In that condition you should use a boom microphone.
The cheapest, with enough quality is the Rode NTG2. Take care to boom it with the right direction and out of the frame. If you don't mind to see the lavalier, that is for sure the best sound for the voice. Anyway, try to take well care with audio in the same way you do with image. I mean if you are in a very soundy place tha audio will be noisy.