Hi Viajero
The weather DOES look rather daunting doesn't it?
The bus from Roncevalles goes to Pamplona via Larrasoana so you could always go to Roncevalles and assess the situation more accurately from there. The bus company website is
http://www.autocaresartieda.com/default.html. Look under 'lineas regulares' and select 'Jaurrieta'. there appears to be 4 buses leaving Roncevalles between 8am an 12noon. The worst that can happen is you bus it back to Larrasoana or even Pamplona instead of walking but at least you will have started in the mountains and seen your preferred starting point rather than missing it out altogether. Please check these details for yourself as I would hate to have got it wrong and sent you on a hopeless journey!!
I loved the walk from Roncevalles to Larrasoana but did it in raging heat so cant offer you any advice on how accessible it is under present conditions. I do remember parts of the path were very steep so be sure to have good grips on your boots!
There are sure to be other pilgrims arriving at Roncevalles with you so whatever decision you make you are unlikely to be heading out on your own which is always reassuring.
Have a wonderful camino and, if possible, let us know how you found the conditions when you get there as that will be helpful to others leaving soon.
Wishing you all the best
Laura