Tap Natural Disaster: 6 shift spirit evocation, requires a natural disaster; by drawing on the immense energy of a natural disaster such as a storm, earthquake or volcano, you apply two sticky aspects to yourself. Effectively, you link with the natural disaster using your magic in order to draw more power for your spells. Unfortunately, using natural disasters is not easy both because they don't come very often and because aspects applied in this way are always colored by the type of energy you used and may have destructive consequences.
On YS294 there's a spell that looks almost exactly like what you're trying to do: Grasping Branches, which uses an invocation or tag on an existing scene aspect to achieve higher-than-normal levels of power.
Also, on YS289, there's a bit on temporary access to sponsored magic. A major storm could be thematically similar to a ley-line in terms of available power - though you'd need to figure out what sort of compel you'd throw at the resulting point of sponsor debt...
edit: Oh, right, and the question on magic circles. I'd suggest doing some searching (google, with options set to limit its scope to this site would probably be better than the forum's built in search engine) as this question has come up before. The short answer is that (depending on GM), magic circles seem to be plot devices - magic, and thus magically animated ectoplasm, literally cannot cross them; they function kinda like an infinitely strong threshold - albeit one that can be casually broken by a mortal. (Breaking a circle would probably require at most a supplemental action; thus why it's not much use against a mortal spellcaster.)
The grasping branches is, at base, a straightforward weapon: 7 evocation. Call up 7 shifts of power, roll to control. However, on the control roll, you're spending a fate point to invoke the scene aspect "nearby tree" - for a +2 bonus on your control. You could, of course, cast the same spell without the invoke, if you had a high enough discipline + control specializations + focus item bonuses to go without.
As to sponsor debt: Maybe the thunderstorm per se isn't a semi-aware entity, but it's still got a (largely destructive) agenda to it. Maybe the compel comes immediately in the form of not letting the player accept a concession that doesn't involve killing the targetted foe. Maybe the compel comes in the form of a lightning strike taking out power to the PCs home. Maybe it comes a few minutes later when the PC is trying to cast something and there's just too much running water around. Or maybe it doesn't show up until, sometime in the middle of next game session, the local Thunderbird spirit asks the PC to pay off the favor...
Similar things go with ley-lines; there should be a definite agenda to it - for example, a ley line following an active fault might have an agenda of motion, with compels relating to being unable to *stop* moving when you needed to, or compels involving the earth softening into inconvenient quicksand... If it helps you, make up a face for the place; a spirit tied to the location's power, with its own goals and desires (like).(click to show/hide)
It's one exchange if you're spending a fate point to invoke the aspect.Wrong, it's one exchange.
Two if you don't have a fate point to spare and need to declare the aspect so you can get a free tag on it (though that declaration could, for example, have come prior to combat start, if you were picking your ground for a fight ahead of time.)
Does that happen in two exchanges? First, invoke the aspect "Nearby Tree" in order to create an animated tree (Maneuver to create the aspect "Arboreal Servant"). Second, tag the aspect "Arboreal Servant" for the Discipline roll as an attack.
The alternative would be everything in one exchange, but how can cast a spell to animate the tree and then tag he new aspect and make an attack in one exchange?