Running water erodes any magic. Since evocation is happening instantly it is going to be in the rain when it happens, so the effect is much more obvious there. Thaumaturgy is just as much affected, though there is usually a lot more power or finesse in a thaumaturgy spell, that it lasts despite the rain. A bit of rain won't wash away a ward, and it is just one of many eroding effects on lasting magic. Like dusk and dawn. That's why thaumaturgy doesn't last indefinitely and has to be recast every now and again.
If the wardens are putting up a veil during heavy rain, they'll have to account for it, since it makes it harder to do magic. If they already put up the veil, and the rain threatens to wash it away, that's a great hook for a compel.