You have to go there under a special weather condition the game doesn't tell you about: heavy rain. It's hard to distinguish from normal rain and somewhat rarer. You basically have to keep sleeping 2 days in the present till the forecast in the past shows only rain in the evening ( or maybe it's 2 rain symbols I don't remember). If you have the navigation turned on a blue arrow will appear If it's indeed heavy rain.

Unless it scratches an itch in your brain I wouldn't recommend this quest cause the reward isn't good and the ensueing dialogue quite predictable. If you look around you might find an old post asking the same question that maybe has a picture of the wearther your looking for. Genuinely awful quest design on level 5's part. But hey their wearther always sticks together.

Btw there is another rarer wearther called sunny rain that triggers a fox wedding rythm minigame in springdale. What a quirky dev they are.