I brewed an IPA Saturday using Wyeast's Northwest Ale. I've been fermenting it at 62 degrees with lots of activity - a decent amount of krausen travelling through the blowoff tube. Looking at the Wyeast website I noticed the temperature range is 65-75. How can fermenting at a lower temp than recommended affect the brew? I know fermenting at too high a temperature can cause some fruity off flavors (I learned this after accidently fermenting my last IPA around 78 due to lack of temp control).

When changing out from an airlock to the blowoff I smelled the krausen on the bottom of the airlock and it smelled delicious. Much more aroma than that at anypoint along the process with my last IPA.

Fermenting lower will generally cause the yeast to develop less esters and leave you with a cleaner beer. However it can also lead to underattenuation if the yeast flocks out too soon, which can leave residual acetylaldehyde (green apple) and diacetyl (buttery). This is most common when a beer is crash cooled before it had completed its fermentation.

I think you're probably going to be ok at 62, but you might try and raise it up a few degrees towards the end to help the yeast clean itself up.

+1
Give it a couple days up at 68-70 when the krausen STARTS to fall. That should re-kick it and you'll end up with a really nice clean beer.

And if 62 is your ambient temp, your actual ferment temp inside the beer is higher, maybe 66 or so. You are fine at 62, the beer, as others have said, might have less fruity esters than normal. I ferment my beers with wlp001 at 62. I love that temp setting.