I'll show the initial situation with the jump map on so you have an idea where the area shown is located on the map.
The map is huge, those interested in viewing it in its entirety can download the planning map from JTS (or check the scenario in the game, of course).
The contrast on the unit names that I placed on the map is not great, so I'll name the units in the post for clarity's sake.
First, the OOB.
The vast majority of the Serbian forces start on-map, the exceptions being The New Cavalry Brigade that teleports into the map and some New Territories border guard forces.
I cut the border guard forces from the OOB as they're companies and only the total strength in men is shown. Each Serbian army controls a number of border guard companies. They withdraw on the 10th day of action in their sector. That means your eyes and ears can disappear from one turn to the next, so you have to plan ahead to avoid becoming operationally blind.
The bulk of the Montenegrin forces arrive as reinforcements, only the 10th Independent Kolashinka Brigade and the 6th Pec Division start on-map.
All Entente forces aside from those in Beograd arrive as reinforcements, mostly (entirely?) by rail. Entente forces may or may not have 100% arrival chances and trickle into the map. I'll be able to count on solid French assistance but the arrival of the British is a bit iffy. The Grand Campaign variant includes strategy options for making them arrive in a more orderly manner, whilst also increasing Bulgarian forces.
Update 14th of May:
The weather offers nearly every possible variety of conditions possible in this part part of Europe.
Day/month/year followed by normal/soft/mud/snow/frozen conditions chance, followed by min/max visibility in hexes and the storm chance.
Visibility can change to any value between the values shown during the day. The ground conditions and storm conditions (if any) don't change during the day.
After the opening Normal/Soft conditions, there's a Storm with Mud conditions followed by mostly Mud until late October with a chance of Soft conditions.
November starts with excellent campaigning weather, which is why I expect both sides to make big moves then, followed by the weather slowly becoming colder and the weather becoming rather unpredictable.