Early December -- evening temperatures on the Track

[Yoknapatawpha]

A friend and I are traveling from the other hemisphere to walk the Track during the first week of December.

I will bring a light three-season tent with me.

I am trying to determine what weight sleeping bag to bring with me. A ranger suggested one rated to at least -10 C.

I know the weather is quite fickle, even in the late spring and early summer. Assuming I sleep in a tent and not a hut, do I really need such a warm bag at that time of the year, though?

[tasadam]

It's so variable on a number of things so it's hard to say.
-10 sounds pretty solid, though you are planning on being in a light tent, so better to have it and not need it, than to not have it and need it.
It depends on how you sleep too - whether you sleep warm, whether you are prepared to open the bag and use it like a doona wrapped around you if too warm, whether you will have enough dry layers that you can use at night if you take a less warm bag, how good your sleeping mat is.
The track is fairly high in altitude as well, another consideration.
The two areas of recorded data for weather atr at Waldheim near Cradle to the north, and Lake St Clair to the south.
You really should be prepared to cope with the worst of it, not the average...
HERE is the historical weather for Lake St Clair, November 2008.
HERE is the historical weather for Lake St Clair, December 2008.
HERE is the historical data for Lake St Clair
HERE is the historical data for Waldheim.

[Yoknapatawpha]

-10 sounds pretty solid, though you are planning on being in a light tent, so better to have it and not need it, than to not have it and need it.
<snip>
You really should be prepared to cope with the worst of it, not the average...

These two lines and the weather records confirm my thoughts.

I had hoped to carry a lighter bag; no biggie in the grand scheme of things, though.

Thanks for the insight and the links. That is precisely what I was looking for.

[corvus]

A friend and I are traveling from the other hemisphere to walk the Track during the first week of December.

I will bring a light three-season tent with me.

I am trying to determine what weight sleeping bag to bring with me. A ranger suggested one rated to at least -10 C.

I know the weather is quite fickle, even in the late spring and early summer. Assuming I sleep in a tent and not a hut, do I really need such a warm bag at that time of the year, though?

If it is of any help I use a -5 bag all year here in Tassie both in Tents and Huts, in the coldest time recently (on Snow whilst snowing) in a Tent I wore my fleece pants and down Puffer vest, (Bag zip was undone) tent was a 4 season 2 man however as tasadam said I is all relative and dependant on your own personal body core temperature.
corvus