The first 3 days of Colorado 3rd season las year was in low 70s. The next 5 days was 20s in the mornings and 50s thereafter. Unfortuantely I was actually using RL19 in my 338 last fall and honestly never got a shot off so I can not attest to RL19 performance in that cold temperature nor the H's performance.
For what it is worth, I am reloading hunting party's custom 300 Win Mag with H1000 but 2 for a 30-06 and a 7mm Rem Mag I am going with IMR 4350 and IMR 4831 respectively. For my 338 Win Mag I am not sure but chances are not with the H powders.
Let us know your approach.