I'm not a commercial pilot but do hold a PPL and a do believe you may be incorrect.
89F is hot and combined with altitude the aircraft has diminished performance. Since no airline wants a tech stop for fuel because of the cost associated with it, I wouldn't doubt the required the fuel to divert around a weather system.
I doubt it was poor planning, this is an airline that flies over 3000 flights a day so I think they have the planning part down to a science.
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