"There is snow outside"
Translation:Pana theluji nje
Kuna should probably also be accepted, but generally if you also say where something is, then you'll often use pana.
Kuna aina nyingi za ndizi. = There are many kinds of bananas.
Pana ndizi nyingi
juu ya meza. = There are many bananas on the table.
It's not quite a hard rule though, and you can definitely use "kuna" for "round about ..."
Kuna ndizi huku. = There are bananas around here.
But my guess is, if you're saying that if you go outside, you'll see the snow and won't have to look around for it, pana might be a bit more natural.
tl;dr I'm not sure, both are probably right.