Mountain weather will more usually be very different to that around a low laying airfield, even one close by. Treat weather as 'what you see is what you get' in mountainous areas.
One aspect of valley flying in Australia that was never a consideration in PNG is power lines...particularly the high tension variety that cross valleys from one large tower, that may be obscured in cloud, to another on the opposite side that may well be similarly obscured.
There was one set in PNG that crossed the Markam River just inland from Lae...that I didn't find out about until I landed in Lae in VERY crap weather and a Talair pilot (I was not in Talair yet at the time) said "How did you get in?" "Down the river at 30' and then a rate one turn from the river mouth until I saw the threshold" "What about the power lines?"
"Power lines?"
They were in cloud above me...the towers were on the cloud shrouded river banks either side of me
BTW did you know cloud tends to dome up over rivers?
Valley/mountain flying (in other than calm clear weather) requires extreme levels of local knowledge to be consistently 'safe' ish. I would suggest having very high personal minimas until you REALLY get to know 'your' local valley systems...down to individual tree/rock outcrop/funny bend in river/creek level...and I mean that literally.
The VERY first landmark I ever learned in PNG was an
individual tree on a ridge line near my home base in the Central Highlands - Chimbu Province. That tree and I developed a very special relationship in bad weather and good (but mostly bad
) over many years....probably 10 years after first having it pointed out to me on my first ever training flight ex Kundiawa, and having used it in anger 100s of times, and just smiled at it in passing 1000s of times, I flew past in a DHC7 to see that the locals had cut it down...I was near incandescent with anger
Wind is, like water, a fluid and behaves around mountains EXACTLY the way water flows around rocks and over waterfalls in a fast flowing river. Visualising what a wind is doing as it blows through/across mountains is a skill you'll need to develop to an instinctive level.
Crossing ridge lines/gaps/saddles at an angle is important if you're low and/or the weather is marginal...it allows you to assess conditions and reject crossing the ridge line with only a gentle turn away through a small angle as opposed to a 180 turn. With a 180 turn too terrain that may not have been in your focus can suddenly jump out and grab you...much less likely to happen crossing at a relatively shallow angle...30-40 degrees.
What you are looking for as you approach a ridge line is seeing more and more of what is beyond the ridge line. That means you're above it...less and less obviously means you're below it and roughly the same means you're at the same elevation as the terrain.
The other thing you will notice when down in valley systems is you're 'inside the map' effectively making them marginally useful...your visual horizon also goes as the terrain is all above you and attitude control becomes difficult until you get used to visualising where the actual horizon would be if a mountain wasn't in the way...it is
almost instrument flying.
Above all it is GREAT, if somewhat SERIOUS, FUN.
S8 makes a good point re performance...a C182/180-185/206 etc is really the minimum aircraft for regular/serious mountain flying...serious meaning anything beyond low elevation rolling hills.