2000 Subaru Legacy -Hesitation

Usually when it is raining or drizzling my car has a hard time starting and once it finally starts it may sputter and backfire for several days until weather becomes warmer. When I keep gas tank as full as possible and use dry gas it is better but not perfect. A friend of mine looked at it and said when trying to start engine wet the area around spark plugs was arching. He used wd40 around plugs and it was better for a while, but on reapplication of WD 40 things have not returned to normal. It is like there is no power and then when you step on gas it may lurch ahead…It especially sputters and chokes on hills and inclines. HELP!

Install new spark plug wires.

Warmer has nothing to do with it, it’s moisture/humidity.

Your spark plug wires are long overdue for replacement. Once you replace them, this problem is likely to disappear.

And, you might want to see how long it has been since the plugs themselves were replaced.
Is this car maintained according to the mfr’s maintenance schedule?