The ideal weather conditions for building a snow fort are those that produce snow that is sticky and packable. Generally, snow that is good for building a snow fort has the following characteristics:
Slightly wet: The snow should be slightly wet, as dry powdery snow does not pack well and can be difficult to work with. Wet snow is more cohesive and easily moldable, making it ideal for building snow structures.
Not too cold: The temperature should be cold enough to prevent the snow from melting but not too cold, as extremely low temperatures can make the snow too dry and powdery to pack.
Not too windy: Wind can cause snow to become too hard-packed, making it difficult to shape and mold.
Depth: There should be enough snow to work with, typically at least 6 inches deep.
When snow has these properties, it is easier to compact and shape into blocks for building snow forts. Additionally, slightly wet snow will freeze together overnight, making the snow fort more stable and durable. So, the best weather conditions for building a snow fort are those that produce slightly wet, packable snow that is not too cold or windy, and with sufficient depth of snow.