So
the big question is:
What does ADG mean by tracing
from?
- Do they mean you trace
from a supply source,
to the unit in need of supply? Sounds very logical, from a logistical POV.
- Or do they mean you start tracing
from the unit in need of supply, all the way down
to a primary supply source? This is how I've always interpreted the rules. It is literally stated so in the rules (2.4.2 tracing supply): "You trace a supply path
from a unit
to a primary supply source".
I think the last example of 2.4.2 (tracing supply) also provides an answer:
In this picture, the INF unit is 4 hexes away from Kiev, a supply source
in another weather zone. The unit is in supply if the weather in its hex is
fine. The weather in Kiev’s hex is irrelevant.
As I see it, this example states as a general rule: it's the weather in the hex occupied by a unit/source that needs supply that matters. In this post, Yamamoto's hex, so he has 3 hexes to trace and therefore is OOS for now.
To sum up the entire supply chain as I understand it (Yamamoto -> Umezu -> primary source in Japan):
Umezu is in supply because he gets 4 hexes of a
railways supply path and needs only 3 to trace back to any Japanese city but Yamamoto is OOS because he gets 3 hexes (because he is in snow) of a
basic supply path which is not enough to reach Umezu.