If the nearest alternate is a long distance away it can lead to issues where the aircraft will exceed max landing weight if not weight restricted on takeoff. In the past there were some work arounds for this however the FAA has eliminated those after some airlines abused the option.

I'm still not buying the diversion angle here (maybe it is diversion risk in combination with heavy cargo transport). We're talking about an A319 here. A319s with low density configs can theoretically fly JFK<->LHR (not that anyone would actually do that). MSP->BOS can't be more than 1200 miles and that's 1200 miles with tailwinds.

Perhaps Delta has contractually limited MTOW, but it still has to be something more than just having to fly a few hundred miles to a diversion airport.

IRROPS last Wednesday (please note it all worked out for me in the end) due to storms in ATL and was #1 on standby for an MD-90 PBI-ATL. Red coat said no go for me and the flight was going out with 4 empty seats due to weight restrictions.
I did notice 2 non-revs got on at the end (in unifrom) and it went to 2 seats available on the monitor. I will refrain from commenting on that since I don't have all the information.

You can't dispatch a flight that is planned to arrive at destination over it's Max Landing Weight. Doesn't matter what kind of range the jet has or how much fuel it can carry.. On a day like yesterday with widespread low ceilings and visibility you are likely going to have to select an alernate that is a significant distance from BOS. In fact, they likely had 2 alternates on the flight plan. When dispatched, the flight has to arrive at its destination with sufficient fuel to fly to the furthest alternate while still being under max landing weight.

Are there ever situations where it's impossible to satisfy both constraints simultaneously? In other words, can the fuel required for the farther alternative airport be too much to landing weight at the destination or other alternative airport?

Fair enough, but that must be a hell of a diversion. MZFW versus MLW on an A319 allows for 4 tons of fuel which is enough to fly ~500 mi (admittedly they would need reserve on top of that) - but still, that would be hell of a diversion (and an odd one given the density of airports in the NE).

Your numbers are pretty good there. A 319 that was loaded with pax and cargo at close to its Max Zero Fuel Weight would have to land with not much more than 8000 lbs of fuel to be under MLW. But I did leave out something in the dispatch requirements that you mentioned and that is standard reserve fuel (in domestic ops, that's an amount that allows 45 minutes of flight at normal cruise). So domestic fuel planning requires fuel to fly to destination, then to fly to the furthest alternate, plus standard reserves.

I don't fly the Bus but I believe standard reserves are in the 3500 to 4000 lb range. On a max zero fuel weight takeoff day, that leaves a little over 4000 lbs for an alternate. On a day like yesterday when you have to look at something like Syracuse for Boston or even further, and fuel gets tight. You can't take less fuel, so you have to remove pax or cargo. Keep in mind that weather at destination determines if you even need an alternate using the 1-2-3 rule and often It's not required.

Sure, it's possible, but fairly rare. Payload Optimization tends to fall into two instances: One is when trying to protect max landing weight on bad weather days at destination that require distant alternates. The other is trying to protect Takeoff weight that is limited by runway or climb restraints often on hot days or high altitude airports

I'll chime in with the other replies (as an amateur. )

With virtually no passengers, I think this would be impossible (or nearly so with some really bizarre weather around). In other words, with little cargo, you only need sufficient fuel to carry the nearly-empty plane and that fuel, which is not as much as the fuel needed to carry 100+ people and their stuff and the plane and that fuel.

A lighter plane needs less fuel to get off the ground and fly to the destination plus possibly fly around to find a place to land, and if such diversion is unnecessary, due to the lighter load of people/cargo and correspondingly less fuel, that plane may be below maximum landing weight.

One time I was on United Express going DAY-ORD, and they had to offload 6 people! That is a lot for a RJ (forgot what type but it was 2-2 seating). The reason was bad weather in ORD. They didn't explain why they would need to offload 6 people for such a short flight, but I was able to figure it out later by reading about this type of situation. I volunteered, got a UA voucher and was put on USAirways via PIT to my destination. I'm sure today they would just tell people to sit there in Dayton until tomorrow.

When a flight is payload optimized, cargo is pulled way before bags. Bags are almost the last option (behind revenue pax), so if they're being pulled, the aircraft is very close to max takeoff weight.