The wording of the spell description "desperate and uncaring " makes it sound like the direction is random, though that would render the spell useless in my opinion, so I would rule it like you choose the direction when you have first action in a round. The flame burst itself could perhaps be force meters in length and start at the casters feet and expand outwards and be force meters wide at its max range so it created a cone like area of effect. Either that or just play it as a flamethrower spell that travels in a straight line from the caster again up to a max range of force meters hitting everything in its path.