Tex Paclite or an equivalent membrane handles four. The shoulder cut is the crucial part — the jacket must allow a full turn. If it pulls across your back you will shorten the swing and push everything right.
Small things that matter: spare socks in the car (not the bag, where they get wet too), a plastic bag for the wet glove, and headcovers that do not freeze solid. Keep a spare cap in a bag pocket as well — a soaked peak drips onto the ball while you putt.
Finally, tactics. In rain the ball gets no run and carry drops around five per cent from damp air and a wet face. Take an extra club, aim at the middle of the green, and forget shaping anything. Greens are softer so the ball stops sooner, which is the only good news a wet round offers.
