Wanted to just give a follow up. Despite being tempted to switch to LAX-LHR for the longer flight in the better seat, we decided to leave things as-is as the weather forecast improved and since there were lots of backup options along the way and the change would cost us the better part of the day at the destination. Everything turned out fine.

Thanks for the advice -- the reminder that thunderstorm forecasts are more variable than snow forecasts as was good, and the suggestions how to approach a reroute will probably help on some future trip.