I agree with most of the comments here. I think the OP said his "significant other" was his fiance, and it makes a difference if that sig. other is a new girlfriend or a wife. If she knows it's important to you, and she knows you love doing it, she really shouldn't a problem letting you go do something you love. Now, the OP is new to chasing, so I could certainly see why his siggy other would be hesitant. The above (she should let you go if she knows that it's a part of you) refers to those who chase regularly and have for a while. Many folks have misconceptions about chasing, so you getting a few chases under your belt should help show her what chasing is really like (which, much of the time, is sitting under a hot sun in some desolate location for a few hours before driving hours to set up for the next day's wishcast LOL). Build up her trust and make her confident that you can keep yourself safe by chasing several times, and I think it'll be easier for her to stop worrying.
From my own experience, I've found it easiest to call her during the "down-time" of the chase to touch bases. In addition, when you are home, make sure you give a little extra attention to your relationship (cards and flowers help). My wife knows chasing is a passion of mine, and she's never told me not to go out. That said, I know that she sometimes gets disappointed when I have to back out of a movie-night or something for a last-minute chase, so make sure that you make extra effort to show her that, just because you back out of a datenight or something else, you still value her OVER chasing (assuming you want this to be a long-term relationship). My wife understands that I obviously value her over chasing, but she realizes that chasing opportunities are limited (usually to a few months a year, a few times a week, with several long-term downtimes). As such, I can't always plan to chase 4-5 days ahead of time, and the extended chase times will be followed by extended downtime. My wife has chased with me numerous times through the years, though most frequently during the terrible 2002 year. Actually, I'm insanely frustrated that my success ratio when she's in the car is terrible (1 or maybe 2 "good" chases through probably 15-20 total chases with her). I really, really want to get her on a 'blockbuster'-type of chase, since she's given the time and energy to keep me company on many chases in the past and hasn't seen much a reward in terms of beautiful storms or tornadoes. Statistically, it's bound to happen sometime, I suppose...