I look at a few things.
1. Heights around AK and th Beaufort Sea
2. Heights in the nao region
3. Temperatures (850) in our part of the hemisphere.
as a weenie, I look at snow maps. SV maps which tend to be the most conservative have 12+ for most of us during the run. Heights are above normal but there seems to be enough of a PNA and EPO working with nao to keep us in reasonable shape. It’s a huge improvement vs 0z which had low heights on the Pacific side in a place we don’t want them.