Biggest thing is the 2d 4th, 28th, 99th, & 106th Inf Did are not taken by suprise & the initial 12 hours confusion in the 1st Army are waived away.
With a weeks notice: Bradley could ease up on current attacks and detach at least two armored divisions & perhaps even two infantry divisions to form a reserve and rest a day or two. Taking a armored division from the 3rd Army would probablly be necessary to make this work. If Eisenhower releases the 82d & 101st AB Div to Bradley those could pair up with the two Arm Div to make a hefty reserve.
Hodges might choose to leave the 2d ID in place & slip the 106th in along side it. This eliminates the weak point of the 104th Arm Cav Group. On the down side it leaves the 99t ID cover the high ground of Elsenborn ridge alone. 1st Army can also have its local reserves in the form of combat commands of the 7th & 9th Arm Div rested and ready.
None of this is a pancea, but it makes for a tougher 16-17 Dec battle, & the 1st Army loses a lot less ground. The breakthroughs are smaller & few or no US units surrounded anywhere. Casualties are just as high as OTL. Hitler probablly ends the offensive a week or two earlier. No spectacular manuver & attack by 3rd Army. More likely 3rd Army executes spoiling attacks in the direction of Trier to threaten the flank/rear of the German attack & drawing off the German reserve.