Katrina hesitated once she stood before the great oak doors of the infamous alchemist’s home, her anxiety getting the best of her. Looking back down Maker’s Street, she reminded herself that this was what her mistress desired and so it was her duty to obtain it. A flash of lightning illuminated the storm-soaked street immediately followed by a roar of thunder. With a start, Katrina forgot her nervousness and turned towards the door, rapping upon it with a fist.

The sound echoed on the other side of the door, but there came no answer. No windows lit by lanterns, not sounds of curious footsteps coming to question the visitor who arrived at such a late hour. Katrina waited until the next flash and rumble before raising her hand to pound once more against the door. It swung open slowly before her.

A new anxiety replacing the old, Katrina peeked inside the darkened foyer. “H-Hello? Sir Alchemist?” There was no answer. Looking back at the storm behind her, Katrina knew that her mistress would be furious if she returned empty-handed. Gathering what courage she had and drawing her wet cloak closer about her body, Katrina took a tentative step inside the house.

The storm is a terrible cliche, I'm afraid, as an omen of fear/terror/portentous events. Generally, thunderstorms are quite rare, and when they are used dramatically (particularly in films) I tend to think it's just lazy writing, employing the weather to do the work of creating an atmosphere, instead of working on it through subtler but more effective means.

I did have a problem with the fact that you don't start with the fact that the POV character is cold, wet, and standing in the rain. It may be a minor problem, but it was jarring to discover that when I did.