Brewing lager at a low temperature allows the yeast to ferment the wort producing a very low ester profile. That's to say there is less contribution to the flavour from the yeast so we get a "cleaner" flavour. Many lager yeasts will ferment satisfactorily at ale temperatures, too, but the beer will taste more like ale and seeing as how your malt profile is balanced towards lager, it probably won't be excellent.
Try to keep a max temperature of 10-12 C during fermentation- or even less. You can let it rise a couple of degrees later for a diacetyl rest, but there's really no need as the beer's going to sit on the yeast, anyway, for at least 6 weeks.
Sorry I should have posted this as a reply to Rog. T.B.