Remember, you can check the full document Paschale Solemnitatis on this site, among many on the internet.
There are two alternatives to the big procession. Though PS 29 provides for only one procession per parish, the document does seem inclined to urge pastors to avoid minimalism on this day:
30. The Missal, in order to commemorate the entrance of the Lord into Jerusalem, in addition to the solemn procession described above, gives two other forms, not simply for convenience, but to provide for those situations when it will not be possible to have the procession.
The second form seems to be preferred to the third:
The second form is that of a solemn entrance, when the procession cannot take place outside of the church. The third form is a simple entrance such as is used at all Sunday Masses which do not have the solemn entrance. (Cf. Roman Missal, Passion Sunday (Palm Sunday), 16)
The more northern climates might experience rain, cold, or even snow. Are these reasons enough to go to a default plan B or C?
My current parish opens onto a busy and narrow one-way city street. Having a processional gathering means gathering on the street, so city police have to be notified and the street temporarily blocked, which ricochets through part of the neighborhood with many one-way streets. This is done on Palm Sunday and at the Easter Vigil. This year, however, the walls of snow along the street made things more, um, difficult.
My general attitude is, we can have the big procession, just don’t expect everyone inside the church to join it….
PS: there are a lot of large American Catholic churches with relatively small (often, only one) portals where a procession outside would naturally gather*. So moving people out and back in through those portals tends to drain the vitality from the procession.
* They don’t look like, say, this: